Last Updated: August 23, 2020
The foramen magnum is located in the occipital bone, which has three parts: a squamosal part located behind the foramen magnum; a basal (clival) portion located anterior to the foramen magnum; and a condylar part that connects the squamosal and clival parts (Fig. 6.1). The suboccipital approaches are directed through the squamosal part and the anterior approaches through the clival part. The condylar part, which includes the occipital condyle, posterior margin of the jugular foramen, and hypoglossal canal, is exposed in the far-lateral approach and its transcondylar, retrocondylar, and supracondylar modifications described in the chapter on the far lateral approach. Structures involved in foramen magnum lesions include the lower cranial and upper spinal nerves, the caudal brainstem and rostral spinal cord, the vertebral artery and its branches, the veins and dural sinuses at the craniovertebral junction, and the ligaments and muscles uniting the atlas, axis, and occipital bone (5, 26). The foramen magnum is most commonly approached from posteriorly through the suboccipital and upper cervical region or from anteriorly through the nasal and oral cavities, the pharynx, or maxilla.
The Foramen Magnum
The osseous structures that must be considered in planning an approach to the region of the foramen magnum are the occipital bone, the atlas, and the axis.
The occipital bone surrounds the foramen magnum (Fig. 6.1). The foraminal opening is oval shaped and is wider posteriorly than anteriorly. The wider posterior part transmits the medulla, and the narrower anterior part sits above the odontoid process. The occipital bone is divided into a squamosal part located above and behind the foramen magnum, a basal part situated in front of the foramen magnum, and paired condylar parts located lateral to the foramen magnum.
The squamous part is an internally concave plate located above and behind the foramen magnum. Its upper margins articulate with the parietal bones at the lambdoid sutures and its lower margins articulate with the mastoid portion of the temporal bones at the occipitomastoid sutures. The convex external surface has several prominences on which the muscles of the neck attach. The largest prominence, the external occipital protuberance or inion, is situated at the central part of the external surface. The inion is located an average of 1 cm below the apex of the internal occipital protuberance and the inferior margin of the confluence of the sagittal and transverse sinuses. Two parallel ridges radiate laterally from the protuberance: the highest nuchal line is the upper and thinner ridge, and the superior nuchal line is the lower and more prominent one. The area below the nuchal lines is rough and irregular and serves as the site of attachment of numerous muscles. A vertical ridge, the external occipital crest, descends from the external occipital protuberance to the midpoint of the posterior margin of the foramen magnum. The inferior nuchal lines run laterally from the midpoint of the crest.
The internal surface of the squamous part is concave and has a prominence, the internal occipital protuberance, near its center. The internal surface is divided into four unequal fossae by the sulcus of the superior sagittal sinus that extends upward from the protuberance, the internal occipital crest, a prominent ridge that descends from the protuberance, and the paired sulci for the transverse sinuses that extend laterally from the protuberance. The sulcus for the right transverse sinus is usually larger than the one on the left. The upper two fossae are adapted to the poles of the occipital lobes. The inferior two fossae conform to the contours of the cerebellar hemispheres. The internal occipital crest bifurcates above the foramen magnum to form paired lower limbs, which extend along each side of the posterior margin of the foramen. A depression between the lower limbs, the vermian fossa, is occupied by the inferior part of the vermis. The falx cerebelli is attached along the internal occipital crest.
The basilar part of the occipital bone, which is also referred to as the clivus, is a thick quadrangular plate of bone that extends forward and upward, at an angle of about 45° from the foramen magnum. It joins the sphenoid bone at the sphenoccipital synchondrosis just below the dorsum sellae (7). The superior surface of the clivus is concave from side to side and is separated on each side from the petrous part of the temporal bone by the petroclival fissure. This fissure has the inferior petrosal sinus on its upper surface and ends posteriorly at the jugular foramen. On the inferior surface of the basilar part, in front of the foramen magnum, a small elevation, the pharyngeal tubercle, gives attachment to the fibrous raphe of the pharynx.
The paired lateral or condylar parts are situated at the sides of the foramen magnum. The occipital condyles, which articulate with the atlas, protrude from the external surface of this part. These condyles are located lateral to the anterior half of the foramen magnum. They are oval in shape, convex downward, face downward and laterally, and have their long axes directed forward and medially. A tubercle that gives attachment to the alar ligament of the odontoid process is situated on the medial side of each condyle. The hypoglossal canal, which transmits the hypoglossal nerve, is situated above the condyle, and is directed forward and laterally from the posterior cranial fossa. The canal may be partially or completely divided by a bony septum. Septated hypoglossal canals were found on one or both sides in 6% of the dry skulls (15).
The condylar fossa, a depression located on the external surface behind the condyle, is often perforated to form the posterior condylar canal through which an emissary vein connects the vertebral venous plexus with the sigmoid sinus. One or both condylar foramina may be absent or incompletely perforated (9). The jugular process, a quadrilateral plate of bone, extends laterally from the posterior half of the condyle to form the posterior border of the jugular foramen. It serves as a bridge between the condylar and squamosal portions of the occipital bone. The jugular process articulates laterally with the jugular surface of the temporal bone. On the intracranial surface of the condylar part an oval prominence, the jugular tubercle, sits just superior to the hypoglossal canal and just medial to the lower extent of the petroclival fissure. The caudal part of the tubercle often presents a shallow furrow above which the glossopharyngeal, vagus, and accessory nerves course. The groove of the sigmoid sinus curves medially and forward around an upwardly directed, hook-shaped process, on the superior surface of the jugular process, and ends at the jugular foramen. The posterior condylar canal opens into the posterior cranial fossa close to the medial end of the groove for the sigmoid sinus.
The jugular foramen is situated lateral and slightly superior to the anterior half of the condyles. It is bordered posteriorly by the jugular process of the occipital bone, and anteriorly and superiorly by the jugular fossa of the petrous portion of the temporal bone (14). The foramen sits at the posterior end of the petroclival suture. The jugular foramen is divided into two parts by the intrajugular processes on the opposing edges of the petrous and occipital bones, which either join directly or are connected by a fibrous band. The smaller anteromedial part, the petrous part, transmits the inferior petrosal sinus, and the larger posterolateral part, the sigmoid part, transmits the sigmoid sinus. The intrajugular part, situated along the intrajugular processes, transmits the glossopharyngeal, vagus, and accessory nerves. The enlarged part of the internal jugular vein located within the foramen is referred to as the jugular bulb. The jugular process also serves as the site of attachment of the rectus capitis lateralis muscle behind the jugular foramen.
The atlas, the first cervical vertebra, differs from the other cervical vertebrae by being ring shaped and by lacking a vertebral body and a spinous process (Fig. 6.2). It consists of two thick lateral masses situated at the anterolateral parts of the ring. The lateral masses are connected in front by a short anterior arch and behind by a longer curved posterior arch. The position of the usual vertebral body is occupied by the odontoid process of the axis. The anterior arch is convexed forward and has a median anterior tubercle. The posterior arch is convex backward and has a median posterior tubercle and a groove on the lateral part of its upper-outer surface in which the vertebral artery courses. The groove may be partly or fully converted into a foramen by a bridge of bone that arches backward from the posterior edge of the superior articular facet of the atlas to its posterior arch. The first cervical spinal nerve also lies in the groove, which is located between the artery and the bone. The upper surface of each lateral mass has an oval concave facet that faces upward and medially and articulates with the occipital condyle that faces downward and laterally. The inferior surface of each lateral mass has a circular, flat, or slightly concave facet that faces downward, medially, and slightly backward, and it articulates with the superior articular facet of the axis. The medial aspect of each lateral mass has a small tubercle for the attachment of the transverse ligament of the atlas, which passes behind the dens. Each transverse foramen, which transmits a vertebral artery, and upon which the nerve root sits, is situated between the lateral mass and the transverse process.
The axis, the second cervical vertebra, more closely resembles the typical vertebrae than the atlas, but is distinguished by the odontoid process (dens), which projects upward from the body (Fig. 6.2). The dens is 1.0- to 1.5-cm long, and approximately 1-cm wide. On the front of the dens is an articular facet that forms a joint with the facet on the back of the anterior arch of the atlas. The dens has a pointed apex that is joined by the apical ligament, has a flattened side where the alar ligaments are attached, and is grooved at the base of its posterior surface where the transverse ligament of the atlas passes. The dens and body are flanked by a pair of large oval facets that extend laterally from the body onto the adjoining parts of the pedicles and articulate with the inferior facets of the atlas. The superior facets do not form an articular pillar with the inferior facets, but are anterior to the latter. The anterior aspect of the body is hollowed out on each side of the midline in the area where the longus colli muscles attach. The lamina are thicker than on any other cervical vertebrae, the pedicles are stout, and the spinous process is large.
The transverse processes of the axis are small. Their blunt tips present a single tubercle, the anterior tubercle, situated at or near the junction of the anterior root of the transverse process and the body. Each transverse foramen faces superolaterally, thus permitting the lateral deviation of the vertebral artery as it passes up to the more widely separated transverse foramina in the atlas. The inferior articular facets are situated at the junction of the pedicles and laminae, and face downward and forward. The spade-shaped vertebral foramen is relatively large.
The Atlantoaxial Joints
The articulation of the atlas and axis comprises four synovial joints: two median ones on the front and back of the dens, and paired lateral ones between the opposing articular facets on the lateral masses of the atlas and axis (Figs. 6.2-6.4). Each of the median joints, situated on the front and back of the dens, has its own fibrous capsule and synovial cavity. The anterior one is situated between the anterior surface of the dens and the posterior aspect of the anterior arch of the atlas. The posterior one has an even larger synovial cavity and lies between the cartilage-covered anterior surface of the transverse ligament of the atlas and the posterior surface of the dens.
The atlas and axis are united by the cruciform ligament, the anterior and posterior longitudinal ligaments, and the articular capsules surrounding the joints between the opposing articular facets on the lateral masses. The cruciform ligament has transverse and vertical parts that form a cross behind the dens. The transverse part, called the transverse ligament, is a thick strong band that arches across the ring of the atlas behind the dens and divides the vertebral canal into a larger posterior compartment containing the dura and the spinal cord and a smaller anterior compartment containing the odontoid process. The transverse ligament is broader in the middle behind the dens than at the ends where it is attached to a tubercle on the medial side of the lateral masses of the atlas. As it crosses the dens, small longitudinal bands are directed upward and downward from its posterior surface. The cranial extension is attached to the upper surface of the clivus between the apical ligament of the dens and the tectorial membrane. The lower band is attached to the posterior surface of the body of the axis. The neck of the dens is constricted where it is embraced posteriorly by the transverse ligament.
In front, the atlas and axis are connected by the anterior longitudinal ligament, which is a wide band fixed above to the lower border of the anterior arch of the atlas and below to the front of the body of the axis. The posterior longitudinal ligament is attached below to the posterior surface of the body of the axis, and above to the transverse part of the cruciform ligament and the clivus. Posterior to the spinal canal, the atlas and axis are joined by a broad, thin membrane in series with the ligamentum flavum that is attached above to the lower border of the posterior arch of the atlas, and below to the upper edges of the laminae of the axis. This membrane is pierced laterally by the second cervical nerve.
The Atlanto-occipital Joints
The atlas and the occipital bone are united by the articular capsules surrounding the atlanto-occipital joints and by the anterior and posterior atlanto-occipital membranes (Figs. 6.2- 6.4). The articular capsules of the atlanto-occipital joints are sometimes deficient medially where the synovial cavities may communicate with the synovial bursa between the dens and the transverse ligament of the atlas. The anterior atlanto- occipital membrane is attached superiorly to the anterior edge of the foramen magnum, inferiorly to the superior edge of the anterior arch of the atlas, and laterally to the capsule of the atlanto-occipital joints.
The posterior atlanto-occipital membrane is a thin sheet connected above to the posterior margin of the foramen magnum and below to the upper border of the posterior arch of the atlas. The lateral border of the membrane is free and arches behind the vertebral artery and the first cervical nerve root. The lateral edge of this membrane may be ossified in the area where it arches over the posterior aspect of the vertebral artery, thus creating a partial or complete osseous ring around the artery on the medial side of the atlanto-occipital joint.
Axis and Occipital Bone
Four fibrous bands, the tectorial membrane, the paired alar ligaments, and the apical ligament, connect the axis and the occipital bone (Figs. 6.3 and 6.4). The tectorial membrane is a cephalic extension of the posterior longitudinal ligament that covers the dens and cruciform ligament. It is attached below to the posterior surface of the body of the axis, above to the upper surface of the occipital bone in front of the foramen magnum, and laterally to the medial sides of the atlanto-occipital joints. The alar ligaments are two strong bands that arise on each side of the upper part of the dens and extend obliquely superolateral to attach to the medial surfaces of the occipital condyles. The apical ligament of the odontoid process extends from the tip of the dens to the anterior margin of the foramen magnum and is situated between the anterior atlanto-occipital membrane and the superior prolongation of the cruciform ligament.
The foramen magnum is surrounded by the muscles attached to the occipital bone and upper cervical vertebrae (Figs. 6.4 and 6.5). The trapezius covers the back of the head and neck. It extends from the medial half of the superior nuchal line, the external occipital protuberance, and the spinous processes of the cervical and thoracic vertebrae and converges on the shoulder to attach to the scapula and the lateral third of the clavicle. The sternocleidomastoid passes obliquely downward across the side of the neck from the lateral half of the superior nuchal line and mastoid process to the upper part of the sternum and the adjacent part of the clavicle. This muscle divides the side of the neck into an anterior triangle and a posterior triangle. The anterior triangle is bounded posteriorly by the anterior border of the sternocleidomastoid, above by the mandible, and anteriorly by the median line of the neck; the posterior triangle is bounded in front by the posterior border of the sternocleidomastoid, below by the middle third of the clavicle, and behind by the anterior margin of the trapezius. The splenius capitis, situated deep to and partially covered by the trapezius and sternocleidomastoid, extends from the bone below the lateral third of the superior nuchal line to the spinous processes of the lower cervical and upper thoracic vertebrae. Two muscles, both of which are situated deep to the splenius capitis and sternocleidomastoid and attach below to the upper thoracic and lower cervical vertebrae, are the semispinalis capitis, which attaches above in the area between the superior and inferior nuchal lines beginning medially at the external occipital crest and extending laterally to the occipitomastoid junction, and the longissimus capitis muscle, which attaches above to the posterior margin of the mastoid process.
The suboccipital muscles, located in the next layer, are a group of muscles situated deep to the splenius, semispinalis, and longissimus capitis in the suboccipital area. This group includes the superior oblique, which extends from the area lateral to the semispinalis capitis between the superior and inferior nuchal lines to the transverse process of the atlas; the inferior oblique, which extends from the spinous process and lamina of the axis to the transverse process of the atlas; the rectus capitis posterior major, which extends from and below the lateral part of the inferior nuchal line to the spine of the axis; and the rectus capitis posterior minor, which is situated medial to and is partially covered by the rectus capitis posterior major, extends from the medial part and below the inferior nuchal line to the tubercle on the posterior arch of the atlas.
The suboccipital triangle is a region bounded above and medially by the rectus capitis posterior major, above and laterally by the superior oblique, and below and laterally by the inferior oblique (Fig. 6.5). It is covered by the semispinalis capitis medially and by the splenius capitis laterally. The floor of the triangle is formed by the posterior atlanto-occipital membrane and the posterior arch of the atlas. The structures in the triangle are the terminal extradural segment of the vertebral artery and the first cervical nerve.
The platysma is a broad sheet extending downward from the lower part of the face and across the clavicle to the fascia covering the pectoralis major and deltoid. The anterior vertebral muscles insert on the clival part of the occipital bone anterior to the foramen magnum. This group includes the longus colli, which attach to the anterior surface of the vertebral column between the atlas and the third thoracic vertebra; the longus capitis, which extends from the clivus in front of the foramen magnum to the transverse processes of the third through the sixth cervical vertebrae; the rectus capitis anterior, which is situated behind the upper part of the longus capitis and extends from the occipital bone in front of the occipital condyle to the anterior surface of the lateral mass and transverse process of the atlas; and the rectus capitis lateralis, which extends from the jugular process of the occipital bone to the transverse process of the atlas.
The muscles described above are embedded in the cervical fascia. This fascia is divided into superficial and deep layers. The superficial layer is a lamina of loose connective tissue below the dermis, which invests the platysma. The deep layer lies internal to the platysma, invests the muscles, and condenses into fibrous sheaths that bind the arteries and accompanying veins together. The superficial lamina of the deep fascia attaches in the posterior midline to the ligamentum nuchae, thinly invests the trapezius, continues forward covering the posterior triangle of the neck, divides at the posterior border of the sternocleidomastoid to enclose the muscle, and at its anterior margin again forms a lamina that covers the anterior triangle of the neck and reaches the median plane, to be continuous with the corresponding lamina from the opposite side. The carotid sheath is a condensation of the cervical fascia, which invests the common and internal carotid arteries, the internal jugular vein, and the vagus nerve. The pre-vertebral lamina of the cervical fascia covers the prevertebral muscles, extends laterally to connect with the carotid sheath, and covers the scalene muscles to form a fascial floor for the posterior triangle of the neck. Superiorly it is attached to the base of the skull, and inferiorly it continues downward behind the pharynx and in front of the longus colli into the superior mediastinum. The deep fascia is fused above to the superior nuchal line, mastoid process, zygomatic arch, styloid process, and mandible, and below to the scapula, clavicle, and sternum.
The neural structures situated in the region of the foramen magnum are the caudal part of the brainstem, cerebellum and fourth ventricle, the rostral part of the spinal cord, and the lower cranial and upper cervical nerves (Figs. 6.3 and 6.6) (5, 19).
The spinal cord blends indistinguishably into the medulla at a level arbitrarily set to be at the upper limit of the dorsal and ventral rootlets forming the first cervical nerve (Figs. 6.3 and 6.6). It is easier to differentiate this level on the ventral than on the dorsal surface because the ventral rootlets of the first cervical nerve are always present, whereas the dorsal rootlets are absent in many cases. The fact that the junction of the spinal cord and medulla is situated at the rostral margin of the first cervical root means that the medulla, and not the spinal cord, occupies the foramen magnum.
The spinal cord immediately below the level of the foramen magnum is round, and it is divided by one fissure and several sulci. The anteromedian fissure and the posteromedian sulcus divide the spinal cord into symmetrical halves. The anteromedian fissure reaches a depth of several millimeters. The posteromedian sulcus is much shallower, and from it the posteromedian septum penetrates the spinal cord, almost reaching the central canal. The posterior lateral sulcus is situated along the line where the dorsal roots enter the spinal cord. The posterior funiculus is situated between the posteromedian and posterior lateral sulci. At the upper cervical level, the surface of each posterior funiculus is divided by another shallow longitudinal furrow, the posterior intermediate sulcus, into the fasciculus gracilis medially and the fasciculus cuneatus laterally. The region of the spinal cord between the posterior lateral sulcus and the anteromedian fissure is divided into anterior and lateral funiculi by the exiting ventral rootlets of the spinal nerves. The anterior funiculus includes the zone of emergence of the ventral roots. The lateral funiculus lies between the ventral roots and the posterior lateral sulcus. In the upper cervical region, the rootlets that unite to form the spinal part of the accessory nerve emerge through the lateral funiculus.
The dentate ligament is considered with the spinal cord because it is attached to it (Figs. 6.3 and 6.6). This ligament is a white fibrous sheet that is attached to the spinal cord medially and to the dura mater laterally. The medial border of the dentate ligament, which is attached to the pia mater between the dorsal and ventral rootlets along the length of each side of the spinal cord, presents a series of triangular toothlike processes on each side that are attached at intervals to the dura mater. At the craniocervical junction, the dentate ligament is located between the vertebral artery and the ventral roots of C1 anteriorly and the branches of the posterior spinal artery and the spinal accessory nerve posteriorly; in addition, it is often incorporated into the dural cuff around the vertebral artery at the site of dural penetration. The most rostral attachment of the dentate ligament is located at the level of the foramen magnum, above where the vertebral artery pierces the dura. The ligament courses behind the accessory nerve at that level, although the dentate ligament is located anterior to the accessory nerve at lower levels. The second triangular process is attached to the dura below the site at which the vertebral artery and the roots of C1 pierce the dura. Sectioning the upper two triangular processes will increase access anterior to the spinal cord. The first cervical nerve courses along the posteroinferior surface of the vertebral artery as it pierces the dura. The ventral root is located anterior to the dentate ligament, and the dorsal root, which is infrequently present, passes posterior to the dentate ligament. There are frequently communications between the C1 nerve root and the spinal accessory nerve.
The lower medulla blends indistinguishably into the upper spinal cord at the level of the C1 nerve roots (Figs. 6.3, 6.4, and 6.6). The anterior surface of the medulla is formed by the medullary pyramids, which face the clivus, the anterior edge of the foramen magnum, and the rostral part of the odontoid process. The lateral surface is formed predominantly by the inferior olives. The posterior surface of the medulla is divided into superior and inferior parts. The superior part is composed in the midline of the inferior half of the fourth ventricle, and laterally by the inferior cerebellar peduncles. The inferior part of the posterior surface is composed of the gracile fasciculus and tubercle medially, and the cuneate fasciculus and tubercle laterally.
The suboccipital cerebellar surface rests above the posterior and lateral edge of the foramen magnum. Only the lower part of the hemispheres formed by the tonsils and the biventral lobules, and the lower part of the vermis formed by the nodule, uvula, and pyramid, are related to the foramen magnum. The biventral lobule sits above the lateral part of the foramen magnum, and the tonsils rest above the level of the posterior edge (Figs. 6.3 and 6.6). The cerebellar surface above the posterior part of the foramen magnum has a deep vertical depression, the posterior cerebellar incisura, which contains the falx cerebelli and extends inferiorly toward the foramen magnum. The tonsils, which sit above the posterior edge of the foramen magnum, are commonly involved in herniations through the foramen magnum. Each tonsil is an ovoid structure that is attached along its superolateral border to the remainder of the cerebellum. The cerebellomedullary fissure extends superiorly between the cerebellum and the medulla and is situated rostral to the posterior margin of the foramen magnum.
The accessory nerve is the only cranial nerve that passes through the foramen magnum (Figs. 6.3 and 6.6). It has a cranial part composed of the rootlets that arise from the medulla and join the vagus nerve, and a spinal portion formed by the union of a series of rootlets that arise from the lower medulla and upper spinal cord. In the posterior fossa, the accessory nerve is composed of one main trunk from the spinal cord and three to six small rootlets that emerge from the medulla. The most rostral medullary rootlets are functionally inferior vagal rootlets, since they arise from the vagal nuclei (25). The lower medullary rootlets join the spinal portion of the nerve. The upper medullary rootlets enter the jugular foramen without joining the spinal portion, but once inside the jugular foramen, they join either the vagus or accessory nerve. The spinal contribution arises from the cervical portion of the spinal cord as a series of rootlets situated midway between the ventral and dorsal rootlets. The lowest level of origin of the rootlets contributing to the accessory nerves was at the C7 root level in 2 of the 50 nerves examined, C6 in 10, C5 in 13, C4 in 11, C3 in 7, C2 in 5, and Cl in 2(5). These rootlets unite to form a trunk with a diameter of approximately 1.0 mm, which ascends through the foramen magnum between the dentate ligament and the dorsal spinal roots to enter the posterior cranial fossa behind the vertebral artery.
Of the 50 accessory nerves examined in our previous study, all had connections with the dorsal roots of the upper cervical nerves. The most common and largest anastomosis was with the dorsal root of the first cervical nerve (5, 22). Twenty-eight of the C1 dorsal roots arose solely from the accessory nerve without there being a contribution from the C1 level of the spinal cord. All of the 15 Cl dorsal roots that received rootlets arising from the spinal cord at the C1 level also had anastomotic fibers from the accessory nerve. Four of the 50 accessory nerves had an anastomotic connection with the C2 nerve root, 10 with the C3, 8 with the C4, and 2 with the C5.
The lower four cranial nerves are sufficiently close to the foramen magnum that they may be involved by lesions arising there (Figs. 6.3 and 6.6). Their intradural anatomy is described in the chapter of this issue on the cerebellopontine angle and posterior fossa cranial nerves.
Cervical Nerve Roots
Each dorsal and ventral root is composed of a series of six to eight rootlets that fan out to enter the posterolateral and anterolateral surfaces of the spinal cord, respectively (Figs. 6.3 and 6.6). The dorsal and ventral roots cross the subarachnoid space and transverse the dura mater separately, then unite close to the intervertebral foramen to form the spinal nerves. The rootlets in the region of the foramen magnum pass almost directly lateral to reach their dural foramina. The neurons of the dorsal roots collect to form ganglia located just proximal to the union of the dorsal and ventral root in the intervertebral foramina, however the first cervical dorsal root and associated ganglion may be absent. The C1, C2, and C3 nerves, distal to the ganglion, divide into dorsal and ventral rami. The dorsal rami divide into medial and lateral branches that supply the skin and muscles of the posterior region of the neck. The C1 nerve, termed the suboccipital nerve, leaves the vertebral canal between the occipital bone and atlas and has a dorsal ramus that is larger than the ventral ramus. The dorsal ramus courses between the posterior arch of the atlas and the vertebral artery to reach the suboccipital triangle, where it sends branches to the rectus capitis posterior major and minor, superior and inferior oblique, and the semispinalis capitis, and occasionally has a cutaneous branch that accompanies the occipital artery to the scalp. The C1 ventral ramus courses between the posterior arch of the atlas and the vertebral artery and passes forward, lateral to the lateral mass of the atlas and medial to the vertebral artery, and supplies the rectus capitis lateralis. The C2 nerve emerges between the posterior arch of the atlas and the lamina of the axis where the spinal ganglion is located extradurally, medial to the inferior facet of C1 and the vertebral artery. Distal to the ganglion, the nerve divides into a larger dorsal and a smaller ventral ramus. After passing below and supplying the inferior oblique muscle, the dorsal ramus divides into a large medial and a small lateral branch. It is the medial branch that is most intimately related to this suboccipital operative field and that forms the greater occipital nerve. It ascends obliquely between the inferior oblique and the semisplenius capitis, pierces the latter and the trapezius muscle near their attachments to the occipital bone, and is joined by a filament from the medial branch of C3. It supplies the semispinalis capitis muscle, ascends with the occipital artery, and supplies the scalp as far forward as the vertex, and occasionally the back of the ear. The lateral branch sends filaments that innervate the splenius, longissimus, and semisplenius capitis, and is often joined by the corresponding branch from the C3 nerve. The C2 ventral ramus courses between the vertebral arches and transverse processes of the atlas and axis and behind the vertebral artery to leave this operative field. Two branches of the C2 and C3 ventral rami, the lesser occipital and greater auricular nerves, curve around the posterior border and ascend on the sternocleidomastoid muscle to supply the skin behind the ear.
The first cervical nerve, located just below the foramen magnum, deserves special attention (Figs. 6.3 and 6.6). It differs from the other cervical nerves in the consistency and origin of the dorsal rootlets forming the nerve. The C1 ventral root is composed of four to eight rootlets that joined and coursed laterally. Before entering the dural foramina, the C1 ventral root, and the corresponding dorsal root if present, attaches to the posteroinferior surface of the initial intradural part of the vertebral artery, and both exit the dural sac through the funnel-shaped dural foramen around the vertebral artery. The ventral root joins the dorsal root in or external to the dural foramen.
The dorsal root of the first cervical nerve is more complicated than the ventral root because of the variations in its composition and its connections with the accessory nerve. In the 25 cervical spinal cords examined, in which one would expect to find 50 C1 dorsal roots arising from the posterior lateral sulcus, only 15 were found (5). The accessory nerve contributed a root to the C1 nerve in 28 of the 35 roots lacking a dorsal root arising from the spinal cord. In the remaining 7 cases, the C1 dorsal root was absent. Each of the 15 dorsal roots that arose from the spinal cord also had a contribution from the accessory nerve.
The major arteries related to the foramen magnum are the vertebral and posteroinferior cerebellar arteries (PICA), and the meningeal branches of the vertebral, and external and internal carotid arteries (Figs. 6.3, 6.4, and 6.6) (16, 20, 21).
The paired vertebral arteries arise from the subclavian arteries, ascend through the transverse processes of the upper six cervical vertebrae, pass behind the lateral masses of the axis, enter the dura mater behind the occipital condyles, ascend through the foramen magnum to the front of the medulla, and join to form the basilar artery at the pontomedullary junction. Each artery is divided into intradural and extradural parts (Figs. 6.3-6.6).
The extradural part is divided into three segments. The first segment extends from the origin at the subclavian artery to the entrance into the lowest transverse foramen, usually at the C6 level. The second segment ascends through the transverse foramina of the upper six cervical vertebrae in front of the cervical nerve roots. This segment deviates laterally just above the axis to reach the laterally placed transverse foramen of the atlas. The third segment, the one most intimately related to the foramen magnum, extends from the foramen in the transverse process of the atlas to the site of passage through the dura mater. The artery, after passing through the transverse process of the atlas, is located on the medial side of the rectus capitis lateralis. The third segment passes medially behind the lateral mass of the atlas and atlanto-occipital joint and is pressed into the groove on the upper surface of the lateral part of the posterior arch of the atlas, where it courses along the floor of the suboccipital triangle. It enters the vertebral canal by passing anterior to the lateral border of the atlanto-occipital membrane. It is partially covered by the posterior atlanto-occipital membrane and semispinalis capitis, the rectus capitis posterior major, and the superior and inferior oblique muscles. It is surrounded by a venous plexus composed of anastomoses between the deep cervical and epidural veins. The C1 nerve root passes through the dura mater on the lower surface of the vertebral artery between the artery and the groove on the posterior arch of the atlas with the vertebral artery. This bony groove is frequently transformed into a bony canal that completely surrounds a short segment of the artery. Of the 50 arteries we examined, 24 (48%) were in a shallow groove, 12 (24%) were partially, but incompletely, surrounded by bone, and 14 (28%) coursed through a bony ring that completely surrounded the artery (Fig. 6.6) (5). The terminal extradural segment of the vertebral artery gives rise to the posterior meningeal and posterior spinal arteries, branches to the deep cervical musculature, and infrequently the PICA.
The intradural segment begins at the dural foramina just inferior to the lateral edge of the foramen magnum. The dura in this region is much thicker than in other areas, and it forms a funnel-shaped foramen around a 4- to 6-mm length of the artery. The first cervical nerve exits the spinal canal, and the posterior spinal artery enters the spinal canal through this dural foramen with the vertebral artery. These three structures are bound together at the foramen by fibrous dural bands. The initial intradural segment of the vertebral artery passes just superior to the dorsal and ventral roots of the first cervical nerve, and just anterior to the posterior spinal artery, the dentate ligament, and the spinal portion of the accessory nerve.
Once inside the dura mater, the artery ascends from the lower lateral to the upper anterior surface of the medulla. The intradural part of the artery is divided into lateral and anterior medullary segments (5, 16). The lateral medullary segment begins at the dural foramen and passes anterior and superior along the lateral medullary surface to terminate at the preolivary sulcus. The anterior medullary segment begins at the preolivary sulcus, courses in front of, or between, the hypoglossal rootlets, and crosses the pyramid to join with the other vertebral artery at or near the pontomedullary sulcus to form the basilar artery. In its ascending course, the anterior and lateral surfaces of the lateral medullary segments face the occipital condyles, the hypoglossal canals, and the jugular tubercles. The anterior medullary segment rests on the clivus. The branches arising from the vertebral artery in the region of the foramen magnum are the posterior spinal, anterior spinal, PICA, and anterior and posterior meningeal arteries.
Posterior Spinal Artery
The paired posterior spinal arteries usually arise from the posteromedial surface of the vertebral arteries, just outside the dura mater, but they may also arise from the initial intradural part of the vertebral arteries, or from the PICA (Figs. 6.3 and 6.6) (5, 16, 21). Care should be taken to preserve the posterior spinal artery during dural opening because it may be incorporated into the dural cuff around the vertebral artery. As each posterior spinal artery passes through the dura mater, it is surrounded by the same fibrous tunnel as the vertebral artery and the first cervical nerve root. In the subarachnoid space, it courses medially behind the rostral-most attachments of the dentate ligament, and on reaching the lower medulla, it divides into ascending and descending branches. The ascending branch courses through the foramen magnum and supplies the restiform body, the gracile and cuneate tubercles, the rootlets of the accessory nerve, and the choroid plexus near The foramen of Magendie, and may give rise to branches that anastomose with branches of the PICA. The descending branch passes downward between the dorsal rootlets and the dentate ligament on the posterolateral surface of the spinal cord, and supplies the superficial part of the dorsal half of the cervical spinal cord. It anastomoses with the posterior branches of the radicular arteries that enter the vertebral foramen at lower levels. The descending branch gives rise to collateral branches, each lower one being smaller and less constant than the last one, which course medially across the posterior surface of the spinal cord, and join to form an artery that courses in the midline, parallel to the posterior spinal arteries.
Posteroinferior Cerebellar Artery
The PICA is the largest branch of the vertebral artery (Figs. 6.3 and 6.6). It usually originates with the dura mater, but it may infrequently originate from the terminal extradural part of the vertebral artery. It may arise at, above, or below the level of the foramen magnum; of the 42 arteries found in 50 cerebellae examined, 35 arose above and 7 arose below the foramen (16). The tonsillomedullary PICA segment, which forms the caudal loop related to the lower part of the tonsil, is most intimately related to the foramen magnum. The lower end of the caudal loop was found to be above the edge of the foramen magnum in 37 of the 42 arteries examined, below the edge in 4, and at the level of the edge of the foramen in 1.
Anterior Spinal Artery
The anterior spinal artery is formed by the union of the paired anterior ventral spinal arteries, which originate from the anterior medullary segment of the vertebral arteries near the origin of the basilar artery (Figs. 6.3, 6.4, and 6.6). The junction of the anteroventral spinal arteries was located above the level of the foramen magnum near the lower end of the olives in 84% of our specimens (5). In some cases, one of the anterior ventral spinal arteries continued inferiorly as the anterior spinal artery, and the other terminated on the anterior surface of the medulla or in a rudimentary channel connected the smaller anterior ventral spinal artery with a dominant one.
The anterior spinal artery descends through the foramen magnum on the anterior surface of the medulla and the spinal cord in or near the anteromedian fissure. On the medulla, it supplies the pyramids and their decussation, the medial lemniscus, the interolivary bundles, the hypoglossal nuclei and nerves, and the posterior longitudinal fasciculus (17). It anastomoses with the anterior branches of the radicular arteries entering the cervical foramina. There are few anastomoses with the anterior radicular branches if the descending channel is large, but it has frequent connections with the anterior radicular arteries if it is small.
The dura mater around the foramen magnum is supplied by the anterior and posterior meningeal branches of the vertebral artery, and the meningeal branches of the ascending pharyngeal and occipital arteries (Figs. 6.3 and 6.6) (5, 20). These arteries, plus the dorsal meningeal branch of meningohypophyseal trunk that arises from the intracavernous segment of the internal carotid artery, supply all of the dura lining the posterior cranial fossa. Infrequently, the PICA, the posterior spinal artery, and the intradural part of the vertebral artery give rise to meningeal branches.
The anterior meningeal branch of the vertebral artery arises from the medial surfaces of the extradural part of the vertebral artery immediately above the transverse foramen of the third cervical vertebra (Fig. 6.3). The artery enters the spinal canal through the intervertebral foramen between the second and third cervical vertebrae, and ascends between the posterior longitudinal ligament and the dura mater. At the level of the apex of the dens, each artery courses medially to join its mate from the opposite side and forms an arch over the apex of the dens. Its branches supply the dura mater in the region of the clivus and the anterior part of the foramen magnum and upper spinal canal, and they anastomose with the branches of the ascending pharyngeal and dorsal meningeal arteries that supply the dura mater covering the anterior and anterolateral part of the posterior fossa. The anterior meningeal artery also gives rise to muscular and osseous branches that supply the body and odontoid process of the axis and the articulate plate of the atlanto-occipital and atlantoaxial joints.
The posterior meningeal artery arises from the posterosuperior surface of the vertebral artery as it courses around the lateral mass of the atlas, above the posterior arch or just before penetrating the dura; however, it may have an intradural origin, in which case, it penetrates the arachnoid to reach the dura (Fig. 6.6) (5). It pursues a tortuous ascending course and penetrates the dura before reaching the posterior edge of the foramen magnum. After passing through the foramen magnum, it ascends near the falx cerebelli and divides near the torcula into several branches that terminate in the posterior part of the tentorium and cerebral falx. It supplies the dura mater lining the posterolateral and posterior part of the posterior cranial fossa, and anastomoses with the meningeal branches of the ascending pharyngeal and occipital arteries.
The ascending pharyngeal branch of the external carotid artery usually sends two branches to the dura above the foramen magnum. One branch passes through the hypoglossal canal and the other enters through the jugular foramen (14). The branch passing through the hypoglossal canal divides into an ascending branch that passes upward in the dura covering the clivus and anastomoses with the branches of the dorsal meningeal artery, and a descending branch that courses inferomedially toward the anterior edge of the foramen magnum and anastomoses with branches of the arcade above the odontoid process formed by the anterior meningeal arteries. This anastomotic rete in the dura anterior to the foramen magnum and on the clivus gives osseous branches to the clivus. The branches that enter through the jugular foramen divide into branches that course posteriorly and posterosuperiorly to anastomose with the meningeal branches of the occipital and posterior meningeal arteries, and supply the dura mater in the posterior and posterolateral parts of the posterior cranial fossa.
The meningeal branch of the occipital artery is inconstant and, if present, it penetrates the cranium through the mastoid emissary foramen. It divides into one branch that courses posterosuperiorly to join the branches of the posterior meningeal artery that supplies the dura mater in the posterior part of the posterior fossa, and another branch that courses anterolaterally and joins the meningeal branches of the ascending pharyngeal artery.
The venous structures in the region of the foramen magnum are divided into three groups: one composed of the extradural veins, another formed by the intradural (neural) veins, and a third constituted by the dural venous sinuses (13, 18). The three groups anastomose through bridging and emissary veins.
Venous flow in this area empties into two systems: one drained by the internal jugular vein and another draining into the vertebral venous plexus. The internal jugular vein and its tributaries form the most important drainage system in the craniocervical area. The internal jugular vein originates at the jugular foramen by the confluence of the sigmoid and inferior petrosal sinuses (14, 18, 25). The venous plexus surrounding the vertebral artery in the suboccipital triangle is formed by numerous small channels that empty into the internal vertebral plexuses (between the dura and the vertebrae), which issue from the vertebral canal above the posterior arch of the atlas. This vertebral venous plexus and multiple small veins from the deep muscles communicate with the dense venous plexus, which accompanies the vertebral artery into the foramen in the transverse process of the atlas and descends through the transverse foramina of successive cervical vertebrae into the brachiocephalic vein. The posterior condylar emissary vein, which passes through the posterior condylar canal, forms a communication between the vertebral venous plexus and the sigmoid sinus. The venous plexus of the hypoglossal canal passes along the hypoglossal canal to connect the basilar venous plexus with the marginal sinus, which encircles the foramen magnum. Obliteration of a portion of the venous plexus exposes the upper extradural segment of the vertebral artery.
Dural Venous Sinuses
The venous channels in the dura mater surrounding the foramen magnum are the marginal, occipital, sigmoid, inferior petrosal, and basilar venous plexus. The marginal sinus is located between the layers of the dura in the rim of the foramen magnum. It communicates anteriorly, through a series of small sinuses, with the basilar sinus on the clivus, and posteriorly with the occipital sinus. It is usually connected to the sigmoid sinus or jugular bulb, by a sinus that passes across the intracranial surface of, and communicates with, the veins in the hypoglossal canal. These anastomoses provide an alternative route for venous drainage in the case of obstruction of the internal jugular vein. The occipital sinus courses in the cerebellar falx. Its lower end divides into paired limbs each of which courses anteriorly around the foramen magnum to join the sigmoid sinus or the jugular bulb and its upper end joins the torcula.
The basilar venous plexus is located between the layers of the dura mater on the upper clivus. It is formed by interconnecting venous channels that anastomose with the inferior petrosal sinuses laterally, the cavernous sinuses superiorly, and the marginal sinus and epidural venous plexus inferiorly. The inferior petrosal sinuses extend along the petroclival fissure and communicate above with the basilar sinus and below with the jugular bulb. The sigmoid sinus descends along the sigmoid groove and exits the cranium through the sigmoid part of the jugular foramen, and descends anterolateral to the occipital condyle, and anterior to the transverse process of the atlas.
Intradural (Neural) Veins
The intradural veins in the region of the foramen magnum drain the lower part of the cerebellum and brainstem, the upper part of the spinal cord, and the cerebellomedullary fissure. The veins of the medulla and spinal cord form longitudinal plexiform channels that anastomose at the foramen magnum. The median anterior spinal vein that courses in the anteromedian spinal fissure deep to the anterior spinal artery is continuous with the median anterior medullary vein that courses on the anteromedian sulcus of the medulla. The lateral anterior spinal vein courses longitudinally along the origin of the ventral roots and superiorly joins the lateral anterior medullary vein that courses longitudinally in the anterolateral medullary (preolivary) sulcus along the line of origin of the hypoglossal rootlets. The lateral posterior spinal vein, which courses along the line of origin of the dorsal roots in the posterior lateral spinal sulcus, is continuous above with the lateral medullary vein that courses along the retro-olivary sulcus, dorsal to the olive. The median posterior spinal vein, which courses along the posteromedian spinal sulcus, is continuous above with the main vein on the posterior surface of the medulla, the median posterior medullary vein that courses along the posteromedian medullary sulcus. The transverse medullary and transverse spinal veins cross the medulla and spinal cord at various levels, interconnecting the major longitudinal channels. Bridging veins may connect the neural veins with the dural sinus in the region of the foramen magnum.
Herniation of cerebellar tissue into the foramen magnum may cause neural compression and even death. These herniations are commonly referred to as tonsillar herniations (8, 27), but the herniation usually involves the tonsils and biventral lobules, both of which are deeply grooved by the edge of the foramen magnum. The herniation may compress the medulla and be so severe that the herniated tissue undergoes necrosis. Patients with herniation at the foramen magnum may be asymptomatic; or may present with pain, signs of neural compression, increased intracranial pressure, and sudden unexpected death. Symptoms caused by dysfunction of the cerebellum, brainstem, and lower cranial and upper spinal nerves include pain in the neck and upper arms, dizziness, ataxia, disturbances of gait, diplopia, dysphagia, tinnitus, decreased hearing, nystagmus, weakness up to the degree of quadriparesis, and sensory deficit in the extremities. Coughing or sneezing may aggravate the symptoms and cause syncope. Some patients without previous symptoms who die suddenly are found to have herniations through the foramen magnum at autopsy. The occurrence of sudden death in these patients means that herniation at the foramen magnum is a precarious situation that can be aggravated by minor stresses (8). The common denominator in these cases with sudden death is herniation of the tonsils and adjacent part of the biventral lobule into the foramen magnum. The herniation may be bilateral and symmetrical, although more commonly it is not strictly symmetrical and may be unilateral. The herniated tonsils are tightly pressed against the medulla. Acute or chronic herniations may be seen with space-occupying lesions, such as cerebellar astrocytomas or cystic tumors. Chronic herniation is seen with the Arnold-Chiari malformation.
Tumors arising in the region of the foramen magnum are divided by Cushing and Eisenhardt (4) into a craniospinal group that arises above and grows downward toward the foramen magnum, and a spinocranial group that arises below and grows upward toward the foramen magnum. The intradural extramedullary tumors in this region are usually benign, with meningiomas and schwannomas being the most frequent. The intramedullary tumors are represented mainly by astrocytomas and ependymomas. Cerebellar tumors, especially those originating in the fourth ventricle and those arising in the lower part of the cerebellar hemisphere or vermis, may extend into or through the foramen magnum into the upper spinal canal. Chordomas and metastases are the most common extradural tumors. The chordomas usually arise at the level of the clivus and may extend caudally into the foramen magnum.
Foramen magnum tumors have frequently eluded early diagnosis because they cause bizarre symptoms that simulate cervical, spondylosis, multiple sclerosis, or degenerative diseases (1, 23, 30). Symptoms or signs, common in other disorders that should also suggest the presence of a tumor in the region of the foramen magnum include neck stiffness and pain, involvement of the lower cranial nerves, especially the spinal accessory nerve, unilateral upper extremity weakness and atrophy, incoordination of the hands, gait disturbances, vague sensory disturbances or paresthesia in the extremities, objective sensory loss in a nonanatomic pattern, incoordination in the upper extremities, and pyramidal tract findings with spastic gait. Those tumors arising in the caudal part of the fourth ventricle or cerebellum may cause increased intradcranial pressure by obstructing cerebrospinal fluid drainage at the level of the fourth ventricle.
The foramen magnum is most commonly approached from posteriorly or anteriorly, and less frequently from laterally (Fig. 6.7). The posterior operative approach is commonly selected for intradural lesions, and an anterior approach is frequently selected for extradural lesions situated anterior to the foramen magnum. A lateral approach may be selected for lesions located lateral to or in front of the brainstem, especially if they involve, or are located contiguous to the temporal bone and clivus. The lateral approaches directed through the temporal bone are reviewed in the chapter on the temporal bone.
The vertical midline incision is used for lesions situated in the upper spinal canal and posterior or posterolateral at the level of or above the foramen magnum (Figs. 6.3, 6.6, and 6.8). The vertical midline skin incision is of sufficient length to complete a craniectomy above the foramen magnum and a laminectomy of the axis and atlas. The subcutaneous tissues are separated from the underlying fascia near the inion to gain room for a Y-shape muscle incision. The upper limbs of the “Y” begin at the level of the superior nuchal line, lateral to the external occipital protuberance, and join several centimeters below the inion, leaving a musculofascial flap along the superior nuchal line for closure. The inferior limb of the “Y” incision extends downward in the midline. The major extracranial hazard is injury to the vertebral artery as it courses along the lateral part of the posterior arch of the atlas. This artery is not encountered if the incision is strictly midline, but it is frequently encountered in the floor of the suboccipital triangle if the muscle incision deviates laterally, or when the muscles are stripped from the lateral part of the posterior arch of the atlas. The emissary veins and vertebral venous plexus should be obliterated quickly if they are opened.
The hockey-stick incision is selected if the lesion extends anterior or anterolateral to the brainstem toward the jugular foramen or the cerebellopontine angle. The skin incision extends from the mastoid process along the superior nuchal line to the inion, and downward in the midline. A muscular cuff is left attached along the superior nuchal line to facilitate the closure. This incision permits removal of the full posterior rim of the foramen magnum, the posterior elements of the atlas and axis, and, in addition, to complete a unilateral suboccipital craniectomy of sufficient size to expose the anterolateral surface of the brainstem and the nerves in the cerebellopontine angle.
In opening the dura mater, using either the midline or hockey-stick approach, the marginal and occipital sinuses, along with the bridging veins passing from the neural surfaces to these and the sigmoid sinus, are encountered. Posterior intradural lesions may separate easily from the surface of the brain and spinal cord. On the other hand, they may be attached to the nerve roots and spinal cord, or they may extend upward through the cerebellomedullary fissure to be attached to the inferior medullary velum, choroid plexus, or the floor of the fourth ventricle. Opening the tela choroidea and inferior medullary velum may facilitate the exposure of tumors in this area. Care is required to avoid injury to the PICA as it courses around the tonsil and through the cleft between the superior pole of the tonsil and inferior medullary velum and tela choroidea.
Laterally situated tumors may be attached to the initial intradural segment of the vertebral artery and the thick dural cuff around the artery, which also incorporates the posterior meningeal and posterior spinal arteries, Cl nerve root, accessory nerve, and the dentate ligament. Dealing with these lesions may be facilitated by using a far-lateral approach, which is extended to include exposure of the atlanto-occipital joint, extradural vertebral artery, and transverse process of C1, combined with drilling of the occipital condyle, as described in detail in the chapter on the far lateral approach (29, 33). Dividing the attachments of the upper triangular processes of the dentate ligaments may facilitate the exposure of anteriorly situated lesions. Structures encountered in exposing superiorly along the lateral surface of the medulla include the PICA and the glossopharyngeal, vagus, accessory, and hypoglossal nerves. The vertebral artery may be followed upward to its junction with the basilar artery through the hockey-stick exposure. The most difficult lesions to remove are those situated anterior to the glossopharyngeal, vagus, and accessory nerves and the lateral medullary segment of the vertebral artery. Before sacrificing any rootlets of these nerves, an attempt should be made to gently separate the rootlets and to operate through the interval between the rootlets. Often, tumors expand and widen the interval between the rootlets, thus providing some access to medially placed lesions. Another route through which it may be easier to reach a lesion anterior to the medulla and pons is the interval between the lower margin of the vestibulocochlear and facial nerves and the upper margin of the glossopharyngeal nerve. It is uncommon to be able to work between the vagal rootlets; however, the lower cervical rootlets of the accessory nerve are very fine and are often separated by a wide interval. Consideration might be given to sacrificing a few of the lower accessory rootlets if it will make an otherwise incurable lesion curable. The intracapsular contents of the tumor are removed, and the remaining tumor capsule is separated from the surface of the brainstem and nerves rather than attempting to deliver the whole intact tumor through the limited exposure. Extreme care should be used when cutting into tumors situated anterolateral to the brainstem, since these tumors, especially meningiomas, may encase a segment of the vertebral artery or the PICA. The dura mater is closed with a dural substitute if closure of the patient’s dura mater constricts the cerebellar tonsils or the cervicomedullary junction. A pseudomeningocele may form at the operative site if there is any tendency toward the development of hydrocephalus. Spinal drainage, repeated spinal punctures, or a shunting procedure may be required to decompress a postoperative pseudomeningocele.
Anterior Operative Approaches
The anterior approach was first used to reach lesions anterior to the spinal cord, and was subsequently used to expose lesions anterior to the brainstem (Figs. 6.4, 6.5, and 6.9-6.11). The greatest advantage of the anterior approach is the direct route to the lesion, and the major disadvantages are the contaminated field and the frequency of cerebrospinal fluid fistula, pseudomeningocele, and meningitis after the exposure of intradural lesions by this approach. The depth of the operative field was once considered a disadvantage, but the use of the operating microscope has reduced the importance of that factor.
Anterior approaches have been used to reach tumors of the atlas, axis, and clivus; for the resection and fixation of the odontoid process after ligamentous and osseous injury; for decompressing bony malformations of the craniovertebral junction, such as basilar invagination, which compress the medulla or spinal cord from anteriorly; and for approaching aneurysms of the lower third of the basilar artery, the vertebrobasilar junction, and the upper part of the vertebral arteries.
The transoral route through the mouth and the posterior pharyngeal wall, referred to as the buccopharyngeal approach, is the anterior approach most commonly selected. The basic transoral approach may be modified to include a transpalatine approach in which the soft palate, or both the soft and hard palates, are opened, and a labiomandibular or labioglossomandibular approach in which the lip, mandible, and possibly the tongue and floor of the mouth are split to increase the exposure. Other types of anterior approaches are: the transcervical approach directed through the submandibular area along the anterior border of the sternocleidomastoid muscle (31); the transcranial-transbasal approach in which the clivus is reached through a bifrontal craniotomy after resection of the sphenoid and ethmoid sinuses (6); the extended frontal approach in which the bifrontal craniotomy is combined with an osteotomy of the orbital rims; and the transsphenoidal approach directed under the lip, along the nasal septum, and through the sphenoid sinus to the upper part of the clivus.
For the transoral approach, the soft palate is retracted to reach the anterior part of the atlas and axis, and the posterior pharyngeal wall is incised longitudinally in the midline (Figs. 6.4, 6.12, and 6.13). The mucosa and prevertebral muscles are elevated as a single mucoperiosteal layer using subperiosteal dissection, and are retracted laterally. To expose the clivus, it is often necessary to split the soft palate in the midline. If added craniad exposure is needed, laterally based mucoperiosteal flaps may be elevated from the lower surface of the hard palate, and the posterior part of the hard palate may be removed. The mucosa covering the upper surface of the hard palate should be retracted and not opened. This permits the pharyngeal incision to be extended upward through the vault of the nasopharynx to the posterior border of the vomer. When elevating the mucoperiosteal layer from the clivus, the lateral margins slope dorsally into “gutter-like” depressions in which the tissue becomes thicker and more adherent. Depending on the lesion, the clivus, the anterior arch of the atlas, the dens, and bodies of C2 and C3 may be removed with a drill and rongeurs. The clival exposure between the occipital condyles is 2- to 2.5-cm wide and 2.5- to 3.0-cm long. Care must be taken to avoid the sixth through the twelve cranial nerves, the internal carotid arteries, the internal jugular veins, and the inferior petrosal sinuses that are on the periphery of the exposure. The most common lesions approached by this route are in an extradural location. Opening the dura mater will expose both vertebral arteries and the lower part of the basilar artery.
To increase the exposure and reduce the operative depth, the lip and chin may be incised vertically and a step-like mandibular osteotomy accomplished in the midline after removal of a central incisor tooth. Spreading the mandibular edges laterally, without splitting the tongue, permits the tongue to be depressed downward between the mandibular halves. If the exposure is still inadequate, the tongue and floor of the mouth may be split in the midline. Spreading the mandibular-lingual halves exposes the pharyngeal wall down to the level of the arytenoid cartilages. After dealing with the lesion, the mucosa and musculature of the tongue and floor of the mouth are reapproximated, the mandibular osteotomy is repositioned with wire, and the lip, chin, and submandibular region are carefully closed.
Transmaxillary approaches have been advocated for pathology extending to the upper and middle third of the clivus, which is difficult to reach by the transoral approach (Figs. 6.12 and 6.14-6.16). Four different types of transmaxillary approaches have been used (2, 3). In one approach, a LeFort I osteotomy is completed, and the maxilla and hard palate are down-fractured into the oral cavity. In the second approach, called the extended maxillectomy, the LeFort osteotomy is combined with a midline incision of the hard and soft palate and the halves of the maxilla are swung laterally. In the third approach, the unilateral lower subtotal maxillotomy, half of the maxilla, and the hard palate are hinged on the soft palate and folded downward into the floor of the mouth (6). The medial maxillotomy is a fourth and less extensive approach permitting exposure of the clivus. It involves removing the medial part of the anterior maxillary wall and the part of the maxilla bordering the anterior piriform aperture (Fig. 6.15). This provides an opening through the sinus and adjacent part of the nasal cavity that exposes the clivus above the level of the upper side of the hard palate. The sinus wall and the anterior piriform aperture can be reconstructed at the end of the procedure. It can commonly be performed through a degloving incision, although a lateral rhinotomy incision would be used if there is a need to extend the approach to the medial orbit (11, 12).
In the first approach, with a LeFort osteotomy, the upper lip is elevated and a mucosal incision is made along the upper alveolar margin, extending around the molars on both sides (Fig. 6.16). The mucosa is stripped off the anterior face of the maxilla below the infraorbital foramen. The saw cuts extend into the maxillary sinuses below the infraorbital foramen and high enough to avoid the dental roots, and extending into the nasal cavity leaving the branches of the internal maxillary artery and the nerves to the maxilla and palate intact. The mucosa on the nasal surface of the maxilla is dissected off, and the nasal septum is divided just above its attachment to the palate. The freed bone block includes, in one piece, the part of both maxilla and the maxillary teeth situated below the infraorbital foramen with their intact blood and nerve supply, which enters in the region of the infratemporal fossa and pterygoid plates. The fact that the soft palate is left intact reduces the incidence of speech and swallowing disorders. The intact maxillary block, however, blocks access to the craniovertebral junction, although it provides reasonable access to the upper and middle third of the clivus. In an effort to increase access to the craniovertebral junction, the LeFort osteotomy has been combined with a midline incision of the hard and soft palate, thus allowing the maxillary halves, with their attachment, to be reflected laterally (3). The disadvantage of the procedure is the difficulty obtaining good dental occlusion and proper functioning of the hard and soft palate.
In the lower subtotal maxillotomy approach, the part of half of the maxilla, located below the orbital floor and infraorbital canal, is folded into the floor of the mouth on a hinge of vascularized tissue, including the internal maxillary artery and leaving the soft palate intact (Fig. 6.14) (2, 11). The hard palate is divided in the midline, care being taken to preserve the soft palate. This opens a route through the nasal and oral cavities to the clivus, foramen magnum, and upper cervical area.
In each of the approaches, the posterior part of the nasal septum and turbinates may be removed to expose the posterior pharyngeal wall and provide access to the clivus and upper cervical vertebrae. These approaches also provide access to the sphenoid and ethmoid sinuses and the sella, and the medial part of the floor of the anterior fossa. The posterior part of the mucosa on both sides of the nasal septum may be prepared to provide flaps that can be folded into the clival defect for closure. In addition, planning will allow for a temporalis muscle graft to be folded into the clival defect for closure. The incidence of swallowing and speech difficulties is significantly greater with those approaches in which the soft palate is divided than when it is left intact. In each approach, plates and screws are positioned before making the bone cuts to achieve satisfactory dental occlusion after the procedure. The unilateral lower maxillotomy provides a more rapid recovery of oropalatal function because only half of the maxilla is disturbed, and the soft palate remains intact. That approach to the clivus is slightly oblique, but can provide as wide an exposure as is achieved with the approaches involving a bilateral maxillotomy.
The transsphenoidal approach along the nasal septum may be used to expose the upper third of the clivus (Figs. 6.9-6.11 and 6.17) (10). The vomer is resected to enter the sphenoid sinus and expose the floor of the sella turcica and the ventral surface of the clivus. The anterior nasal spine and the anterior part of the septal cartilage are preserved. In approaching the clivus, the floor of the sella turcica may be removed and the bony opening extended downward on the clivus to the inferior margin of the sphenoid sinus. Lesions extending to the upper third of the clivus may be biopsied or partially removed through this approach. The sellar and clival openings are closed with fat or muscle and nasal septal cartilage. The advantage of this approach is the low complication rate, and the disadvantage is the small operative field limited to the superior third of the clivus.
The transcervical approach, as performed by Stevenson et al., is directed through the fascial planes of the neck to the region of the foramen magnum (Fig. 6.18) (31). It avoids opening the oropharyngeal mucosa, but is selected infrequently because of the depth of the exposure and because it is not a direct midline exposure. A tracheostomy, which allows the jaws to be closed tightly, facilitates the exposure. The T-shaped skin incision includes a submandibular incision from the mastoid tip to the symphysis menti and an inferior extension carried from the midpoint of the submandibular incision across the sternocleidomastoid muscle. The fascial plane between the pharynx and the prevertebral muscles is reached through an exposure directed along the anterior border of the sternocleidomastoid muscle and between the carotid sheath laterally and the esophagus and trachea medially. The prevertebral fascia and muscles are retracted laterally to expose the ventral aspect of the clivus, atlas, and axis. Structures that may be divided from below to above to increase the exposure include the ascending pharyngeal and superior thyroid arteries, external laryngeal nerve, ansa hypoglossi, internal laryngeal nerve, lingual artery, hypoglossal nerve, stylohyoid muscle, anterior belly of the digastric muscle, stylohyoid ligament, glossopharyngeal nerve, and the stylopharyngeus and styloglossus muscles. The anterior arch of the atlas and the odontoid process, and a 2 cm width of clivus extending from the foramen magnum to the sphenooccipital synchondrosis may be removed. Deviation laterally may damage the internal jugular vein, internal carotid artery, eustachian tube, and the ninth through the twelfth cranial nerves.
The subfrontal-transbasal approach may be used to approach tumors of the anterior side of the foramen magnum if the tumor also involves and requires resection of part of the ethmoid and sphenoid bones, and the clivus (Figs. 6.19 and 6.20). The transbasal approach, as performed by Derome (6), is made through a bicoronal scalp incision placed behind the hairline and a bifrontal free bone flap situated strictly supraorbital without regard for the frontal sinuses. The subfrontal dura mater is separated from the orbital roofs, the olfactory nerves are divided at the cribriform plates, and the extradural dissection is carried posteriorly to the lesser wings of the sphenoid bone, the tuberculum sellae, and the base of the anterior clinoid processes. Attempts have been made to leave the olfactory bulbs attached to the cribriform plate, but this has usually not prevented the loss of the sense of smell seen commonly after these procedures. The clivus is reached after resecting the posterior part of the floor of the anterior cranial fossa, the upper part of the walls of the ethmoid and sphenoid sinuses, and the floor of the sella turcica. Proceeding downward from the sellar floor, the clivus is removed to open the anterior margin of the foramen magnum. Separation of the pharyngeal mucosa from the front of the spine permits exposure of the anterior arch of the atlas, and even the C2 and C3 vertebral bodies. The nasal and pharyngeal mucosa are not opened in the transcranial transbasal approach, but are commonly exposed in those procedures that include a supraorbital osteotomy in addition to a bifrontal flap. Dural defects are closed with a leak-proof dural substitute, more than twice the size of the defect, which is sutured to the dura mater at the most remote margins of the exposure. The orbital roofs and the remainder of the cranial base are reconstructed using autogenous bone grafts. If the clivus has been removed, the graft above the ethmosphenoidal space is fitted into the edge of a vertical graft extending from the anterior margin of the foramen magnum or the anterior arch of the atlas to the floor of the sella. The advantages of the transbasal approach are that a tighter closure of the dura mater is possible than can be achieved through the transoral approaches, the subcranial mucosal planes can be preserved, and it can be combined with another intradural approach without the high risk of infection associated with the transoral approaches. The transbasal approach may be combined with a transbasal-transsphenoidal route to gain access to the sella turcica. In the transbasal approach the clivus and sphenoid bone can be resected more extensively than by the transsphenoidal approach, but the subsellar area is hidden by the bulging dura in the transbasal approach. Both approaches may be combined to permit removal of all of the clivus below the level of the dorsum sellae. Anosmia is the only certain side effect. The most frequent complications are cerebrospinal fluid leaks, meningitis, and pseudomeningoceles.
Extended Frontal Approach
The extended frontal approach is similar to the transcranial-transbasal approach, except that it includes an orbitofrontoethmoidal osteotomy (Figs. 6.19 and 6.21) (28). It may also be used to approach tumors of the anterior side of the foramen magnum, especially if the tumor requires resection of part of the ethmoid and sphenoid bones as well as the clivus. The approach uses a souttar scalp incision, bifrontal bone flaps, and an orbitofrontoethmoidal osteotomy in which the supraorbital ridges, and part of the orbital roofs and possibly the upper nasion, the roof of the ethmoid sinuses, and the cribriform plate are removed in a single block. The resection of the lesion may involve an extradural or combined intradural-extradural approach. The clivus and foramen magnum are reached after resecting the posterior part of the floor of the anterior cranial fossa, the upper walls of the ethmoid and sphenoid sinuses, and the floor of the sella. If needed, the supraorbital osteotomy can even be tailored in size and site to include the lateral orbital rims.
Selection of Operative Approach
Anterior extradural lesions of the clivus or upper cervical vertebrae are best reached by one of the anterior approaches. The transoral approach is selected for most anterior extradural lesions involving the foramen magnum because it provides a midline exposure and is the most direct route to the pathology. For more extensive lesions, a transmaxillary approach may be considered. Before selecting an anterior approach that would require that the dura mater be opened through the oropharynx, one should consider choosing a posterior approach since the incidence of cerebrospinal fluid leak, meningitis, and pseudomeningocele is high if the dura mater is opened through the oropharynx. The transcervical approach has the advantage of reaching the foramen magnum through the deep fascial planes of the neck rather than through the oropharynx; however, the depth of the exposure, the length of the time required to complete the dissection, and the fact that the foramen magnum is not approached from the midline have prevented it from gaining common usage. The transcranial-transbasal and extended frontal approaches offer another anterior route for reaching the foramen magnum, however these approaches should not be considered for approaching a tumor strictly localized in the region of the foramen magnum, but might be used for an extensive lesion involving the ethmoid and sphenoid sinuses as well as the clivus and foramen magnum. The transsphenoidal approach provides an easy route for biopsying lesions in the region of the foramen magnum if they extend to the upper third of the clivus, but it does not provide adequate exposure for removing larger lesions of the region. The transsphenoidal approach may be combined with another approach in removing lesions involving the clivus and foramen magnum.
The posterior approaches are preferred for most intradural lesions. The vertical midline incision, and a bilateral suboccipital craniectomy and upper cervical laminectomy is used for lesions situated in the upper spinal canal and posterior or posterolateral in the area above the foramen magnum. The hockey-stick incision and a unilateral suboccipital craniectomy and upper cervical laminectomy is selected if the lesion extends anterolateral or anterior to the brainstem toward the jugular foramen or cerebellopontine angle. The far-lateral modification of the lateral suboccipital approach, described in the next chapter on the far lateral approach, gives a more direct approach to lesions ventral to the brainstem and along the anterior rim of the foramen magnum, while reducing the need for retraction of neural structures (32, 33). The foramen magnum can also be reached through the approaches directed through the temporal bone, the subject of the chapter on the temporal bone; however, for reaching the foramen magnum and clivus, these approaches may require repositioning of the carotid artery or facial nerve, and possibly resection of the auditory and vestibular labyrinth.
Content from Rhoton AL. The Posterior Cranial Fossa: Microsurgical Anatomy and Surgical Approaches. Neurosurgery 47(3), 2000, 10.1097/00006123-200105000-00065. 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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