Temporal Bone and Transtemporal Approaches
Last Updated: August 23, 2020
The temporal bone is divided into squamosal, petrous, mastoid, tympanic, and styloid parts (Figs. 8.1 and 8.2). The squamosal part helps enclose the brain. The mastoid part is trabeculated and pneumatized to a variable degree and contains the mastoid antrum. The petrous part is compact and encloses the cochlea, the vestibule, and the semicircular, facial, and carotid canals (Fig. 8.3). The tympanic part forms part of the wall of the tympanic cavity and the external acoustic meatus. The styloid projects downward and serves as the site of attachment of several muscles. This section examines these parts in greater detail and defines the anatomic basis of the approaches directed through the temporal bone to the posterior fossa and petroclival region. The approaches examined are the middle fossa, translabyrinthine, transcochlear, combined supra- and infratentorial presigmoid, subtemporal anterior transpetrosal, subtemporal preauricular infratemporal, and the postauricular transtemporal approaches.
The approaches directed through the surface of the temporal bone forming the middle fossa floor include 1) the very limited middle fossa exposure of the internal acoustic meatus; 2) the anterior petrosectomy approach directed medial to the internal acoustic meatus through the petrous apex to access the upper anterior part of the posterior fossa and clivus; 3) the extended middle fossa approach, which may include not only resection of the roof of the internal acoustic meatus and petrous apex, but is extended lateral to the internal acoustic meatus to include resection, as needed, of the semicircular canals, vestibule, roof of the mastoid antrum and tympanic cavity, and the posterior face of the temporal bone; and 4) the subtemporal preauricular infratemporal fossa approach in which the middle fossa exposure is combined with exposure of the infratemporal fossa and, if needed, the petrous carotid, petrous apex, pterygopalatine fossae, and orbit.
The approaches directed through the mastoid in front of the sigmoid sinus vary in the amount of temporal bone resected. They include 1) the minimal mastoidectomy variant in which only enough presigmoid dura is exposed to open the dura in front of the sigmoid without exposing the labyrinth; 2) the retrolabyrinthine approach, which exposes the bony capsule of the labyrinth; 3) the partial labyrinthectomy, which includes removal of one or more of the semicircular canals; 4) the translabyrinthine approach, which includes resection of the semicircular canals and vestibule; and 5) the transcochlear modification, which includes removal of all the labyrinth, including the cochlear and possibly the petrous apex. These variants of the transmastoid approaches can all be combined, as needed, with the supra- and infratentorial presigmoid approaches to the middle and posterior fossa.
The final approach to be reviewed is the postauricular transtemporal approach, which allows lesions involving the mastoid, tympanic cavity, petrous apex, and jugular foramen to be followed backward to the areas exposed by the retrosigmoid and far-lateral approaches and forward to the infratemporal, pterygopalatine and middle fossae, lateral maxilla, and orbit. Selecting an approach directed through the temporal bone requires an understanding of its complex anatomy and its relationship to the petroclival region, the infratemporal fossa, and parapharyngeal space. Protecting and preserving the facial nerve, the petrous carotid artery, and the sensory organs of the inner ear that are contained within the temporal bone are important elements in operative approaches directed through the lateral aspect of the cranial base.
The Temporal Bone and Transtemporal Approaches
When the skull and temporal bone are viewed from a lateral perspective, some landmarks useful in performing approaches directed around and through the temporal bone can be identified (Fig. 8.2). The posterior end of the superior temporal line continues inferiorly as the supramastoid crest and blends into the upper edge of the zygomatic arch. The supramastoid crest is located at the level of the floor of the middle fossa. The junction of the supramastoid crest with the squamous suture is located at the lateral end of the petrous ridge. The meeting point of the parietomastoid and squamous sutures is located a few millimeters below the lateral end of the petrous ridge. The anterior edge of the junction of the sigmoid and transverse sinuses is located at the junction of the squamous and parietomastoid suture.
The mastoid antrum, a pneumatized space opening into the tympanic cavity, is located about 1.5 cm deep to the suprameatal triangle, a depression in the mastoid surface located between the posterosuperior edge of the external meatus, the supramastoid crest, and the vertical tangent along the posterior edge of the meatus. The suprameatal spine of Henle is located at the outer end of the posterosuperior edge of the external canal along the anterior edge of the suprameatal triangle and corresponds to the level of the lateral semicircular canal and tympanic segment of the facial nerve at a depth of approximately 1.5 cm. Several landmarks are also helpful in identifying the location of the junction of the transverse and sigmoid sinuses at the posterior aspect of the mastoid. The asterion located at the junction of the lambdoid, occipitomastoid, and parietomastoid sutures is usually located over the junction of the lower part of the transverse and sigmoid sinuses. A burr-hole placed at this site will usually expose the lower edge of this junction. A burr-hole located at the junction of the supramastoid crest and the squamosal suture will be located at the posterior part of the middle fossa floor just above and anterior to the upper edge of the junction of the transverse and sigmoid sinuses.
The Tympanic Part
The tympanic part of the temporal bone is a curved plate anterior to the mastoid process (Figs. 8.1, 8.2, and 8.4). Its concave posterior surface forms the anterior wall, floor, and part of the posterior wall of the external acoustic meatus. The roof and upper posterior wall are formed by the squamosal part. Its surface contains a portion of the tympanic sulcus for attachment of the tympanic membrane, which closes the medial end of the external canal. The anterior surface, which is concave, forms the posterior wall of the mandibular fossa. Its lateral border forms most of the margin of the external acoustic meatus. Medially, it joins the petrous part at the petrotympanic fissure through which the chorda tympani passes. The carotid canal and the jugular foramen are located medial to the tympanic part.
The styloid process, a slender spicule ensheathed by the inferior border of the tympanic bone, projects into the infratemporal fossa and is the site of attachment for the styloglossus, stylopharyngeus, and stylohyoid muscles (Fig. 8.5). It is located immediately anterior to the emergence of the facial nerve from the stylomastoid foramen and is covered laterally by the parotid gland. The stylomastoid foramen, the external end of the facial canal, opens between the styloid and mastoid processes. The facial nerve crosses the lateral surface of the styloid process, and the external carotid artery crosses the tip. Resecting the styloid process and reflecting the attached muscles downward exposes the internal jugular vein as it exits the jugular foramen and the carotid artery as it enters the carotid canal medial to the tympanic bone.
The Squamous Part
The externally convex surface of the squamosal part gives attachment to the temporalis muscle (Figs. 8.1, 8.2, and 8.5). The supramastoid crest extends backward across its posterior part, giving attachment to the temporalis muscle and fascia. The suprameatal triangle, a depressed area, located below the anterior part of the crest and behind the posterosuperior margin of the external meatus, marks the deep location of the mastoid antrum. The cerebral surface of the squamosal part is concave, accommodating the temporal lobe and joining the greater wing of the sphenoid anteriorly. The zygomatic process of the squamosal part projects forward and with the zygomatic bone completes the zygomatic arch. The attachment of the zygomatic process to the squama is wide giving it anterior and posterior edges, referred to as the anterior and posterior roots. The temporalis fascia attaches to the superior border of the arch and the masseter attaches to the lower border. The posterior root of the zygomatic process blends posteriorly into the suprameatal crest. The anterior root is located at the anterior margin of the temporomandibular joint, with the joint forming a rounded fossa on the lower margin of the zygomatic process between the anterior and posterior roots. The upper margin of the zygomatic process between the two roots gives attachment to the posterior part of the temporalis muscle. The mandibular fossa, located on the lower margin of the process between the two roots, is delimited in front by the articular tubercle and posteriorly by the postglenoid tubercle adjacent to its junction with the tympanic bone. The squamotympanic fissure is located between the medial part of the squamosal part of the mandibular fossa and the medial part of the tympanic bone. The petrotympanic fissure is situated between the tympanic plate and the petrosal part and leads into the tympanic cavity; it contains the anterior ligament of the malleus and the anterior tympanic branch of the maxillary artery. The anterior canaliculus for the chorda tympani exits the tympanic cavity in the petrotympanic fissure. The rootlets of the temporal branch of the facial nerve cross the lateral aspect of the zygomatic arch and course through the subcutaneous tissues on the superficial layer of the temporal fascia. During resection of the zygomatic arch, the superficial temporalis fascia should be carefully dissected from the underlying deep fascia, starting as close as possible to the tragal cartilage, and carried forward, reflecting the superficial fascia anteriorly to avoid damage to the filaments of the temporal branch to the frontalis muscle.
The Mastoid Part
The mastoid is the posterior part of the temporal bone (Figs. 8.1, 8.2, and 8.4). It projects downward to form the process that is the site of attachment, from superficial to deep, of the sternocleidomastoid, splenius capitis and longissimus capitis muscles, and the posterior belly of the digastric muscle (Fig. 8.5). The lower surface medial to the mastoid process is grooved by the mastoid notch to which the posterior belly of the digastric attaches. Medial to the notch, the occipital groove gives passage to the occipital artery. The fascia covering the anterior margin of the posterior belly of the digastric is continuous anteriorly with the connective tissue surrounding the emergence of the mastoid segment of the facial nerve from the stylomastoid foramen and can be used as a landmark for identifying the initial extracranial segment of the nerve. After exiting the stylomastoid foramen, the nerve divides in the substance of the parotid gland into temporal, zygomatic, buccal, marginal mandibular, and cervical branches (Fig. 8.5). The temporal and zygomatic branches cross the zygomatic arch and the superficial fascia of the temporalis muscle. Keeping the connective tissue surrounding the nerve at the stylomastoid foramen intact during mobilization of the facial nerve will reduce the risk of facial nerve damage. The posterior border of the mastoid process is perforated by one or more foramina through which an emissary vein to the sigmoid sinus and a dural branch from the occipital artery pass.
The medial aspect of the mastoid process is grooved by the sigmoid sinus (Figs. 8.1-8.4). The sinus represents the posterior limit of the mastoid cavity. The sinus meets the roof of the cavity at the level of the petrous ridge. The angle between the superior petrosal and sigmoid sinuses and the middle fossa dura delimits a dural space called the sinodural angle. The sinodural angle is an important landmark when exposing the contents of the mastoid. Inferiorly, the sigmoid sinus curves medially and forward, crossing the occipital bone to enter the jugular foramen. The superior aspect of the jugular foramen corresponds to the apex of the jugular bulb and constitutes the inferior limit of the mastoid cavity.
The medial limit of the mastoid cavity is formed by the block of solid bone, the otic capsule, containing the bony labyrinth (Figs. 8.4 and 8.6). The area of posterior fossa dura mater that can be exposed through the mastoid cavity between the sigmoid and superior petrosal sinuses, the otic capsule, and the jugular bulb is called Trautman’s triangle. The size of this dural triangle is important in surgical procedures in which the dura delimited by the triangle must be opened medial to the sigmoid sinus. The distance from the anterior margin of the sigmoid sinus to the otic capsule at the level of the posterior semicircular canal averages 8 mm (range, 6–9 mm) on the right side, and 7 mm (range, 4–9 mm) on the left (44).
The distance between the apex of the jugular bulb and the superior petrosal sinus is also an important determinate of the size of exposure that can be achieved by opening Trautman’s triangle. This distance is reduced if there is a high jugular bulb. The jugular bulb usually lies inferior to the ampulla of the posterior semicircular canal, but it can project superiorly as far as the level of the lateral semicircular canal (27). The average distance from the jugular bulb to the superior petrosal sinus is 14 mm (range, 10–19 mm) on the right side, and 16 mm (range, 11–21 mm) on the left (44).
The mastoid interior is composed of trabeculated bone, which coalesces to form a cavity, the mastoid antrum, that communicates through an opening, the aditus, that leads forward to the epitympanic part of in the tympanic cavity (Figs. 8.4 and 8.6). The lateral semicircular canal is medial to the epitympanic recess. The medial wall of the antrum faces the posterior semicircular canal. The roof is formed by the tegmen in the floor of the middle cranial fossa. The mastoid segment of the facial canal courses adjacent to the anteroinferior margin of the antrum. The lateral wall of the mastoid antrum, through which it is usually approached surgically, is formed by the postmeatal part of the squamous temporal bone. The lateral wall of the antrum is located deep to the suprameatal triangle, which is demarcated superiorly by the suprameatal crest, located at the level of the floor of the middle fossa; anteroinferior by the posterosuperior margin of the acoustic meatus, which indicates approximately the position of the descending or mastoid part of the facial canal; and posteriorly by a posterior vertical tangent to the posterior margin of the external meatus. The air cells in the mastoid may extend behind the sigmoid sinus and into the squamosal part of the temporal bone, the posterior root of the zygomatic process, the osseous roof of the external acoustic meatus, the floor of the tympanic cavity near the jugular bulb, and the petrous apex surrounding the carotid canal, eustachian tube, and labyrinth.
The tympanic cavity is a narrow air-filled space between the tympanic membrane laterally and the promontory containing the auditory and vestibular labyrinth medially (Figs. 8.4, 8.6, and 8.7). It communicates posteriorly with the mastoid antrum and anteriorly through the eustachian tube with the nasopharynx. It contains the malleus, incus, and stapes. The tympanic cavity opens upward into the epitympanic recess, which contains the heads of the malleus and the incus. The roof of the tympanic cavity is formed by a thin plate, the tegmen tympani, which separates the middle fossa and tympanic cavities, and also roofs the mastoid antrum and the tensor tympani. The thin floor of the tympanic cavity separates the cavity from the jugular bulb. The medial part of the floor is perforated by an opening for the tympanic branch of the glossopharyngeal nerve. The lateral wall is formed by the tympanic membrane and the osseous ring to which the membrane attaches. The ring is deficient above near the openings of the anterior and posterior canaliculi for the chorda tympani (Figs. 8.4 and 8.6). The posterior canaliculus for the chorda tympani arises from the facial canal a few millimeters above the mastoid foramen and ascends in front of the facial canal to open into the tympanic cavity at the level of the upper part of the handle of the malleus. The chorda tympani passes in close relation to the tympanic membrane and the medial aspect of the neck of the malleus and forward to enter its anterior canaliculus at the medial aspect of the petrotympanic fissure, and descends vertically medial to the sphenoid spine and lateral pterygoid muscle to join the lingual nerve.
The medial wall of the tympanic cavity, which forms the lateral boundary of the inner ear and the petrosal part of the temporal bone, is the site of the promontory, the oval and round windows, and the prominence over the facial nerve (Figs. 8.2 and 8.4). The tympanic nerve plexus grooves the promontory overlying the lateral bulge of the basal turn of the cochlea. The apex of the cochlea lies near the medial wall of the cavity anterior to the promontory. The oval window is posterosuperior to the promontory and connects the tympanic cavity to the vestibule, and is occupied by the footplate of the stapes. The round window is posteroinferior to the oval window and opens under the overhanging edge of the promontory. The prominence of the facial canal is located above the oval window. The posterior wall of the tympanic cavity is mainly the site of the aditus, the opening of the tympanic cavity, into the mastoid antrum. The medial wall of the aditus has a round prominence overlying the lateral semicircular canal. The pyramidal eminence, which houses the stapedial muscle, is located just behind the oval window and anterior to the mastoid part of the facial canal. The stapedius extends forward from the eminence to attach to the neck of the stapes. The fossa incudis is a small depression low and posterior in the epitympanic recess; it contains the short process of the incus, which is fixed to the fossa by ligamentous fibers.
The anterior wall of the tympanic cavity narrows and leads into the eustachian tube, which communicates the nasopharynx with the tympanic cavity (Figs. 8.4, 8.7, and 8.8). It has bony and cartilaginous parts. The bony part begins in the anterior part of the tympanic cavity and is directed anteriorly and medially. It joins the cartilaginous part at the junction of the squamous and petrous parts of the temporal bone. The cartilaginous part of the tube is attached to the lower margin of the sphenopetrosal groove, which is situated between the petrous bone and the greater wing of the sphenoid bone, and its base lies directly under the mucous membrane of the lateral wall of the nasaopharynx. Both the petrous carotid and eustachian tube are directed anteromedially, with the eustachian tube being located along the anterior margin of the carotid canal (Figs. 8.7 and 8.8). The tensor tympani muscle and its bony semicanal are located above the eustachian tube, parallel to the horizontal segment of the petrous carotid. The canals for the tensor tympani superiorly and the osseous part of the eustachian tube inferiorly open into the upper part of the anterior wall of the tympanic cavity. These canals are inclined downward, anteriorly, and medially; they open into the angle between the squamous and petrous parts of the temporal bone and are separated by a thin, bony septum. The canal for the tensor tympani extends posterolaterally on the medial wall of the tympanic cavity, to end above the oval window where the posterior end of the canal curves laterally to form a pulley, the trochleariform process, around which the tensor tympani tendon turns laterally to attach to the handle of the malleus.
The Petrous Part
The petrous part of the temporal bone is wedged between the sphenoid and occipital bones (Figs. 8.1 and 8.3). It contains the acoustic and vestibular labyrinth and is the site of the jugular fossa and the facial and carotid canals (Figs. 8.3, 8.4, and 8.7). It has a base, apex, three surfaces and margins. The apex is located in the angle between the greater wing of the sphenoid and the occipital bone and is the site of the carotid canals medial opening. It forms the posterolateral limit of the foramen lacerum. The anterior surface faces the floor of the middle cranial fossa and its surface is grooved by the trigeminal impression for the trigeminal ganglion; anterolateral to this, it forms the roof of the carotid canal (Figs. 8.1 and 8.7). Lateral to the trigeminal impression is a shallow depression, which partially roofs the internal acoustic meatus and is limited laterally by the arcuate eminence, which overlies the superior semicircular canal. The posterior slope of the arcuate eminence overlies the posterior and lateral semicircular canals. Farther laterally, the roof covers the vestibule and part of the facial canal. The tegmen extends laterally from here and roofs the mastoid antrum and tympanic cavities and the canal for the tensor tympani. Opening the tegmen from above exposes the heads of the malleus, incus, the tympanic segment of the facial nerve, and the superior and lateral semicircular canals (Fig. 8.7). The tympanic segment of the facial nerve begins at the geniculate ganglion and ends at the level of the stapes, where the nerve turns downward below the lateral semicircular canal. The tegmen anteriorly is grooved by the greater petrosal nerve extending anterior and medial from the area in front of the arcuate imminence and crossing the floor of the middle fossa toward the foramen lacerum (Figs. 8.7 and 8.8). The greater petrosal nerve can be identified medial to the arcuate eminence as it leaves the geniculate ganglion by passing through the facial hiatus to reach the middle fossa floor. It runs beneath the dura of the middle fossa in the sphenopetrosal groove formed by the junction of the petrous and sphenoid bones, immediately superior and anterolateral to the horizontal segment of the petrous carotid. In a previous study, we found that bone of the middle cranial fossa was absent over the geniculate ganglion in 16% of the specimens, thus exposing the facial nerve and geniculate ganglion to the danger of injury during elevation of the dura from the floor of the middle fossa (31). Facial nerve injury can also result from damaging the branch of the middle meningeal artery, which passes through the facial hiatus to supply the nerve, or from traction applied to the ganglion when manipulating the greater petrosal nerve (30).
The lesser petrosal nerve from the tympanic plexus passes through the tympanic canaliculus, which is located anterior to the facial hiatus and courses in an anteromedial direction parallel to the greater petrosal nerve (Fig. 8.8). The cochlea lies below the floor of the middle fossa in the angle between the labyrinthine segment of the facial nerve and the greater petrosal nerve, just medial to the geniculate ganglion, anterior to the fundus of the internal acoustic meatus, and posterosuperior to the lateral genu of the petrous carotid artery. The cochlea is separated from the petrous carotid by a 2.1 mm (range, 0.6–10.0 mm) thickness of bone and can be injured during exposure of the petrous carotid. The middle meningeal artery, an important landmark when approaching the structures of the middle fossa, enters the cranial cavity through the foramen spinosum of the sphenoid bone. The foramen spinosum is an average of 4.5 mm (range, 3–6 mm) anterolateral to the carotid canal and 14.0 mm (range, 11.0–17.0 mm) anterolateral to the geniculate ganglion (44).
The posterior surface of the petrosal part faces the posterior cranial fossa and cerebellopontine angle and is continuous with the mastoid surface (Figs. 8.1-8.3). The opening for the internal auditory meatus is situated midway between the base and the apex on the posterior surface. The lateral end of the meatus is divided into superior and inferior halves by the transverse crest. The area above the transverse crest is further divided by the vertical crest, also called Bill’s bar, which separates the anteriorly located facial canal from the posteriorly located superior vestibular area (29). The cochlea and inferior vestibular nerves penetrate the lateral end of the meatus below the transverse crest, with the cochlear nerve being located anteriorly. The posterior wall of the meatus, lateral to the porus is the site of a small bony opening, the subarcuate fossa, which gives passage to the subarcuate artery, a branch of the anteroinferior cerebellar artery (AICA), which usually ends blindly in the region of the superior semicircular canal. Inferolateral to the porus of the meatus is the opening for the vestibular aqueduct, which transmits the endolymphatic duct that opens below into the endolymphatic sac located between the dural layers. The opening of the cochlear aqueduct, also called the cochlear canaliculus and occupied by the perilymphatic duct, is situated inferior to the porus of the internal meatus at the anteromedial edge of the jugular foramen, just superior and lateral to where the glossopharyngeal nerve enters the intrajugular part of the jugular foramen.
The inferior surface is very irregular. The apex is connected medially to the clivus by fibrocartilage and gives attachment to the levator veli palatini and the cartilaginous portion of the eustachian tube (Figs. 8.1 and 8.9). Behind this is the opening of the carotid canal, behind which is the jugular fossa that contains the jugular bulb. The small foramen for the tympanic branch of the glossopharyngeal nerve is located on the ridge between the carotid canal and jugular foramen (Fig. 9.2). On the lateral wall of the jugular bulb is the mastoid canaliculus for the auricular branch of the vagus nerve. The superior border, located along the petrous ridge, is grooved by the superior petrosal sinus and serves as the attachment of the tentorium cerebelli, except medially where it is crossed by the posterior trigeminal root. The lower posterior border, located along the petroclival fissure, is the site of a groove in which resides the inferior petrosal sinus that connects the cavernous sinus and the medial wall of the jugular bulb. Behind this, the jugular fossa of the temporal bone joins with the jugular notch on the jugular process of the occipital bone to form the margins of the jugular foramen.
The jugular foramen is located at the lower end of the petro-occipital fissure and is divided into a larger lateral opening, the sigmoid part, that receives the drainage of the sigmoid sinus, and a small medial part, the petrosal part, that transmits the inferior petrosal sinus (Fig. 9.1). The intrajugular part, located between the sigmoid and petrosal parts, transmits the glossopharyngeal, vagus, and accessory nerves. The anterior border is joined laterally to the temporal squama at the petrosquamosal suture and medially articulates with the sphenoid’s greater wing.
The bony labyrinth consists of three parts: the vestibule, the semicircular canals, and the cochlea. The vestibule, located in the central part of the bony labyrinth, is a small cavity at the confluence of the ampullate and nonampullated ends of the semicircular canals. It is situated lateral to the meatal fundus, medial to the tympanic cavity, posterior to the cochlea, and superior to the apex of the jugular bulb (Figs. 8.3, 8.4, and 8.7).
The floor of the vestibule is separated from the apex of the jugular bulb by a thickness of bone that averages 6 mm (range, 4–8 mm) on the right side and 8 mm (range, 4–10 mm) on the left side (44). This distance is particularly important during translabyrinthine approaches since the height of the jugular bulb is a major determinant of the size of the exposure of the cerebellopontine angle that can be achieved with this approach. A high-placed jugular bulb may be the source of troublesome bleeding and air emboli if it is opened during exposure of the labyrinth or internal acoustic meatus.
The semicircular canals are situated posterosuperior to the vestibule (Figs. 8.3, 8.4, and 8.7). The anterior part of the lateral semicircular canal is situated above the tympanic segment of the facial nerve and can be used as a guide to locating that segment of the nerve. The posterior semicircular canal lies parallel to and in close proximity with the posterior surface of the petrous bone in the area just behind and lateral to the lateral end of the internal acoustic meatus. The superior semicircular canal projects toward the floor of the middle fossa, usually in close relation to the arcuate eminence. Each canal has an ampullated and a nonampullated end that opens into the vestibule. The anterior end of the lateral and superior canals and the inferior end of the posterior canal are the site of the ampullae, which are innervated by the vestibular nerves. The posterior ends of the superior and posterior canals, the ends opposite the ampullae, join to form a common crus that opens into the vestibule. The superior vestibular nerve innervates the ampullae of the superior and lateral canals, and the singular branch of the inferior vestibular nerve innervates the posterior ampulla. The vestibular nerves also have branches to the utricle and saccule located within the vestibule. The internal auditory meatus can be found medial to the arcuate eminence at an angle of about 60 degrees medial from the long axis of the superior semicircular canal. The superior canal is the most susceptible to damage in completing the middle fossa approach to the internal acoustic meatus. The posterior canal may be damaged in removing the posterior wall to expose the meatal contents by the retrosigmoid approach (Fig. 8.3).
During surgical approaches to the cerebellopontine angle in which the posterior meatal lip is removed, care should be taken to avoid opening the vestibular aqueduct, vestibule, posterior semicircular canal, or the common crus (Figs. 8.2 and 8.3). In our studies, we observed that there is a constant set of relationships among the structures around the posterior meatal lip. The common crus of the posterior and superior semicircular canals is located lateral to the entrance of the subarcuate artery into the subarcuate fossa. The vestibular aqueduct has an oblique orientation. It leaves the vestibule and runs in a posterior direction to open beneath the dura mater at a level corresponding to that of the posterior semicircular canal. The average distance between the posterior semicircular canal, at the level with the junction of the common crus, and the lateral edge of the porus was 7 mm (range, 5–9 mm) (44).
The carotid artery, at the point where it enters the carotid canal, is surrounded by a strong layer of connective tissue that makes it difficult to mobilize the artery at this point (Figs. 8.9 and 8.10) (38, 39). The vertical segment of the artery passes upward in the canal toward the genu, where it curves anteromedially to form the horizontal segment. The eustachian tube and the tensor tympani muscle are located parallel to and along the anterior margin of the horizontal segment, where they are separated from the artery by a thin layer of bone.
The trigeminal ganglion and the adjacent part of the posterior root and their surrounding dural and arachnoidal cavern, called Meckel’s cave, sit in an impression on the upper surface of the petrous apex above the medial part of the petrous carotid (Figs. 8.1, 8.7, and 8.8). The length of the horizontal segment of the petrous carotid that can be exposed by removing bone lateral to the trigeminal ganglion averages 8.1 mm (range, 4.0–11.0 mm) (44). The length that can be exposed can be increased if the mandibular branch of the trigeminal nerve is retracted or divided, after which the average length that can be exposed increases to 20.1 mm (range, 17.5–28.0 mm) (Figs. 8.7 and 8.8) (10, 17). Gaining this added exposure can be particularly helpful during surgical procedures that are directed through the petrous apex to complete a vascular anastomosis, to occlude the artery for control of bleeding, and to allow for mobilization of the vertical and horizontal segments of the artery (40). A venous plexus of variable size, an extension of the cavernous sinus within the periosteal covering of the distal part of the canal, surrounds the artery.
The facial nerve in the temporal bone, which often blocks access to lesions within and deep to the temporal bone, is divided into three segments (Figs. 8.4, 8.5, and 8.7). The first, or labyrinthine segment, which is located in the petrous part, extends from the meatal fundus to the geniculate ganglion and is situated between the cochlea anteromedially and the semicircular canals posterolaterally. The labyrinthine segment ends at the site at which the greater superficial petrosal nerve arises from the facial nerve at the level of the geniculate ganglion. From there, the nerve turns laterally and posteriorly along the medial surface of the tympanic cavity, thus giving the name tympanic segment to that part of the nerve. The tympanic segment runs between the lateral semicircular canal above and the oval window below. As the nerve passes below the midpoint of the lateral semicircular canal, it turns vertically downward and courses through the petrous part adjacent to the mastoid part of the temporal bone; thus the third segment, which ends at the stylomastoid foramen, is called the mastoid or vertical segment.
These transtemporal operative approaches are often directed to the petroclival region located where the posterior surface of the petrous temporal bone meets the clival part of the occipital bone along the petroclival fissure. The junction of the two bones forms a line that extends from the jugular foramen to the petrous apex (Fig. 8.1). From a surgical standpoint, the intradural compartments of the petroclival region are divided along this petroclival line into 1) an inferior space related to the medulla and to the structures around the region of the foramen magnum; 2) a middle space related to the pons and to the structures in the prepontine and cerebellopontine angle; and 3) a superior space related to the contents of the interpeduncular cistern, and to the sellar and parasellar regions.
The Inferior Petroclival Space
The inferior petroclival space corresponds to the anterior surface of the medulla and adjacent part of the clivus and anterior margin of the foramen magnum (4). The neurovascular structures in this region are those contained in the premedullary cistern. The superior limit is the junction of the pons and medulla. The inferior limit is the rostral margin of the first cervical nerve root, the site of the junction of the spinal cord and the medulla. The inferior petroclival space includes the lower four cranial nerves, lower part of the cerebellum, the vertebral artery and its branches, and the structures around the occipital condyle.
The Middle Petroclival Space
The middle petroclival space corresponds to the anterolateral surface of the pons and cerebellum. Its superior limit is at the pontomesencephalic sulcus and the lower limit is at the pontomedullary sulcus. The lateral limits are formed by the posterior surface of the petrous bone and by the contents of the cerebellopontine angle including the trigeminal, abducens, facial, and vestibulocochlear nerves, the basilar artery, and the AICA and the superior petrosal veins.
The Superior Petroclival Space
The superior petroclival space is located anterior to the midbrain and corresponds to the anterior part of the tentorial incisura. It extends anteriorly and laterally to the sellar and parasellar regions. Its roof is formed by the diencephalic structures forming the floor of the third ventricle. The posterior limit is formed by the cerebral peduncles and the posterior perforated substance. The inferior limit is situated above the origin of the trigeminal nerve at the pontomesencephalic sulcus. It includes the intradural segment of the oculomotor and trochlear nerves, the basilar artery and its branching into the posterior cerebral artery (PCA) and superior cerebellar artery (SCA), and the cavernous carotid and its intracavernous branches to the dura of the upper clivus. The medial edge of the tentorium divides the superior petroclival space into infra- and supratentorial compartments.
The structures important in accessing the temporal bone from posteriorly and laterally have already been reviewed. This section reviews the structures located in front of the temporal bone that are important in reaching lesions that involve the bone or involve both the bone and areas anterior to it. They include several muscles, like the temporalis and masseter, the infratemporal fossa, and the parapharyngeal spaces.
The temporalis muscle, along with the deep temporal vessels, passes between the gap formed by the zygomatic arch and the floor of the temporal fossa (Fig. 8.5). The muscle attaches to the coronoid process of the mandible. The superficial and the deep temporalis fasciae attach, respectively, to the lateral and medial aspects of the upper border of the zygomatic arch. Inferiorly, the parotid fascia invests the parotid gland and the masseter muscle and attaches to the lower border of the zygomatic arch. The masseter muscle has two superimposed layers. A superficial layer which attaches to the zygomatic process of the maxilla and anterior part of the lower border of the zygomatic arch and a deep layer which attaches to the medial aspect of the whole zygomatic arch. Inferiorly it inserts onto the angle and ramus of the mandible.
The parotid gland, the parotid duct, and the branches of the facial nerve are located superficial to the masseter muscle (Figs. 8.5, 8.9, and 8.10). In surgical procedures in which the mandibular condyle is resected or displaced inferiorly, the parotid gland, along with the branches of the facial nerve, can be dissected from the underlying masseter to avoid excessive traction on the facial nerve and to reduce the risk of facial palsy (33).
Muscles commonly encountered in operative approaches to the region of the temporal bone include the posterior belly of the digastric muscle and the muscles attached to the styloid process. The posterior digastric belly originates in the digastric groove, lateral to the occipital groove in which the occipital artery courses, and inserts onto the hyoid bone. The muscles attached to the styloid process, the stylohyoid, styloglossus, and stylopharyngeus muscles, extend to the hyoid bone, tongue, and pharyngeal wall, respectively.
The infratemporal fossa, a route through which some temporal bone lesions can be reached, is a not uncommon site of involvement by lesions that also involve the temporal bone (11). The osseous boundaries of the infratemporal fossa are the posterolateral maxillary surface anteriorly, the lateral pterygoid plate anteromedially, the mandibular ramus laterally, and the tympanic part of the temporal bone and the styloid process posteriorly. The fossa is domed anteriorly by the infratemporal surface of the greater sphenoid wing, the site of the foramina ovale and spinosum, and posteriorly by the squamous part of the temporal bone (Figs. 8.8-8.10). The inferior, posteromedial, and superolateral aspects are open without bony walls.
The structures located in the infratemporal fossa are the pterygoid muscles and venous plexus and the branches of the maxillary artery and mandibular nerve. The lateral pterygoid muscle crosses the upper part of the infratemporal fossa, originating from the upper and lower heads; the upper head arises from the infratemporal surface of the greater sphenoid wing, and the lower head originates from the lateral pterygoid plate (Figs. 8.8-8.10). Both heads pass posterolaterally and insert on the neck of the mandibular condylar process and the articular disc of the temporomandibular joint. The medial pterygoid muscle crosses the lower part of the infratemporal fossa and arises with superficial and deep heads; the superficial head arises from the lateral aspect of the palatine pyramidal process and the maxillary tuberosity and passes superficial to the lower head of the lateral pterygoid; and the deep head originates from the medial surface of the lateral pterygoid plate and the pterygoid fossa between the two pterygoid plates and passes deep to the lower head of the lateral pterygoid. Both heads descend backward and laterally to attach to the medial surface of the mandibular ramus below the mandibular foramen. The sphenomandibular ligament, located medial to the mandibular condylar process, descends from the sphenoid spine to attach to the lingula of the mandibular foramen. The structures located or passing between the sphenomandibular ligament and the mandible are the lateral pterygoid and the auriculotemporal nerve superiorly, and the inferior alveolar nerve, the parotid gland, the maxillary artery and its inferior alveolar branch inferiorly.
The maxillary artery is divided into three segments: mandibular, pterygoid, and pterygopalatine (Figs. 8.8-8.10). The mandibular segment arises from the external carotid artery near the posterior border of the condylar process, passes between the process and the sphenomandibular ligament, along the inferior border of the lower head of the lateral pterygoid, and gives rise to the deep auricular, anterior tympanic, middle and accessory meningeal, and the inferior alveolar arteries. The middle meningeal ascends medial to the lateral pterygoid to enter the foramen spinosum, the accessory meningeal arises from the maxillary or middle meningeal to enter the foramen ovale, and the inferior alveolar descends to enter the mandibular foramen. The pterygoid segment usually courses lateral to, but occasionally medial to, the lower head of the lateral pterygoid and gives rise to the deep temporal, pterygoid, masseteric, and buccal arteries. The pterygopalatine segment courses between the two heads of the lateral pterygoid and enters the pterygopalatine fossa by passing through the pterygomaxillary fissure. Its branching will be described with the pterygopalatine fossa.
The pterygoid venous plexus is located in the infratemporal fossa and has two parts: a superficial part located between the temporalis and lateral pterygoid; and a deep part situated between the lateral and medial pterygoids anteriorly, and between the lateral pterygoid and the parapharyngeal space posteriorly. The deep part is more prominent and connects with the cavernous sinus by emissary veins passing through the foramina ovale and spinosum, and occasionally through the sphenoidal emissary foramen (foramen of Vesalius). The main drainage of the pterygoid plexus is through the maxillary vein to the internal jugular vein.
The mandibular nerve enters the infratemporal fossa by passing through the foramen ovale on the lateral side of the parapharyngeal space, where it gives rise to several smaller branches, and then divides into a smaller anterior trunk and a larger posterior trunk (Figs. 8.8-8.10). The anterior trunk gives rise to the deep temporal and masseteric nerves, which supply the temporalis and the masseter, respectively, and the nerve to the lateral pterygoid. The buccal nerve, which conveys sensory fibers, passes anterolaterally between the two heads of the lateral pterygoid, and descends lateral to the lower head to reach the buccinator and the buccal mucosa. The posterior trunk gives off the lingual, inferior alveolar, and auriculotemporal nerves, which descend medial to the lateral pterygoid. The lingual and inferior alveolar nerves, the former coursing anterior to the latter, pass between the lateral and medial pterygoids. The auriculotemporal nerve usually splits to encircle the middle meningeal artery and passes posterolaterally between the mandibular ramus and the sphenomandibular ligament. The chorda tympani nerve, which contains the taste fibers from the anterior two-thirds of the tongue and the parasympathetic secretomotor fibers to the submandibular and sublingual salivary glands, enters the infratemporal fossa through the petrotympanic fissure, descends medial to the auriculotemporal and inferior alveolar nerves, and joins the lingual nerve. The otic ganglion is situated immediately below the foramen ovale on the medial side of the mandibular nerve. The ganglion receives the lesser petrosal nerve, which crosses the floor of the middle fossa anterolateral to the greater petrosal nerve to exit through the foramen ovale or the more posteriorly situated canaliculus innominatus and conveys parasympathetic secretomotor fibers to the parotid gland via the auriculotemporal nerve. The medial pterygoid nerve arises from the medial aspect of the mandibular nerve close to the otic ganglion and descends to supply the medial pterygoid and tensor veli palatini. The nervus spinosus, a meningeal branch, also arises near the otic ganglion and ascends through the foramen spinosum to innervate the middle fossa dura.
The parapharyngeal space is located in the lateral pharyngeal wall and is shaped like an inverted pyramid, with its base on the skull base superiorly and its apex at the hyoid bone inferiorly. The parapharyngeal space is subdivided into prestyloid and poststyloid compartments by the styloid diaphragm, a fibrous sheet that also constitutes the anterior part of the carotid sheath (Figs. 8.5 and 8.9). The prestyloid part, situated anteriorly between the fascia covering the opposing surfaces of the medial pterygoid and tensor veli palatini, is a thin fat-filled compartment separating the structures in the infratemporal fossa from the eustachian tube and the tensor and levator veli palatini muscles in the lateral nasopharyngeal wall. The upper portion of the prestyloid part is situated between two fascial sheets, which are oriented in a sagittal plane. The lateral sheet arises from the medial surface of the medial pterygoid, passes upward, backward, and medial to the mandibular nerve and the middle meningeal artery, incorporating the sphenomandibular ligament posteriorly, and reaching the retromandibular deep lobe of the parotid gland. The medial sheet is formed by the fascia overlying the lateral surface of the tensor veli palatini and is continuous inferiorly with the fascia over the superior pharyngeal constrictor and posteriorly with the thick styloid diaphragm, which envelopes the stylopharyngeus, styloglossus, and stylohyoid and blends into the carotid sheath. The superior border is located where the two fascial sheets fuse together and insert in the skull base along a line extending backward from the pterygoid process lateral to the origin of the tensor veli palatini, medial to the foramina ovale and spinosum to the sphenoid spine and the posterior margin of the glenoid fossa. The sharply angled inferior boundary is situated at the junction of the posterior digastric belly and the greater hyoid cornu. The poststyloid part, which contains the internal carotid artery, internal jugular vein, and the initial extracranial segment of cranial nerves IX through XII, is separated from the infratemporal fossa by the posterolateral portion of the prestyloid part. The glossopharyngeal nerve exits the skull through the intrajugular part of the jugular foramen, anterior to the vagus and accessory nerves, and passes forward, medial to the styloid process in close relationship to the lateral surface of the carotid artery as the artery enters the carotid canal (Fig. 8.9). Care is required to avoid injury to the glossopharyngeal nerve if the artery is to be mobilized at the carotid canal. The vagus nerve leaves the skull through the anteromedial edge of the intrajugular part of the foramen and courses deep within the carotid sheath, between the internal carotid artery and the jugular vein. The accessory nerve exits the intrajugular part and runs backward, lateral to the jugular vein and medial to the styloid process and the posterior belly of the digastric muscle, to innervate the sternocleidomastoid muscle.
The hypoglossal nerve exits through the hypoglossal canal, deep to the jugular vein and to the nerves emerging from the jugular foramen, and runs downward, between the carotid artery and the jugular vein (Figs. 8.9 and 8.10). It becomes superficial at the level of the angle of the jaw where it crosses the internal and external carotid arteries, close to the level of the common carotid bifurcation, to innervate the tongue.
The pterygopalatine fossa, which opens laterally into the medial part of the infratemporal fossa, is bounded posteriorly by the sphenoid pterygoid process, medially by the palatine perpendicular plate, that bridges the interval between the maxilla and pterygoid process, and opens superiorly through the medial part of the inferior orbital fissure into the orbital apex (Figs. 8.5, 8.9, and 8.10) (11). The fossa contains the maxillary nerve, pterygopalatine ganglion, maxillary artery, and their branches, all embedded in fat tissue. Its lateral boundary, the pterygomaxillary fissure, opens into the infratemporal fossa and allows passage of the maxillary artery from the infratemporal into the pterygopalatine fossa, where the artery gives rise to its terminal branches. The lower part of the fossa is funnel-shaped, with its inferior apex opening into the greater and lesser palatine canals, which transmit the greater and lesser palatine nerves and vessels, and communicate with the oral cavity. The sphenopalatine foramen, located in the upper part of the fossa’s medial wall, conveys the sphenopalatine nerve and vessels, and opens into the superior nasal meatus just above the root of the middle nasal concha. The foramen rotundum opens just below the superior orbital fissure through the superior part of the posterior wall of the fossa. The pterygoid canal opens through the sphenoid pterygoid process inferomedial to the foramen rotundum and conveys the vidian nerve carrying autonomic fibers to the pterygopalatine ganglion. The maxillary nerve, after entering the fossa, gives off ganglionic branches to the pterygopalatine ganglion. It then deviates laterally just beneath the inferior orbital fissure, giving rise to, in order, the zygomatic and posterosuperior alveolar nerves outside of the periorbita. It then turns medially as the infraorbital nerve, passing through the inferior orbital fissure to enter the infraorbital groove, where the anterior and middle superior alveolar nerves arise. Finally, it exits the infraorbital foramen to terminate on the cheek. The pterygopalatine ganglion, located in front of the pterygoid canal and inferomedial to the maxillary nerve, receives communicating rami from the maxillary nerve and gives rise to the greater and lesser palatine nerves from the lower surface of the ganglion, the sphenopalatine nerve and pharyngeal branch from the medial surface, and the orbital branch from the superior surface. The vidian nerve is formed by the union of the greater petrosal nerve, which conveys parasympathetic fibers arising from the facial nerve at the level of the geniculate ganglion, and the deep petrosal nerve, which conveys sympathetic fibers from the carotid plexus, to reach the lacrimal gland and nasal mucosa. The parasympathetic fibers synapse in the pterygopalatine ganglion, whereas the sympathetic fibers do not. The sympathetic fibers synapse in the superior cervical sympathetic ganglion.
The third or pterygopalatine segment of the maxillary artery enters the pterygopalatine fossa by passing through the pterygomaxillary fissure. This segment courses in an anterior, medial, and superior direction and gives rise to the infraorbital artery, which passes through the inferior orbital fissure and courses with the infraorbital nerve; the posterosuperior alveolar artery, which descends to pierce the posterolateral wall of the maxilla; the recurrent meningeal branches, which pass through the foramen rotundum; and the greater and lesser palatine arteries, which descend through the greater and lesser palatine canals; the vidian artery to the pterygoid canal; the pharyngeal branch to the palatovaginal canal; and finally the sphenopalatine artery, which passes through the sphenopalatine foramen to reach the nasal cavity and is considered to be the terminal branch of the maxillary artery because of its large diameter. The arterial structures in the pterygopalatine fossa are located anterior to the neural structures.
The arteries that may be involved in pathological abnormalities involving the temporal bone include the upper cervical and petrous portions of the internal carotid artery, the posteriorly directed branches of the external carotid artery, and the upper portion of the vertebral artery.
Common Carotid Artery
The common carotid artery bifurcates into the internal and external carotid arteries at the level of the upper border of the thyroid cartilage. The internal carotid artery initially ascends relatively superficial in the carotid triangle of the neck, but assumes a much deeper position after passing medial to the posterior belly of the digastric (Figs. 8.9 and 8.10). Below the digastric, it is crossed by the hypoglossal nerve and the ansa cervicalis, and by the lingual and facial veins. Medial to the digastric, it is crossed by the stylohyoid muscle and the occipital and posterior auricular arteries. Superior to the digastric, the internal carotid artery is separated from the external carotid artery by the styloid process and the muscles attached to it. At the entrance into the carotid canal, the artery is involved by a dense sheath of connective tissue and is separated from the internal jugular vein by the hypoglossal nerve and by the nerves exiting from the jugular foramen.
The internal carotid artery passes, almost straightly upward, posterior to the external carotid artery and anteromedial to the internal jugular vein to reach the carotid canal. At the level of the skull base, the internal jugular vein courses just posterior to the internal carotid artery, being separated from it by the carotid ridge. Between them, the glossopharyngeal nerve is located laterally and the vagus, accessory, and hypoglossal nerves medially.
After the internal carotid artery enters the carotid canal with the carotid sympathetic nerves and surrounding venous plexus, it ascends a short distance (the vertical segment), reaching the area below and slightly behind the cochlea, where it turns anteromedially at a right angle (the site of the lateral bend) and courses horizontally (the horizontal segment) toward the petrous apex (Figs. 8.8-8.10). At the medial edge of the foramen lacerum, it turns sharply upward at the site of the medial bend to enter the posterior part of the cavernous sinus.
External Carotid Artery
The external carotid artery ascends anterior to the internal carotid artery on the posteromedial margin of the parotid gland and medial to the digastric and stylohyoid muscles. Proximal to its terminal bifurcation into the maxillary and the superficial temporal arteries, it gives rise to six branches that can be divided into anterior and posterior groups according to their directions. The latter group is related to the region of the temporal bone.
The ascending pharyngeal artery, the first branch of the posterior group, often provides the most prominent supply to the meninges around the jugular foramen (18). It arises either at the bifurcation or from the lowest part of the external or internal carotid arteries. Rarely, it arises from the origin of the occipital artery. It courses upward between the internal and the external carotid arteries, giving rise to numerous branches to neighboring muscles, nerves, and lymph nodes. Its meningeal branches pass through the foramen lacerum to be distributed to the dura lining the middle fossa and through the jugular foramen or the hypoglossal canal to supply the surrounding dura of the posterior cranial fossa. The ascending pharyngeal artery also gives rise to the inferior tympanic artery, which reaches the tympanic cavity by way of the tympanic canaliculus along with the tympanic branch of the glossopharyngeal nerve.
The occipital artery, the second and largest branch of the posterior group, arises from the posterior surface of the external carotid artery and courses obliquely upward between the posterior belly of the digastric muscle and the internal jugular vein, and then medial to the mastoid process and either superficial or deep to the longissimus capitis muscle (Fig. 8.5). It courses deep to the latter muscle if it courses in the occipital groove of the mastoid bone, which is located medial to the digastric groove. After passing the longissimus capitis muscle, the occipital artery courses deep to the splenius capitis muscle, finally reaching a subcutaneous location by piercing the fascia between the attachment of the sternocleidomastoid and the trapezius muscles to the superior nuchal line. The occipital artery gives rise to several muscular and meningeal branches, anastomoses with other branches of the external carotid including the ascending pharyngeal and also with branches of the vertebral artery. Its meningeal branches, which enter the posterior fossa through the jugular foramen or the condylar canal, may make a significant contribution to tumors of the jugular foramen.
The posterior auricular artery, the last branch in the posterior group, arises above the posterior belly of the digastric muscle and travels between the parotid gland and the styloid process. At the anterior margin of the mastoid process, it divides into auricular and occipital branches, which are distributed to the postauricular and the occipital regions, respectively. The stylomastoid branch, which arises below the stylomastoid foramen, enters the stylomastoid foramen to supply the facial nerve. Its loss can lead to a facial palsy, even though it anastomoses with the petrosal branch of the middle meningeal artery. The posterior auricular branch may share a common trunk with the occipital artery, or sometimes it is absent, in which case, the occipital artery gives rise to the stylomastoid artery. Members of the anterior group, whose origins may be visualized in exposing lesions in the region, include the superior thyroid, lingual, and facial arteries.
The superficial temporal artery arises from the external carotid artery in the substance of the parotid gland behind the neck of the mandible where it is crossed by the temporal and zygomatic branches of the facial nerve (Fig. 8.5). It ascends over the posterior root of the zygoma and divides into anterior and posterior branches that run with the superficial temporal vein and the auriculotemporal nerve over the superficial temporalis fascia.
The vertebral artery and its meningeal, posterior spinal, and posteroinferior cerebellar branches, which may be exposed in approaches directed through the temporal bone, are detailed in the chapter on the foramen magnum (4, 20, 24).
The venous drainage of the structures of the skull base is through the internal jugular veins, the sinuses in the dura mater, and a series of emissary veins communicating the intra- and extracranial compartments (25). The superior petrosal sinus sits on the petrous ridge and connects the cavernous and transverse sinuses. It receives tributaries from the inferior surface of the temporal lobe and from the petrosal veins that drain the cerebellum and brainstem. The inferior petrosal sinus courses along the petro-occipital fissure and drains the clival area. It consists of one or more channels that, at its lower end, course rostral or caudal to or between the nerves passing through the jugular foramen. It enters the medial wall of the jugular bulb just anterior to where the cranial nerves descend in the anteromedial wall of the jugular bulb (18). It joins the cavernous sinus at its upper margin. The transverse sinus begins at the level of the internal occipital protuberance and passes laterally and forward to the posterolateral part of the temporal bone where it joins the superior petrosal sinus and continues as the sigmoid sinus. It receives drainage from the tentorial surface of the cerebellum through the tentorial sinuses and from the temporal lobe through the vein of Labbe. The basilar venous plexus consists of multiple interconnecting channels situated between the layers of dura mater on the clivus. It forms the largest communication between the paired cavernous sinus and communicates through the inferior petrosal sinuses with the sinuses in the region of the foramen magnum (10).
The suboccipital retrosigmoid approach, the traditional neurosurgical route to intradural pathologies arising in the region of the cerebellopontine angle, lower clivus, and foramen magnum, is reviewed in the chapter on the cerebellopontine angle. The approaches reviewed here are those directed through the temporal bone.
Middle Fossa Approach
This section focuses on the middle fossa approach to the internal acoustic meatus rather than on the more extensive approaches directed through the petrous apex to the petroclival region or the more extended approaches directed through the temporal bone lateral to the internal acoustic meatus. The middle fossa approach to the internal acoustic meatus is usually selected for small tumors that are located predominantly within the internal acoustic meatus in which there is an opportunity to preserve hearing. With this approach, the meatus is approached from above, through a temporal craniotomy located above the ear and zygoma (Figs. 8.7 and 8.11) (2). The dura under the temporal lobe is elevated from the floor of the middle cranial fossa until the arcuate eminence and the greater petrosal nerve are identified. The distance from the inner table of the skull to the facial hiatus, through which the greater petrosal nerve passes, ranges from 1.3 to 2.3 cm (average, 1.7 cm) (42). When separating the dura from the floor of the middle fossa, one should remember that bone may be absent over all or part of the geniculate ganglion. In our previous study of 100 temporal bones, all or part of the geniculate ganglion and the genu of the facial nerve were found to be exposed in the floor of the middle fossa in 15 bones (15%) (31). In 15 other specimens, the geniculate ganglion was completely covered, but no bone extended over the greater petrosal nerve. The greatest length of greater petrosal nerve covered by bone was 6.0 mm. More than 50% of the specimens had less than 2.5 mm of greater petrosal nerve covered. It also is important to remember that the petrous segment of the carotid artery may be exposed without a covering of bone in the floor of the middle fossa deep to the greater petrosal nerve (17) In a previous study, we found that a 7-mm length of petrous carotid artery may be exposed without a bony covering in the area below where the greater petrosal nerve passes below the lateral margin of the trigeminal ganglion to reach the vidian canal at the anterior margin of the anterior margin of the foramen lacerum (30, 31). The foramen spinosum and middle meningeal artery and the foramen ovale and third trigeminal division are situated at the anterior margin of the extradural exposure. The extradural exposure can usually be completed without obliterating the middle meningeal artery at the foramen spinosum.
Two different methods are used for exposing the internal acoustic meatus. One is to remove bone over the greater petrosal nerve and to follow it to the geniculate ganglion and the genu of the facial nerve. From here, the labyrinthine portion of the facial nerve is followed to the lateral end of the internal auditory canal, after which the canal is unroofed. The other method is begun by drilling just in front of the petrous ridge in the area medial to the arcuate eminence. The angle between the long axis of the superior semicircular canal or the greater petrosal nerve and the long axis of the internal acoustic meatus is helpful in selecting the site for drilling. The long axis of the central part of the internal acoustic meatus is located an average of 61 degrees behind the long axis of the greater petrosal nerve and an average of 37 degrees medial to the long axis of the arcuate eminence and superior semicircular canal. The drilling is directed anterolateral to the meatal fundus where the vertical crest is identified.
The lateral part of the bone removal near the meatal fundus is limited posteriorly by the superior semicircular canal, which is located a few millimeters behind and oriented parallel to the labyrinthine segment of the facial nerve (Figs. 8.7 and 8.11). The anteromedial edge of the exposure is limited by the cochlea, which sits only a few millimeters anterior to the site of bone removal, in the angle between the labyrinthine portion of the facial nerve and the greater petrosal nerve. The cochlea and the semicircular canals should be avoided in this approach if hearing is to be preserved. The vertical crest, which is identified at the upper edge of the meatal fundus, provides a valuable landmark for identifying the facial nerve. In the final stage of bone removal, the upper wall of the internal auditory canal is removed to expose the dura lining the entire superior surface of the internal auditory canal from the vertical crest to the porus. The dura is opened to expose the pathology.
The extended middle fossa approach used for the removal of larger acoustic neuromas includes wider opening of the posterior part of the petrous pyramid (21, 28, 42, 43). This approach combines different degrees of resection of the bony labyrinth with the subtemporal transtentorial routes (Fig. 8.12). Extending the resection of the petrous bone posteriorly over the mastoid and the bony labyrinth exposes the whole intrapetrous course of the facial nerve, and provides access to the cerebellopontine angle by a combination of subtemporal, translabyrinthine, and presigmoid routes, all directed through the posterior part of the floor of the middle fossa.
Subtemporal Anterior Transpetrosal Approach
This approach is made through a temporal craniotomy that extends down to the floor of the middle fossa (Figs. 8.12 and 8.13) (19). The dura is carefully elevated from the floor of the middle fossa to expose the middle meningeal artery, which may be obliterated and divided at the foramen spinosum. Further elevation of the dura toward the petrous ridge will expose the arcuate eminence and greater petrosal nerve posteriorly. The cochlea, which is to be preserved, and the anterior wall of the internal auditory canal constitute the lateral limit of the exposure through the petrous apex. The bone layer over the superior wall of the internal auditory canal, which averages 5 mm (range, 3–7 mm) in thickness, can be removed with a drill to improve the exposure (44). The petrous carotid forms the anterior limit of the exposure. The limit above the medial part of the bone resection is the trigeminal nerve in Meckel’s cave. Drilling is directed behind the petrous carotid, through the petrous apex medial to the cochlea and under the trigeminal nerve. The petrous apex is removed and the bone removal is extended to the lateral side of the clivus, exposing the inferior petrosal sinus at the lateral edge of the clivus. Care is required to prevent damage to the abducens nerve as it passes through Dorello’s canal located at the upper edge of the petroclival fissure. The width of the bone resection from the trigeminal impression to the posterior wall of the internal auditory canal averages 13 mm (range, 9–14 mm) (44). The depth of the exposure, from the trigeminal ganglion to the petroclival fissure, averages 13 mm (range, 9–17 mm). The cochlea lies below the floor of the middle fossa near the apex of the angle formed by the greater petrosal nerve anteriorly and the internal acoustic meatus posteriorly. The cochlea is to be avoided if hearing is to be preserved.
After the bone removal is completed, the superior petrosal sinus is obliterated and divided in the area just lateral to the trigeminal nerve, and the dural incision is extended across the tentorium. The dural leaflets of the tentorium are retracted with sutures and the dural incision is carried downward to the lower margin of the opening through the petrous apex. The approach is then directed between the lower margin of the trigeminal nerve above, and the internal acoustic meatus inferiorly and laterally (20).
The exposure is small, as described above, and may require significant temporal lobe retraction, especially if the goal is to reach the lower aspect of the brainstem. To reach the anterior aspect of the pons, the view must be directed from lateral to medial above the internal auditory canal. The angles of view through the area of the petrousectomy can be increased if the cranium is approached at a higher level through a frontotemporal craniotomy combined with zygomatic arch resection.
In the translabyrinthine approach, the internal acoustic meatus and cerebellopontine angle are approached through a mastoidectomy and labyrinthectomy (Fig. 8.6) (16, 29, 38) There are two goals of bone removal in this approach. The first is to expose the dura of Trautman’s triangle on the posterior surface of the temporal bone facing the cerebellopontine angle. The second is to remove enough bone to be able to identify the nerves lateral to the tumor as they course through the internal auditory canal and by the transverse and vertical crests. The approach may also be combined with a retrosigmoid or a supra- and infratentorial presigmoid approach.
A retroauricular incision starts above the pinna and extends inferiorly to the mastoid tip (3). A flap of periosteum and soft tissues overlying the mastoid and retromastoid areas is elevated. The cortical bone over the mastoid is drilled away and the mastoid air cells are removed, exposing the mastoid antrum, the cortical bone around the labyrinth, and the digastric ridge leading anteriorly to the mastoid segment of the facial nerve as it exits the stylomastoid foramen and the sinodural angle. Drilling is continued to expose the semicircular canals and to skeletonize the sigmoid sinus, middle fossa dura, mastoid segment of the facial nerve, and the upper surface of the jugular bulb, leaving only a thin shell of bone over these structures. The lateral semicircular canal is the most laterally projecting canal and is the first one encountered by this approach. It provides a valuable landmark in identifying the tympanic segment of the facial nerve and the other canals. The nerve is found below the lateral canal. The retrofacial air cells are removed and the dome of the jugular bulb is identified inferiorly. In removing bone behind the internal acoustic meatus, it is important to remember that the jugular bulb may bulge upward behind the posterior semicircular canal or internal auditory meatus. The vestibular aqueduct and the endolymphatic sac may be opened and removed during the bone removal between the meatus and the jugular bulb. The cochlear canaliculus will be seen deep to the vestibular aqueduct as bone is removed in the area between the meatus and the jugular bulb. The lower end of the cochlear canaliculus is situated just above the area where the glossopharyngeal nerve enters the medial half of the jugular foramen. The labyrinthectomy portion of the procedure involves removing the semicircular canals and the vestibule to expose the dura lining the internal auditory canal. The lateral and posterior semicircular canals are drilled away. As the bone removal proceeds medially, the ampullae of the lateral and superior semicircular canals are exposed. At this point some bleeding can occur as the subarcuate artery is encountered in the bone near the center of the superior semicircular canal. The vestibule is an oval-shaped cavity located immediately lateral to the internal acoustic meatus, which forms the communication between the semicircular canals and the cochlea. Bone is removed medial and posterior to the vestibule, completely exposing it anterior and inferior to the facial nerve. Care is required to avoid injury to the facial nerve as it courses below the lateral canal and the ampullae of the posterior canal and around the superolateral margin of the vestibule.
The internal auditory canal is located medial and anterior to the tympanic segment of the facial nerve. The dura lining the internal canal is exposed by drilling away the semicircular canals and vestibule and the bone around the superior, posterior, and inferior margins of the internal canal. Further bone removal at the lateral end of the meatus exposes the transverse and vertical crests (Fig. 8.2). The intrameatal portion of the facial nerve is separated from the superior vestibular nerve at the lateral end of the canal by the vertical crest, also called Bill’s bar, that can be used to positively identify the facial nerve (13, 16). The initial part of labyrinthine segment of the facial nerve, which lies just in front of the vertical crest, is exposed at the meatal fundus. After identifying the facial nerve, the dura lining the meatus is opened. The dural incision in Trautman’s triangle is V-shaped with the apex of the “V” extending to the incision along the meatal dura. One limb of the “V” extends below the superior petrosal sinus and the other limb extends above the jugular bulb. The dural flap is then reflected posteriorly to expose the structures in the meatus and the cerebellopontine angle. The subarcuate artery, or the AICA, may be encountered in the dura of Trautman’s triangle. Usually, the subarcuate artery arises from the AICA and passes through the dura on the upper posterior wall of the meatus as a fine stem. Occasionally, however, the subarcuate artery, along with its origin from the AICA, may be incorporated into the dura on the posterior face of the temporal bone. The approach may include transection of the external canal and obliteration of the middle ear with packing of the eustachian tube at closure.
The transcochlear approach is primarily an anteromedial extension of the translabyrinthine approach (Fig. 8.6) (3, 15, 16). It usually includes division and closure of the external canal, resection of at least the posterior part of the osseous external canal, and the tympanic membrane and ossicles, and obliteration of the eustachian tube. After exposing the dura lining the internal auditory canal, as described for the translabyrinthine approach, the incus is removed and the facial nerve is exposed from the geniculate ganglion to the stylomastoid foramen. The greater superficial petrosal nerve is transected and the facial nerve is transposed posteriorly. In the final stage, the bone removal is carried through the facial canal, after nerve transposition, and the cochlea and adjacent part of the petrous apex are drilled away (Fig. 8.6).
Medially, the bone removal extends to the edge of the clivus, exposing the inferior petrosal sinus from the jugular bulb below to the superior petrosal sinus above. The ascending portion of the petrous carotid is exposed at the anterior limit of the dissection. The bone removal, which now extends to the lateral edge of the clivus, could easily be carried medially into the clivus. Extending the dural opening in this area permits visualization of the abducent nerve medial to the internal acoustic meatus, the lower margin of the trigeminal nerve, the nerves entering the jugular foramen, a segment of the basilar artery, and the origin and initial segment of the AICA.
An alternative to transposing the facial nerve is to complete an extensive bone removal in the hypotympanic and retrofacial areas extending forward to the carotid canal, thus skeletonizing the mastoid segment of the facial nerve and leaving it suspended in a shell of bone, as described by Gantz and Fisch (7). In this approach, the external auditory canal is closed as a blind sac and the tympanic membrane, incus, and body of the malleus are removed (7). A mastoidectomy is performed, including the removal of the retrofacial, retrolabyrinthine, and supralabyrinthine compartments. The facial nerve is identified at its tympanic segment and at the stylomastoid foramen. The inferior part of the tympanic bone is removed to expose the infralabyrinthine compartment, the jugular bulb, and the intrapetrous carotid artery. The retrofacial dissection is carried medially and superiorly, removing the semicircular canals and vestibule. The dissection of the posterior fossa dura is carried inferiorly around the internal auditory canal and under the facial canal. The cochlea is drilled away by working inferior and anterior to the facial canal. The facial canal is then left as a bridge over the operative field and the dura is exposed between the carotid artery and the jugular bulb.
Combined Supra- and Infratentorial Presigmoid Approach
The presigmoid approach combines the supra- and infratentorial craniotomy centered on the mastoid and varying degrees of mastoid and labyrinthine resection (Fig. 8.14). The minimal degree of mastoid resection, which we refer to as a minimal mastoidectomy, exposes only enough of the presigmoid dura to open the dura in front of the sigmoid sinus for exposure of the cerebellopontine angle (Figs. 8.15 and 8.16). The next more extensive degree of mastoid resection, the retrolabyrinthine modification, is a more complete mastoidectomy exposing the bony capsule of the semicircular canals and skeletonizing at least a portion of the facial nerve. In the partial labyrinthectomy, one or two of the semicircular canals, commonly the superior and/or posterior canals, are resected with preservation of the lateral canal. Removal of these canals may, but not always, be associated with the loss of hearing (37). The posterior canal may be removed to increase access to the posterior fossa, and removing the superior canal alone gives a more direct access to the petrous apex along the middle fossa. The next more extensive modification is the translabyrinthine approach, in which the semicircular canals and vestibule are resected uniformly, resulting in the loss of hearing. The translabyrinthine approach provides excellent access to the internal auditory canal. The next more extensive modification is the transcochlear approach, in which the cochlea located anteromedial to the fundus of the meatus is removed, thus providing access to the medial part of the petrous apex and the side of the clivus. Another modification, which we call the extended translabyrinthine approach, and is similar to the transcochlear approach, involves drilling bone both anterior and posterior to the facial nerve, leaving the facial nerve skeletonized in a column of bone and working both anterior and posterior to the facial nerve to remove the cochlea and access the side of the clivus. Gaining access for drilling the cochlea anterior to the facial nerve commonly requires that at least part of the posterior part of the external canal be removed, that the tympanic cavity be obliterated, and that the internal carotid artery be exposed below the promontory.
In evaluating these approaches in our laboratory, we have found that the minimal mastoidectomy gives approximately the same exposure as the retrolabyrinthine approach, but is done at reduced risk since the semicircular canals and facial nerve are not skeletonized (Figs. 8.14 and 8.15). Removing the posterior canal increases access to the posterior fossa, but access is only slightly increased over that achieved with the retrolabyrinthine approach. Removing the superior canal increases access to the middle fossa and petrous apex and reduces the needed retraction of the temporal lobe. The translabyrinthine approach does not significantly increase the access to the area medial to the porus of the internal acoustic meatus over that achieved with the minimal mastoidectomy or retrolabyrinthine approach, but does provide access to the internal auditory canal. The transcochlear modification, in which bone is removed up to the edge of the clivus, does significantly increase access to the front of the brainstem and clivus over that achieved with the lesser degrees of bony resection. The retrosigmoid, the presigmoid minimal mastoidectomy, and the retrolabyrinthine approaches were compared and yielded nearly the same exposure of the cerebellopontine angle, but the retrosigmoid approach did not provide the additional exposure of the middle fossa and petrous apex that could be achieved in the combined supra- and infratentorial presigmoid approach.
The skin incision is started in the temporal region above the zygoma, and extends above the ear and downward in the suboccipital area medial to the mastoid process (Figs. 8.14, 8.15, and 8.17). The skin flap is reflected forward to the level of the external auditory canal. The temporal muscle is elevated and reflected anteriorly, and the muscles over the mastoid and suboccipital areas are swept inferiorly. A temporooccipital craniotomy is performed and the transverse sinus is exposed. After the bone flap is elevated, a mastoidectomy is carried out without entering the labyrinth. The sigmoid sinus is skeletonized from the sinodural angle to the jugular bulb. Bone is removed superiorly to expose the floor of the middle fossa and the superior petrosal sinus. Trautman’s triangle is exposed in the area lateral to the otic capsule.
The dura mater is then incised along the base of the temporal craniotomy, while preserving the junction of the vein of Labbe with the transverse sinus. The posterior fossa dura is opened anterior to the sigmoid sinus in Trautman’s triangle. The dural incision is extended across the superior petrosal sinus to join the dural incision in the temporal dura. After division of the superior petrosal sinus, the tentorium is incised parallel to and just behind the petrous ridge and superior petrosal sinus. This dural incision is extended from the site of division of the superior petrosal sinus through the medial edge of the tentorium to the incisura behind where the trochlear nerve enters the tentorial edge. Care is taken to avoid injury to the IVth cranial nerve in its course near the tentorial margin. The posterior portion of the temporal lobe is elevated and the sigmoid sinus is displaced posteriorly along with the cerebellar hemisphere while preserving the junction of the vein of Labbe with the sigmoid sinus. The sigmoid sinus limits the ability for superior retraction of the temporal lobe and can be ligated to improve the exposure if bilateral venous angiography show adequate communication through the torcular to the opposite side (24). The petroclival region can be exposed from the middle fossa and tentorial incisura to near the foramen magnum, although access to the lower petroclival region may be limited by the jugular bulb. The presigmoid exposure provides a shorter working distance to the petroclival area and provides multiple angles for dissection. The major arteries in the posterior fossa are easily accessible. The exposure can also be combined with a far-lateral approach (Fig. 8.17).
Subtemporal Preauricular Infratemporal Fossa Approach
The subtemporal preauricular infratemporal approach is directed through the infratemporal and middle fossae to the part of the anterior surface of the petrous bone located medial to the cochlea and to the petroclival region (Figs. 8.10, 8.13, and 8.18). This description outlines the full extent of the anatomic exposure available through this approach, but it can often be tailored to a smaller, more limited, approach. A curvilinear incision starting in the frontal region turns downward in front of the ear into the cervical region. The incision may be extended downward only to the area just below the tragus if only the petrous apex and upper part of the infratemporal fossa are to be exposed, but it can be extended onto the upper neck if a neck dissection is needed. The skin flap is separated from the underlying tissues and reflected forward. The facial nerve and its major branches are identified distal to the stylomastoid foramen and followed to the parotid gland. The parotid gland is separated from the masseteric fascia to avoid excessive stretching of the facial nerve at the stylomastoid foramen (33, 38, 39). The superficial temporalis fascia in which the upper facial branches course is separated from the temporalis muscle and is reflected forward to prevent damage to the branch of the facial nerve to the frontalis muscle as the zygomatic arch is exposed. The zygomatic arch is divided at its anterior and posterior ends, and the temporalis muscle, with the overlying segment of the zygomatic arch, is reflected downward. The mandibular condyle and the capsule of the temporomandibular joint are either dislocated downward or excised. The temporomandibular joint can be removed in a single piece for later replacement by dividing the mandibular neck below the condyle and osteotomizing the middle fossa floor around the mandibular fossa (Fig. 8.18). The internal carotid artery, the internal jugular vein, and the vagus, accessory, and hypoglossal nerves may be exposed in the neck if needed. The posterior belly of the digastric muscle may be divided and the styloid process resected.
A frontotemporal craniotomy is then performed. The dura is elevated from the floor of the middle fossa to expose and obliterate the middle meningeal artery at the foramen spinosum and to expose the arcuate eminence, the third trigeminal division at the foramen ovale, and the greater petrosal nerve. The greater petrosal nerve is transected if necessary to avoid traction on the facial nerve. The floor of the middle fossa, including the lateral and inferior aspects of the superior orbital fissure, and the lateral margin of the foramina ovale may be removed to expose the structures in the infratemporal fossa.
If needed, bone can be removed medial to the mandibular fossa to expose the eustachian tube and the tensor tympani muscle, both of which may be resected (Figs. 8.10, 8.13, and 8.18). The bone removal is continued inferiorly, exposing the ascending portion of the petrous carotid. In this segment, the carotid artery is surrounded by a periosteal sheath, which encloses a periarterial venous plexus that is an extension of the cavernous sinus. At the entrance of the carotid canal, a dense fibrocartilaginous ring encircles the artery. If mobilization of the artery is required, care must be taken when dividing the ring not to damage the IXth cranial nerve that is in close proximity to the carotid canal as it exits the jugular foramen. After mobilizing the carotid artery and displacing it forward, the petrous apex and the clival region to the level of the foramen magnum can be approached medial to and behind the artery. During drilling, the very hard cortical bone along the petrous apex gives place to a crumbly cancellous bone in the region of the clivus, as the dura of the anterior and lateral aspects of the posterior fossa is being exposed. The area exposed is limited by Meckel’s cave superiorly, by the cochlea and internal auditory canal laterally, by the abducens nerve in its course through the Dorello’s canal medially, and by the hypoglossal canal inferiorly. If the dura is opened, the structures along the lateral and anterior aspects of the upper medulla and lower two-thirds of the pons will be exposed (41). The tentorium can be divided to give access to the upper clival region.
Dividing the third trigeminal division above the foramen ovale will permit exposure of the junction of the petrous and cavernous carotid along with the structures in the inferolateral portion of the cavernous sinus (17, 39). The pterygopalatine fossa, parapharyngeal space, lateral maxilla, and orbit can be exposed farther anteriorly. The lateral aspect of the sphenoid bone and the sphenoid sinus can also be approached by removing bone medial to the maxillary nerve at the root of the pterygoid process.
Postauricular Transtemporal Approach
The postauricular transtemporal approach is most commonly selected for lesions that involve the mastoid and tympanic cavities and track along the nerves and arteries to reach the middle and infratemporal fossa (Figs. 8.19 and 8.20). It can, however, be tailored at its posterior margin to include a retrosigmoid, far-lateral, or presigmoid exposure of the posterior fossa or, at its anterior limits, to include exposure of the pterygopalatine fossa and lateral parts of the maxillary orbit or anterior cranial fossa.
A question mark incision is started behind the hairline in the temporal region, extending behind the ear over the mastoid process and continuing inferiorly in front of the sternocleidomastoid muscle onto the neck. The skin flap is then reflected forward and the external auditory canal is divided at the bone-cartilage junction and closed as a blind sac. The sternocleidomastoid muscle is detached from the mastoid process and reflected inferiorly. The periosteum and posterior portion of the temporalis muscle are reflected anteriorly, thus exposing the temporal, mastoid, and retromastoid areas. The posterior belly of the digastric muscle is divided and reflected inferiorly. At this point, the facial nerve is identified distal to the stylomastoid foramen and is followed, along with its major branches, into the substance of the parotid gland (5). The internal jugular vein, the carotid bifurcation, and the glossopharyngeal, vagus, accessory, and hypoglossal nerves are exposed and isolated in the neck. This allows for proximal control of the internal carotid artery and ligation of the main feeding vessels from the external carotid artery to a neoplasm early in the procedure.
After this, temporal and/or retromastoid craniotomies may be performed with a simple mastoidectomy. The remaining skin of the external auditory canal, the tympanic membrane, the malleus, incus, and stapes arch (leaving the footplate) are removed. The facial nerve is completely skeletonized from the geniculate ganglion to the stylomastoid foramen.
If exposure of the jugular foramen and lower clival region is desired, a new facial canal is created by drilling a groove in the bone of the anterior attic wall, between the geniculate ganglion and the root of the zygoma. The facial nerve is carefully freed at the stylomastoid foramen, while leaving some of the surrounding connective tissue attached to the nerve, and the nerve is transposed anteriorly into the new bony groove of the epitympanum and imbedded for its protection into the parotid tissue (5).
The dura of the middle fossa and the sigmoid sinus from the sinodural angle to the jugular bulb is skeletonized. Then the sigmoid sinus and the jugular vein are ligated in this sequence, and the sigmoid sinus divided. Part of the wall of the sinus, bulb, and/or vein may be excised to increase the exposure. This allows for dissection of the lower cranial nerves at the jugular foramen, as well as for their mobilization and posterior displacement if necessary. The posterior mobilization of the lower cranial nerves allows for a direct exposure of the structures along the lateral and anterior aspects of the medulla and lower pons without the necessity for brain retraction. Dissection in the area of the jugular foramen has proven to be extremely difficult, as the lower cranial nerves are particularly fragile and difficult to isolate from the surrounding tissues.
Exposure of the middle clival structures requires removal of the bony labyrinth, as described for the translabyrinthine approach. The internal auditory canal is exposed, the facial nerve identified, and the cochlear and vestibular nerves divided. The greater superficial petrosal nerve is sectioned at its origin from the geniculate ganglion. The facial nerve is freed from all its attachments in the temporal bone and reflected posteriorly. The bony portion of the external auditory canal and the tympanic bone are drilled away, exposing the ascending portion of the intrapetrous carotid artery medial to the eustachian tube.
The dissection is continued by drilling away the cochlea, starting at its basal turn, to expose part of the horizontal segment of the petrous carotid artery. Anterior displacement of the carotid artery and removal of the cochlea provides a wide exposure of the lateral and anterior portions of the pons and medulla. This exposure extends from the inferior aspect of the trigeminal ganglion to the foramen magnum. The exposure may be carried medially into the clivus and retropharyngeal space and anteriorly to expose the mucosa of the sphenoid sinus.
If the approach is to be extended to the parasellar and parasphenoidal areas, the zygomatic arch is divided and reflected inferiorly with the masseter muscle. The temporalis muscle is separated from its attachment to the coronoid process of the mandible and reflected anteriorly and superiorly. A temporal craniotomy is then performed, and extensive bone is removed along the whole lateral aspect of the middle cranial fossa. The ascending ramus of the mandible is either displaced anteriorly or resected, and the petrous carotid is exposed distally to the proximal portion of the intracavernous segment after removing the cartilaginous portion of the eustachian tube. The cavernous sinus can be approached and the intracavernous carotid artery exposed by dividing the mandibular segment of the trigeminal nerve. The approach can also be extended to the retrosigmoid area and down the vertebral artery to the C1 to C2 level, or to the suboccipital triangle for a far-lateral or transcondylar exposure. The lateral orbit and pterygopalatine fossa can be accessed at the anterior limit of the exposure.
Pathologies can arise anywhere within the petroclival region and frequently are not restricted to a single anatomic compartment of the cranial base. Involvement of multiple cranial nerves and arteries occurs because cranial base tumors tend to achieve considerable size before producing clinical manifestation (32). The distinction between the benign or malignant tumors in this area is not rigid because many benign tumors can have a very invasive characteristic. The selection of the best surgical approach depends on the location, extension, size, and nature of the pathology.
An advantage of these approaches directed through the temporal bone to the petroclival area is that they reach the area through tissue planes outside the oropharynx. They provide another route by which anterior intradural lesions situated medial to the nerves entering the internal acoustic meatus and jugular foramen can be approached without entering the nasopharynx. They also provide an avenue of exposure for lesions that involve the temporal and sphenoid bones in addition to the clivus. One or a combination of the lateral approaches is frequently used to expose intraor extradural clival lesions that also involve the temporal and sphenoid bones. They also provide access to the anterior aspect of the midbrain, pons, and medulla and to the cerebellopontine angle and nerves in the posterior fossa. They may also provide better access to the temporal bone, jugular foramen, and petrous segment of the internal carotid artery than the other anterior or posterior approaches. The area may be approached from directly lateral through the mastoid, labyrinth, and cochlea, as in the translabyrinthine and transcochlear approaches; from above through a subtemporal middle fossa route; from behind in the retrosigmoid suboccipital approach; or from multiple directions using such combined supra- and infratentorial approaches as the presigmoid approach, to which a translabyrinthine or transcochlear approach may be added. Alternative or extended approaches, most of which include some route through the mastoid and petrous parts, include the anterior transpetrosal, the subtemporal preauricular infratemporal, and the far-lateral transcondylar approach.
The retrosigmoid suboccipital approach, described in the chapter on the cerebellopontine angle, offers a wide view of the cerebellopontine angle and of the intradural structures behind the ipsilateral lower clivus, but the dural surface of the petrous apex, upper clivus, and tentorial incisura are not well seen from this exposure (26, 35, 36, 46) (Figs. 8.15 and 8.16). Removal of posterior wall of the internal auditory canal through the retrosigmoid provides access to the contents of the meatus as far lateral as the vertical and transverse crests. The vestibule can be opened if needed to remove a tumor extending into the labyrinth. Care is required to avoid injury to the posterior semicircular canal and common crus if there is the possibility of preserving hearing (29). The retrosigmoid approach provides easy access to the intradural part of cranial nerves V, VII, VIII, and IX through XII. It also provides access to the nerve-related segments of the arteries of the posterior circulation. The vertebrobasilar junction can be exposed in some cases, although the lower cranial nerves and the jugular tubercle are frequent obstacles. Retraction of the pons and working between the cranial nerves is necessary to reach the origin of the AICA from the basilar artery. The far lateral modification of the retrosigmoid approach, described in the chapter on the far lateral approach, was devised to provide a better exposure of the lateral and anterior aspects of the cervicomedullary junction (45).
The presigmoid approach (1, 8, 32) combines a supra- and infratentorial exposure with various degrees of petrousectomy, while preserving the junction of the vein of Labbe with the transverse sinus (Figs. 8.14-8.17). The amount of resection of the petrous bone can vary from a retrolabyrinthine minimal mastoidectomy exposure to a translabyrinthine or transcochlear exposure with posterior displacement of the facial nerve. In selected cases, where angiography shows patency of the communication between the two transverse sinuses across the midline, the sigmoid sinus can be ligated to improve the exposure (24). Preservation of the drainage of the vein of Labbe and avoidance of excessive temporal lobe retraction are major goals of this approach to the upper clival region. Approaching the structures in the inferior petroclival space may be restricted by the jugular bulb, which could be overcome by division of the sigmoid sinus or by working posterior to it (36). The major advantages of this approach are the shorter working distance to clival lesions and the various angles for dissection that are provided. The approach provides access to the ipsilateral cranial nerves III through XII and to the major arteries in the posterior circulation. A major drawback to this exposure is provided by the anatomic variants, described below, that limit the size of the exposure through Trautman’s triangle and the labyrinth.
The translabyrinthine approach provides access to the facial nerve from its origin at the brainstem to the stylomastoid foramen, and exposure of the contents of the internal auditory meatus (Fig. 8.6) (12, 14). The lateral surface of the pons, the inferior aspect of the origin of the trigeminal nerve, and the facial and vestibulocochlear nerve complexes are well visualized, but exposure of the region inferior to the jugular bulb, above the trigeminal nerve, and anterior to the internal acoustic meatus is usually poor. The extent of exposure achieved with the translabyrinthine approach is dependent on several anatomic factors. A high jugular bulb, an anteriorly placed or large sigmoid sinus, or a low middle fossa plate may severely restrict the exposure (22, 27).
The transcochlear approach shares similar limitations with the translabyrinthine exposure, although the posterior transposition of the facial nerve in the transcochlear approach allows better visualization of the structures anterior to the internal auditory canal (15, 16). The area of exposure is very narrow and restricted by the maintenance of the bony external auditory canal, but can be increased by resecting the posterior part of the canal. Transposition of the facial nerve may be followed by a transient or permanent facial palsy.
The subtemporal anterior transpetrosal approach uses extradural resection of the anterior petrous pyramid via a temporal craniotomy (Figs. 8.12 and 8.13). It may be combined with zygomatic resection to increase access to the floor of the middle fossa (20). The area of the petrous apex removal extends from just medial to the internal auditory canal and cochlea to the petrous apex and petroclival junction, and from the petrous ridge posteriorly to the carotid canal anteriorly. A significant degree of temporal lobe retraction may be required. This may be reduced by using a frontotemporal craniotomy with zygomatic resection. Although only a small window in the petrous bone is provided, exposure can be expanded by dividing the adjacent part of the tentorium. The lateral and anterior surfaces of the pons and the upper clivus and adjacent part of the cavernous sinus can be approached through this route (Fig. 8.13). The facial, vestibulocochlear, trigeminal, and abducens nerves can be identified. The petrous carotid may limit the surgeon’s line of vision and restrict access to the inferior part of the petroclival region, but this restriction may be overcome with anterior mobilization of the artery (39, 41). The approach provides access to the anterior aspect of the brainstem and basilar artery in the area between the trigeminal nerve above and the facial and vestibulocochlear nerves below. In approaching the basilar artery through this route, the size and location of the lesion in relation to the petrous ridge is critical. The trigeminal nerve can be mobilized to improve the exposure, although this may result in postoperative facial hypesthesia (19, 20). The anterior transpetrosal approach can be used alone for extradural pathologies restricted to the petrous apex or as a surgical step to approaching intradural pathologies in the petroclival region. It provides a route for resecting extradural lesions that extend from the level of the trigeminal nerve to the foramen magnum.
Removal of the posterior part of the petrous pyramid has been used for acoustic neuroma removal as part of extended approaches directed through the middle fossa (21, 28, 42, 43) (Fig. 8.12). The extended approaches combine different degrees of resection of the bony labyrinth with the subtemporal transtentorial routes. Extending the resection of the petrous bone posteriorly over the mastoid and the bony labyrinth exposes the whole intrapetrous course of the facial nerve, and provides access to the cerebellopontine angle by a combination of subtemporal, translabyrinthine, and presigmoid routes (Figs. 8.12 and 8.13) (9).
The subtemporal preauricular infratemporal approach reaches the skull base from an anterolateral direction (Figs. 8.10, 8.13, and 8.18). Division of the zygomatic arch, resection or displacement of the mandibular condyle, and extensive resection of the lateral part of the middle fossa floor exposes the infratemporal fossa, the nasopharynx, the para- and retropharyngeal areas, and the ethmoid, sphenoid, and maxillary sinuses. The approach also provides access to the upper cervical and petrous carotid. The cavernous sinus also can be approached through its lateral and basal aspects. Anterior displacement of the petrous carotid allows direct access to the clivus and for extensive resection of the petrous bone medial to the cochlea. This exposes the extradural clival region from the level of the trigeminal nerve to the foramen magnum (33, 36, 38, 39). The approach can also provide access to the intradural space ventral to the brainstem (41). The exposure of the cerebellopontine angle and foramen magnum is limited because the approach is carried anterior and medial to cranial nerves VII through XII and the cochlea is not resected (36). Anterior transposition of the petrous carotid artery allows unhindered exposure of the origin of the AICA and the vertebrobasilar junction. The approach could be used as an alternative lateral route to vascular lesions of the midbasilar artery or at the vertebrobasilar junction, when these lesions cannot be exposed through either the retromastoid or subtemporal transtentorial approaches.
The postauricular transtemporal approach, which combines a transcochlear exposure with an infratemporal approach, may be used as an alternative to the preauricular infratemporal approach when the pathology involves the mastoid and the infratemporal fossa and extends to the facial recess, hypotympanic area, and jugular bulb (5, 6, 34) (Figs. 8.19 and 8.20). The structures of the lower and middle clivus can be exposed without the need for brain retraction. The facial nerve is displaced anterosuperiorly and the sigmoid sinus ligated and divided. Displacement of the facial nerve from its bony canal seriously interferes with its vascular supply and temporary or permanent loss of function is to be expected (33). Resection of the jugular bulb allows for exposure of the lower cranial nerves in the jugular foramen. Mobilization of the nerves in the medial part of the jugular foramen is extremely difficult and nerve damage is likely to occur if it is attempted. The lateral and anterior surfaces of the lower pons, medulla, and cervicomedullary junction are well exposed. The extent of exposure of the major arteries is dependent on the different anatomic variants and direction of displacement of the vessels. Exposure of the structures of the middle clivus requires posterior facial nerve displacement and drilling of the labyrinth with consequent destruction of any residual hearing. The lateral and part of the anterior surfaces of the pons can be exposed up to the point of emergence of the trigeminal nerve. Exposure of the superior petroclival space requires that the transtemporal exposure be combined with a subtemporal exposure. The transtemporal approach can easily be extended to the infratemporal fossa, and the same exposure provided by the preauricular approach can be achieved. When this approach is combined with an infratemporal fossa exposure and anterior displacement of the intrapetrous carotid artery, the petrous part of the temporal bone can be completely removed, providing the widest possible exposure of the petroclival region (Figs. 8.19 and 8.20). The retrosigmoid, far-lateral, and transcondylar exposures can be obtained at the posterior margin of the exposure, and the anterior limit can be extended to include the pterygopalatine fossa and lateral part of the maxilla, orbit, and anterior cranial fossa.
Extensive removal of lesions involving the skull base frequently require reconstruction of the resultant bony, neural, and dural defects. The presence of cerebrospinal fluid leaks and the close proximity to contaminated spaces of the oro- or nasopharynx increases the risks of meningitis. Opened sinuses should be obliterated, dural incisions and openings should be sutured and sealed, nerves should be reanastomosed or grafted, and devascularized grafts of bone or dura should be covered with vascularized tissue whenever possible.
Contributor: Albert L. Rhoton, Jr., MD
Content from Rhoton AL. The Supratentorial Cranial Space: Microsurgical Anatomy and Surgical Approaches. Neurosurgery 51(1), 2002, 10.1097/00006123-200210001-00001. With permission of Oxford University Press on behalf of the Congress of Neurological Surgeons
The Neurosurgical Atlas is honored to maintain the legacy of Albert L. Rhoton Jr., MD
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