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Far-Lateral Approach

Last Updated: August 23, 2020

Introduction

The basic far-lateral exposure is carried up to but does not include removal of the posterior part of the occipital condyle. It includes 1) dissection of the muscles along the posterolateral aspect of the craniocervical junction to permit an adequate exposure of the C1 transverse process and the suboccipital triangle; 2) early identification of the vertebral artery either above the posterior arch of the atlas or in its ascending course between the transverse processes of the atlas and axis; and 3) a suboccipital craniectomy or craniotomy with removal of at least half of the posterior arch of the atlas (5, 19, 20). It provides access for the following three approaches: the transcondylar approach directed through the occipital condyle or the atlanto-occipital joint and adjoining parts of the condyle; the supracondylar approach directed through the area above the occipital condyle; and the paracondylar exposure directed through the area lateral to the occipital condyle (Fig. 7.1). The transcondylar extension accompanied by drilling the condyles allows a more lateral approach and provides access to the lower clivus and premedullary area. The supracondylar approach provides access to the region of and medial to the hypoglossal canal and jugular tubercle. The paracondylar approach, which includes drilling of the jugular process of the occipital bone in the area lateral to the occipital condyle, accesses the posterior part of the jugular foramen, and the posterior aspect of the facial nerve and mastoid on the lateral side of the jugular foramen. In the standard posterior and posterolateral approaches, an understanding of the individual suboccipital muscles is not essential. However, these muscles provide important landmarks for the far-lateral approach and its modifications. Important considerations include the relationship of the occipital condyle to the foramen magnum, hypoglossal canal, jugular tubercle, the jugular process of the occipital bone, the mastoid, and the facial canal (1–3, 6, 7, 10, 12, 15–17). 

Stages of Approach

The approach is divided into three anatomic stages (Fig. 7.2). The first stage, the muscular dissection, includes the skin incision, reflection of muscles, including those forming the suboccipital triangle, and examination of the relationship of the muscles to the occipital and vertebral arteries, the vertebral venous plexus, the transverse process of the atlas, and the upper cervical nerves. The second stage, the extradural dissection, examines landmarks for the suboccipital craniectomy, the extent of occipital condyle removal, and the exposure and identification of the hypoglossal canal, jugular process, jugular tubercle, and facial nerve. The final stage, the intradural exposure, reviews the relationships of the intradural segment of the vertebral artery and its branches, including the posteroinferior cerebellar artery (PICA), the lower cranial and upper cervical nerves, and the dentate ligament. 

Muscular Stage

For our study of the region, the exposure was done using a horseshoe scalp flap because it provided a better display of the muscular layers and their relationships to the neural and vascular structures (Fig. 7.2A). The incision began in the midline, approximately 5 cm below the external occipital protuberance, and was directed upward to just above the external occipital protuberance, turned laterally just above the superior nuchal line, reached the mastoid, and turned downward in front of the posterior border of the sternocleidomastoid muscle onto the lateral aspect of the neck to approximately 5 cm below the mastoid tip and below where the transverse process of the atlas can be palpated through the skin. The skin flap was reflected downward and medially to expose the most superficial layer of muscles formed by the sternocleidomastoid and splenius capitis muscles laterally and the trapezius and the semispinalis capitis muscles medially. In this description, the muscles are reflected separately but at an operation, the scalp and muscles superficial to the suboccipital triangle are reflected from the suboccipital area in a single layer, leaving a musculofascial cuff attached along the superior nuchal line for closure. 

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FIGURE 7.1. Osseous relationships. A, inferior view of the occipital condyles and foramen magnum. The occipital condyles are ovoid structures located along the lateral margin of the anterior half of the foramen magnum. Their articular surfaces are convex, face downward and laterally, and articulate with the superior facet of C1. A probe inserted through the hypoglossal canal passes forward approximately 45 degrees from the midsagittal plane in an anterolateral direction. The hypoglossal canal is located above the middle third of the occipital condyle and is directed from posterior to anterior and from medial to lateral. The intracranial end of the hypoglossal canal (small oval) is located approximately 5 mm above the junction of the posterior and middle third of the occipital condyle and approximately 8 mm from the posterior edge of the condyle. The extracranial end of the canal is located approximately 5 mm above the junction of the anterior and middle third of the condyle. The average length of the longest axis of the condyle is 21 mm. The large arrow shows the direction of the transcondylar approach and the cross-hatched area shows the portion of the occipital condyle that can be removed without exposing the hypoglossal nerve in the hypoglossal canal. The condylar fossa is frequently the site of a canal, the condylar canal, which transmits the posterior condylar emissary vein that connects the vertebral venous plexus with the sigmoid sinus just proximal to the jugular bulb. The condylar canal passes above and usually does not communicate with the hypoglossal canal. The jugular process of the occipital bone extends laterally from the posterior half of the occipital condyle to form the posterior margin of the jugular foramen. The portion of the jugular process located immediately behind the jugular foramen serves as the site of attachment for the rectus capitis lateralis muscle. The stylomastoid foramen is situated lateral to the jugular foramen. The styloid process is located anterior and slightly medial to the stylomastoid foramen. B, inferolateral view. A probe has been passed through the hypoglossal canal, which passes above occipital condyle. From its intracranial to its extracranial end it is directed forward, lateral, and slightly upward. C, superior view. The occipital condyle projects downward from the lateral margin of the anterior half of the foramen magnum. The intracranial entrance of the hypoglossal canal is located above the condyle. The jugular tubercles are located above and anterior to the hypoglossal canals. The jugular process of the occipital bone extends laterally from the condyles to form the posterior margin of the jugular foramen. The sigmoid sinus crosses the occipitomastoid suture and turns in a hooklike groove on the upper surface of the jugular process to reach the jugular foramen. Drilling the occipital condyle increases access to the anterolateral margin of the foramen magnum. Drilling in a supracondylar location below the hypoglossal canal accesses the lateral edge of the clivus. Drilling in the supracondylar location above the hypoglossal canal accesses the jugular tubercle, which projects upward and often blocks visualization of the junction of the middle and lower clivus and the region of the pontomedullary junction during the far-lateral approach. Drilling the jugular process in a paracondylar location accesses the posterior margin of the jugular bulb, which is situated in the sigmoid portion of the jugular foramen. D, medial aspect of the occipital condyle and supracondylar region. The inner surface of the mastoid portion of the temporal bone is grooved by the sulcus of the sigmoid sinus. The asterion, the site of the junction of the lambdoid, parietomastoid, and the occipitomastoid sutures, is an important landmark used to define the transition between the transverse and sigmoid sinuses. The sigmoid sulcus crosses the occipitomastoid suture just behind the jugular foramen. The intracranial end of the hypoglossal canal is located above the junction of the posterior and middle thirds of the occipital condyle. The external occipital protuberance is located an average of 2 cm below the apex of the internal occipital protuberance and 1 cm below the lower margin of the torcular herophili. The parietal notch, located at the junction of the squamosal and parietomastoid sutures, defines the upper limit of the petrous portion of the temporal bone and the floor of the posterior portion of the middle fossa. The midportion of the parietomastoid suture approximates the anterior edge of the junction of the transverse and sigmoid sinuses. Ac., acoustic; Artic., articular; Car., carotid; Cond., condyle; Fiss., fissure; For., foramen; Hypogl., hypoglossal; Int., internal; Jug., jugular; Mast., mastoid; Med., medial; Occip., occipital; Parietomast., parietomastoid; Petrocliv., petroclival; Proc., process; Protub., protuberance; Sig., sigmoid; Squam., squamosal; Stylomast., stylomastoid; Tymp., tympanic. 

Muscular Dissection

The sternocleidomastoid and trapezius are in the first layer encountered (Fig. 7.2, B—H). Dividing the sternocleidomastoid just below and with preservation of its upper attachment for closure and reflecting it laterally exposes the upper extension of the splenius capitis. Detaching the trapezius and splenius capitis muscles, while preserving a cuff of their upper attachments for closure, and reflecting them medially exposes the longissimus capitis muscle. Reflecting the longissimus capitis downward exposes the semispinalis capitis and the superior and inferior oblique muscles as well as the transverse process of the atlas, which has a prominent apex palpable through the skin between the mastoid process and mandibular angle. The semispinalis capitis is reflected medially to expose the suboccipital triangle, which is limited by three muscles; 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. 7.2G). 

The triangle deep to these muscles is covered by a layer of dense fibrofatty tissue. The floor in the depth of the triangle is formed by the posterior atlanto-occipital membrane and the posterior arch of the atlas (Fig. 7.2H). The structures in the triangle are the vertebral artery and the C1 nerve, both of which lie in a groove on the upper surface of the lateral part of the posterior arch of the atlas. The suboccipital triangle is opened by reflecting the rectus capitis posterior major inferiorly and medially, the superior oblique laterally, and the inferior oblique medially. Opening the triangle exposes the portion of the vertebral venous plexus that surrounds the vertebral artery as it passes behind the atlanto-occipital joint and across the upper edge of the posterior arch of the atlas (Fig. 7.2I). Reflecting the superior oblique muscle, as described earlier, exposes the rectus capitis lateralis, a short, flat muscle that is an important landmark in identifying the jugular foramen (Figs. 7.2, K and L, and 7.3). It arises from the upper surface of the transverse process of the atlas and attaches above to the rough, lower surface of the jugular process of the occipital bone behind the jugular foramen. The jugular process is a plate of occipital bone extending laterally from the posterior half of the occipital condyle. It is indented in front at the site of the jugular notch, which forms the posterior edge of the jugular foramen (Fig. 7.1). The rectus capitis lateralis, because it is attached to the jugular process at the posterior edge of the jugular foramen, provides a landmark for estimating the position of the jugular foramen and the facial nerve, which exits the stylomastoid foramen just lateral to the jugular foramen. 

Reflecting the muscles forming the suboccipital triangle, as described earlier, exposes the vertebral artery, which is surrounded by a rich venous plexus that must be obliterated and partially removed if the vertebral artery is to be exposed or transposed (Fig. 7.2, H and I). 

The vertebral artery, above the transverse foramen of the axis, veers laterally to reach the transverse foramen of the atlas, which is situated further lateral than the transverse foramen of the axis. The artery, after ascending through the transverse process of the atlas, is located on the medial side of the rectus capitis lateralis muscle. From here it turns medially behind the lateral mass of the atlas and the atlanto-occipital joint and is pressed into the groove on the upper surface of the posterior arch of the atlas, where it courses in the floor of the suboccipital triangle and is covered behind the triangle by the semispinalis capitis muscle. The first cervical nerve courses on the lower surface of the artery between the artery and the posterior arch of the atlas (Fig. 7.2, K–M). After passing medially above the lateral part of the posterior arch of the atlas, the artery enters the vertebral canal by passing below the lower, arched border of the posterior atlanto-occipital membrane, which transforms the sulcus in which the artery courses on the upper edge of the posterior arch of the atlas into an osseofibrous casing that may ossify, transforming it into a complete or incomplete bony canal surrounding the artery (Fig. 7.2H) (5). 

The third segment of the vertebral artery, the segment located between the C1 transverse process and the dural entrance, gives rise to muscular branches and the posterior meningeal arteries. The muscular branches arise as the artery exits the transverse foramen of C1 and courses around the lateral mass of the atlas to supply the deep muscles and anastomose with the occipital and ascending and deep cervical arteries (Fig. 7.2I). Some of the muscular branches may need to be divided to mobilize and transpose the vertebral artery. The posterior meningeal artery arises from the posterior surface of the vertebral artery as it passes behind the lateral mass or above the posterior arch of the atlas or just before penetrating the dura in the region of the foramen magnum, but it may also have an intradural origin from the vertebral artery, in which case it pierces the arachnoid over the cisterna magna to reach the dura (Fig. 7.2L) (20). 

The occipital artery is also exposed as the superficial and deep muscles in the region are reflected (Fig. 7.2, C—G). It originates from the posterior wall of the external carotid artery at the level of the angle of the mandible, ascends parallel and medial to the external carotid artery and lateral to the internal jugular vein to reach the area posteromedial to the styloid process. At that point, it changes its course to posterior and lateral, passing first between the rectus capitis lateralis and the posterior belly of the digastric and then between the superior oblique and the posterior belly of the digastric where it courses in the occipital groove medial to the mastoid notch, in which the posterior belly of the digastric muscle arises. After exiting the area between the superior oblique muscle and the posterior belly of the digastric, it courses medially, being related to the longissimus capitis and semispinalis capitis. If the occipital groove is present, the occipital artery will course deep to the longissimus capitis muscle, but if the groove is absent, the artery will course superficial to the longissimus capitis muscle (Fig. 7.2E). It courses medially behind the semispinalis capitis just below the superior nuchal line in the upper part of the posterior triangle to pass between the upper attachment of trapezius and the semispinalis capitis, where it pierces the attachment of the trapezius muscle to the superior nuchal line and ascends in the superficial fascia of the posterior scalp. 

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FIGURE 7.2. A–D. Far-lateral and transcondylar approach. A, a suboccipital scalp flap is commonly selected for the far-lateral exposure. The medial limb extends downward in the midline so that a wide upper cervical laminectomy can be completed if needed. The lateral limb extends below the C1 transverse process, which can be palpated between the mastoid tip and the angle of the jaw to access the vertebral artery as it ascends through the C1 transverse process. In this dissection, the muscles are reflected separately to show their anatomy; however, at an operation, the muscles superficial to the suboccipital triangle can be reflected from the suboccipital area in a single layer with the scalp flap, leaving a cuff of suboccipital muscle and fascia attached along the superior nuchal line to aid in closure. B, the scalp flap has been reflected to expose the sternocleidomastoid and trapezius, the edges of which form the margins of the posterior triangle of the neck. The splenius and semispinalis capitis are in the floor of the triangle. C, the sternocleidomastoid has been detached from the lateral part of the superior nuchal line and reflected laterally to expose the splenius capitis, which is attached just below the line. The asterion, located at the junction of the lambdoid, occipitomastoid, and parietomastoid sutures, most commonly overlies the lower half of the junction of the transverse and sigmoid sinuses. D, the splenius capitis has been reflected to expose the longissimus capitis and deep cervical fascia. The occipital artery may pass superficial or deep to the longissimus capitis. A., artery; Atl., atlanto; Br., branch; Cap., capitis; CN, cranial nerve; Dent., dentate; Digast., digastric; Dors., dorsal; Gang., ganglion; Hypogl., hypoglossal; Inf., inferior; Lat., lateralis; Lev., levator; Lig., ligament; Long., longissimus; M., muscle; Maj., major; Mas., mastoid; Memb., membrane; Men., meningeal; Min., minor; Musc., muscular; Obl., oblique; Occip., occipital; P.I.C.A., posteroinferior cerebellar artery; Plex., plexus; Post., posterior; Proc., process; Rec., rectus; Scap., scapula; Semispin., semispinalis; Splen., splenius; Suboccip., suboccipital; Sup., superior; Trans., transverse; Vent., ventral; Vert., vertebral. 

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FIGURE 7.2. E–J. Far-lateral and transcondylar approach. E, the fascia has been removed to expose the occipital artery passing behind the superior oblique and semispinalis. F, the longissimus capitis has been reflected to expose the attachment of the superior and inferior oblique muscles to the C1 transverse process. G, the suboccipital triangle, in the depths of which the vertebral artery courses behind the atlanto-occipital joint and across the posterior arch of C1, is situated in the depths of the area between the superior and inferior oblique and the rectus capitis posterior major. H, the superior oblique muscle has been reflected laterally and the rectus capitis posterior major muscle inferomedially. The floor of the suboccipital triangle is formed by the posterior atlanto-occipital membrane and the posterior arch of the atlas. The vertebral artery and the C1 nerve root, which are surrounded by the vertebral venous plexus, course along the upper surface of the posterior arch of the atlas. I, the muscles forming the margins of the suboccipital triangle have been reflected to expose the vertebral artery ascending through the C1 transverse process and behind the atlanto-occipital joint and the surrounding venous plexus. J, the venous plexus around the vertebral artery has been removed. The vertebral artery gives off muscular branches, passes medially behind the atlanto-occipital joint and above the posterior arch of C1, and turns upward and anterior to penetrate the dura. 

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FIGURE 7.2. K–O. Far-lateral and transcondylar approach. K, a suboccipital craniectomy has been completed and the right half of the posterior arch of C1 has been removed. The posterior root of the transverse foramen of the atlas has been removed while preserving the portion of the tip of the transverse process of the atlas to which the rectus capitis lateralis, levator scapulae, and the superior oblique attach. The atlanto-occipital joint and the posterior condylar emissary vein are exposed. The ventral rami of the C1 and C2 nerve roots pass behind the vertebral artery. The dorsal ramus of C2 gives rise to the greater occipital nerve, which passes through the semispinalis capitis to reach the posterior scalp. L, the area above the occipital condyle has been drilled to the depth of the cortical bone surrounding the hypoglossal canal. The change from cancellous to cortical bone indicates that the hypoglossal canal has been reached. M, the hypoglossal canal has been opened to expose the venous plexus, which surrounds the hypoglossal nerve in the canal and connects the basilar venous plexus with the marginal sinus, which encircles the foramen magnum. The dorsal ramus of the C1 nerve root, also termed the suboccipital nerve, passes backward between the posterior arch of the atlas and the vertebral artery, supplies the muscles bordering the suboccipital triangle, and sends fibers to the rectus capitis posterior minor and the semispinalis capitis muscles. N, an upper cervical laminectomy has been completed and the dura opened. The dural incision completely encircles the vertebral artery, leaving a narrow dural cuff on the artery so that the artery can be mobilized. The drilling in the supracondylar area exposes the hypoglossal nerve in the hypoglossal canal, and can be extended extradurally to the level of the jugular tubercle to increase access to the front of the brainstem. O, enlarged view of the site of dural penetration by the vertebral artery in another specimen. The rostral end of the dentate ligament ascends behind the vertebral artery with the accessory nerve and attaches to the dura along the lateral margin of the foramen magnum. 

Osseous Structures

The transverse process of the atlas, an important landmark in these approaches, projects further lateral than the transverse processes on the adjacent cervical vertebrae and has an apex that can be felt through the skin in the area between the mastoid process and angle of the mandible (Fig. 7.2A). Several muscles important in completing the exposure attach to the transverse process of the atlas (Fig. 7.2G). The rectus capitis lateralis arises from the anterior portion, and the superior oblique arises from the posterior portion of the upper surface of the transverse process. The inferior oblique muscle inserts on the lateral tip of the transverse process. The levator scapulae, splenius cervicis, and the scalenus medius attach to the inferior and lateral surface of the transverse process. The levator scapulae is also attached by tendinous slips to the posterior tubercles of the transverse processes of C2 to C4 (Fig. 7.2, F and G). 

Neural Structures

The neural structures encountered during the muscle dissection arise predominantly from the C1 and C2, and to a lesser extent from the C3 spinal nerves that are formed by the united dorsal and ventral roots and are described in the chapter on the foramen magnum (Fig. 7.2, J–M). 

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FIGURE 7.3. Relationships in the transcondylar, supracondylar, and paracondylar exposures. A, right side. The segment of the vertebral artery coursing behind the superior articular process of C1 has been removed. The posterior condylar emissary vein passes through the posterior condylar canal and joins the sigmoid sinus. The rectus capitis lateralis attaches below to the transverse process of C1 and above to the jugular process of the occipital bone that forms the posterior edge of the jugular foramen. The internal jugular vein descends on the anterior side of the rectus capitis lateralis and the C1 transverse process. B, the cancellous bone within the occipital condyle has been drilled away while preserving the cortical and articular surfaces to expose the hypoglossal nerve in the hypoglossal canal. The posterior condylar vein passes above the occipital condyle and hypoglossal canal to empty into the sigmoid sinus. The transition between the sigmoid sinus and jugular bulb is located lateral to the occipital condyle in front of the jugular process of the occipital bone. The posterior third of the occipital condyle can be removed without entering the hypoglossal canal. The extracranial end of the hypoglossal canal is located medial to the jugular foramen. C, the portion of the rectus capitis lateralis that attaches to the jugular process of the occipital bone has been removed to expose the internal jugular vein, and the jugular process of the occipital bone has been removed to expose the jugular bulb. The facial nerve is exposed laterally at the stylomastoid foramen. Several meningeal branches of the occipital artery ascend to pass through the jugular foramen. An emissary vein passes from the jugular bulb to the vertebral venous plexus. A., artery; Atl., atlanto; Cap., capitis; CN, cranial nerve; Cond., condyle; Dent., dentate; Emiss., emissary; Int., internal; Jug., jugular; Lat., lateralis; Lig., ligament; M., muscle; Men., meningeal; Occip., occipital; P.I.C.A., posteroinferior cerebellar artery; Post., posterior; Proc., process; Rec., rectus; Sig., sigmoid; V., vein; Vert., vertebral. 

Extradural Stage

The extradural stage begins with a suboccipital craniectomy or craniotomy, identification of the occipital condyle, and removal of at least half of the posterior arch of the atlas and possibly the posterior root of the transverse foramen, if mobilization of the vertebral artery is needed (Fig. 7.2K). Two osseous landmarks important in planning the suboccipital craniotomy are the asterion located along the lower half of the groove on the inner table of the cranium near the point where the transverse sinus empties into the sigmoid sinus, and the inion (external occipital protuberance) 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. In completing the removal of the posterior arch of the atlas, the tip of the transverse process is preserved along with the attachment of the superior oblique, which is reflected laterally while preserving the attachment of the rectus capitis lateralis. 

At this stage, the segment of the vertebral artery extending from the transverse foramen of C2 to its entrance to the dura is exposed. Removal of the posterior root of the transverse foramen will permit the artery to be displaced downward and medially away from the atlanto-occipital joint to expose the occipital condyle (Fig. 7.2, L—N). The occipital condyles project downward along the lateral edges of the anterior half of the foramen magnum (Figs. 7.1 and 7.3). The articular surfaces, which are ovoid with the long axis in the AP direction, are located on the lower-lateral margin of the condyles. They face downward and laterally to articulate with the superior facets of the atlas, which face upward and medially. 

The intracranial end of the hypoglossal canal is located approximately 5 mm above the junction of the posterior and middle third of the occipital condyle and appropriately 5 mm below the jugular tubercle (Fig. 7.1). The canal is directed forward and laterally at a 45-degree angle with the sagittal plane. The extracranial end of the hypoglossal canal is located immediately above the junction of the anterior and middle third of the occipital condyle and medial to the jugular foramen. The average length of the longest axis of the condyle is 21 mm (range, 18–24 mm) and the average distance between the posterior edge of the occipital condyle and the posterior border of the intracranial end of the hypoglossal canal is 8.4 mm (range, 6–10 mm) (20). The hypoglossal canal is surrounded by cortical bone. The contents of the hypoglossal canal are the hypoglossal nerve, a meningeal branch of the ascending pharyngeal artery, and the venous plexus of the hypoglossal canal, which communicates the basilar venous plexus with the marginal sinus that encircles the foramen magnum (Figs. 7.2M, 7.3B, and 7.4, C and D). Posterior to the occipital condyle, a depression, the condylar fossa, may be pierced by the condylar canal, which transmits the posterior condylar emissary vein, a communication between the vertebral venous plexus and the sigmoid sinus (Fig. 7.3). The canal is directed slightly upward as it proceeds anteriorly to join the sigmoid sinus at the hook-like turn immediately proximal to where the sinus empties into the jugular bulb. The condylar canal does not communicate with the hypoglossal canal. 

The jugular process of the occipital bone serves as a bridge between the condylar and squamosal parts of the occipital bone and forms the posterior margin of the jugular foramen (Fig. 7.1). It extends laterally from the posterior half of the condyle. The jugular process also serves as the site of attachment of the rectus capitis lateralis muscle behind the jugular foramen. The stylomastoid foramen, which transmits the facial nerve, is situated lateral to the jugular foramen at the anterior end of the mastoid notch (Figs. 7.3C and 7.4C). The styloid process is located anterior to the stylomastoid foramen and anterolateral to the jugular foramen. 

After removing the superficial layer of cortical bone covering the occipital condyle, soft cancellous bone will be encountered. Further drilling of the cancellous bone in and above the posterior third of the condyle exposes the second layer of hard, cortical bone that surrounds the hypoglossal canal (Figs. 7.2N and 7.3–7.6). Subsequent drilling of this cortical bone exposes the venous plexus of the hypoglossal canal. The lateral aspect of the intracranial end of the hypoglossal canal is reached with removal of approximately the posterior third of the occipital condyle (8.4 mm of 21 mm) (Fig. 7.1) (20). Further drilling of the occipital condyle can be done after reaching the lateral aspect of the intracranial end of the hypoglossal canal, as the canal is directed anteriorly and laterally, permitting the lateral part of the posterior two-thirds of the condyle to be removed without entering the hypoglossal canal. The distance between the upper surface of the hypoglossal nerve and the roof of the hypoglossal canal averages 4.4 mm. Extensive drilling around the canal may allow the nerve to be transposed from its normal course (Fig. 7.6). 

After exposing the hypoglossal canal above the occipital condyle, the bone of the jugular tubercle situated above the hypoglossal canal can be removed extradurally to gain additional exposure (1–3, 9, 10, 13). The jugular tubercle is a rounded prominence located at the junction of the basilar and condylar parts of the occipital bone (Figs. 7.1, 7.4, C and D, and 7.5, A–C). It is situated above the hypoglossal canal and medial to the lower half of the intracranial end of the jugular foramen. The average distance from the posterior edge of the jugular tubercle (the site of the groove in which the lower cranial nerves course) to the upper border of the hypoglossal canal is 4.5 mm (20). The glossopharyngeal, vagus, and accessory nerves cross the posterior portion of the jugular tubercle in passing from the brainstem to the jugular foramen, sometimes coursing in a shallow groove on the surface of the tubercle (Figs. 7.4 and 7.5). 

The prominence of the jugular tubercle blocks access to the basal cisterns and clivus anterior to the lower cranial nerves. As the jugular tubercle is removed extradurally the cranial nerves, which course along the back margin of the tubercle and are intradural, will not be visualized. As the drilling proceeds, bone will be removed from below the cisternal segment of the accessory and the vagus nerves that course above the tubercle just inside the dura. Caution is required in removing the jugular tubercle to avoid damaging the lower cranial nerves, either by direct trauma, by stretching the dura, or by the heat generated by the drilling (Fig. 7.5, A–C). The lateral margin of the jugular tubercle is situated just medial to and below the medial edge of the jugular bulb. If a more lateral exposure is needed, or the jugular foramen is to be opened from posteriorly, the jugular process of the occipital bone, which extends laterally from the occipital condyle, can be removed after detaching the rectus capitis lateralis muscle from its lower surface (Figs. 7.3 and 7.4). Removing the jugular process, which forms the posterior margin of the jugular foramen, will expose the transition between the sigmoid sinus, jugular bulb, and internal jugular vein. Care is required to avoid damaging the vertebral artery, because it passes upward through the transverse process of the atlas and turns medially in the area directly below the jugular process. For an even more lateral exposure, the posterior belly of the digastric muscle can be separated from the mastoid notch to expose the facial nerve just distal to the stylomastoid foramen (Figs. 7.3C, 7.4, B and C, and 7.6). A partial mastoidectomy can be performed to expose the mastoid segment of the facial nerve in the facial canal at this stage. 

FIGURE 7.4. Relationships in the transcondylar, supracondylar, and paracondylar exposures. A, a left suboccipital craniectomy has been completed and the dura opened. The nerves entering the jugular foramen have been exposed. Bone has been removed above the occipital condyle to expose the hypoglossal nerve entering the hypoglossal canal. A bridging vein passes from the lateral aspect of the medulla to the jugular bulb. B, the rectus capitis lateralis has been detached from the cranial base and the jugular process of the occipital bone, which forms the posterior margin of the jugular foramen, has been removed to expose the jugular bulb. The posterior belly of the digastric muscle has been reflected forward and a mastoidectomy completed to expose the mastoid segment of the facial nerve. C, the jugular bulb and adjoining segment of the internal jugular vein have been removed to expose the glossopharyngeal, vagus, and accessory nerves passing through the jugular foramen and descending behind the internal carotid artery. The cortical bone lining the hypoglossal canal has been removed to expose the hypoglossal nerve and the venous plexus in the canal. The hypoglossal nerve joins the nerves exiting the jugular foramen to descend in the carotid sheath. A mastoidectomy has been completed to expose the bony capsule of the semicircular canals and the mastoid segment of the facial nerve. D, enlarged view of the nerves passing through the hypoglossal canal and jugular foramen in the supracondylar and paracondylar areas. A., artery; Bridg., bridging; Car., carotid; CN, cranial nerve; Cond., condyle; For., foramen; Gl., gland; Hypogl., hypoglossal; Int., internal; Jug., jugular; Lat., lateral, lateralis; Occip., occipital; P.I.C.A., posteroinferior cerebellar artery; Post., posterior; Seg., segment; Sig., sigmoid; Stylomast., stylomastoid; Vert., vertebral. 

FIGURE 7.5. Posterior view of the left cerebellopontine angle. A, the glossopharyngeal, vagus, accessory, and hypoglossal nerves arise from the medulla. The hypoglossal canal has been exposed by drilling the cancellous bone above the occipital condyle. The posterior root of the transverse process of C1 has been removed. The accessory nerve crosses the jugular tubercle, the latter acting as a trochlea around which the accessory nerve courses to reach the jugular foramen. B, enlarged view. The area above the occipital condyle has been drilled to further expose the cortical bone around the hypoglossal canal. The atlanto-occipital joint has been preserved. C, the cortical bone lining the hypoglossal canal has been opened to expose the hypoglossal nerve and the hypoglossal venous plexus in the canal. D, anterior view. The anterior surface of the posterior fossa and the anterior wall of the hypoglossal canal have been removed to expose the hypoglossal nerve in its canal. The rootlets of the hypoglossal nerve originate ventral to the inferior olive and join before exiting the hypoglossal canal. The glossopharyngeal, vagus, and accessory nerves penetrate the dura on the medial side of the jugular bulb. The hypoglossal nerve exits the hypoglossal canal on the medial side of the jugular foramen. A., artery; A.I.C.A., anteroinferior cerebellar artery; Atl., atlanto; Bas., basilar; CN, cranial nerve; Cond., condyle; Dent., dentate; For., foramen; Hypogl., hypoglossal; Jug., jugular; Lig., ligament; Occip., occipital; P.I.C.A., posteroinferior cerebellar artery; Vert., vertebral. 

FIGURE 7.6. A, axial section extending through the occipital condyle and internal jugular vein below the right jugular foramen. The internal jugular vein descends just in front of the rectus capitis lateralis and behind the carotid artery. The occipital condyle is located on the medial side of the jugular foramen, and the styloid process, facial nerve, and parotid gland are on the lateral side. The nerves passing through the jugular foramen and hypoglossal canal collect together on the medial side of the internal jugular vein in an area just below the jugular foramen. B, the parotid gland has been removed. The facial nerve exits the stylomastoid foramen on the lateral side of the internal jugular vein. The styloid process is located along the anterolateral margin of the internal jugular vein. The central third of the occipital condyle has been removed to expose the hypoglossal nerve as it passes through the hypoglossal canal and joins the nerves exiting the jugular foramen on the medial side of the internal jugular vein. The rectus capitis lateralis and some of the jugular process of the occipital bone have been removed to expose the terminal part of the sigmoid sinus. C–D. Transposition of the hypoglossal nerve. C, the vertebral artery has been displaced medially. The occipital condyle, jugular tubercle, and the bone around and in front of the hypoglossal canal have been removed to expose the edge of the lower clivus. The dura ostium of the hypoglossal nerve has been opened so that the nerve can be mobilized. D, enlarged view of the mobilized hypoglossal nerve. A., artery; Atl., atlanto; Cap., capitis; Car., carotid; CN, cranial nerve; Cond., condyle; For., foramen; Gl., gland; Hypogl., hypoglossal; Int., internal; Jug., jugular; Lat., lateralis; M., muscle; Occip., occipital; P.I.C.A., posteroinferior cerebellar artery; Proc., process; Rec., rectus; Sig., sigmoid; Stylomast., stylomastoid; V., vein; Vert., vertebral. 

Intradural Stage

The dural incision begins behind the sigmoid sinus and extends behind the vertebral artery into the upper cervical area. The upper extent of the dural opening depends on how much of the cerebellopontine angle is to be exposed. Possible sources of bleeding during the dural opening are the marginal sinus that encircles the foramen magnum and the posterior meningeal artery, which usually originates from the vertebral artery extradurally, but may infrequently originate intradurally, in which case it crosses the lateral medullary cistern and pierces the arachnoid to reach the dura. Opening the dura exposes the intradural segment of the vertebral artery. As the artery pierces the dura, it is encased in a fibrous tunnel that binds the posterior spinal artery, dentate ligament, first cervical nerve, and the spinal accessory nerve to the vertebral artery (Figs. 7.2, N and O, 7.3) (14). Care should be taken to preserve the posterior spinal artery during the dural opening and mobilization of the vertebral artery because it may be incorporated into the dural cuff around the vertebral artery. 

At the craniocervical junction, the dentate ligament is located between the vertebral artery and ventral roots of C1 anteriorly and the branches of the posterior spinal artery and spinal accessory nerve posteriorly, and is often incorporated into the dural cuff around the vertebral artery (Figs. 7.2O, 7.3, and 7.5). 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 and behind the accessory nerve, although the dentate ligament is located anterior to the accessory nerve at lower levels. Section of 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. The rootlets forming the spinal portion of the accessory nerve, which arise from the cervical portion of the spinal cord midway between the dorsal and ventral rootlets as far caudally as C5, unite to form a trunk that ascends through the foramen magnum between the dentate ligament and the dorsal roots and enters the posterior fossa behind the vertebral artery (5). 

The intradural segment of the vertebral artery, after emerging from the fibrous dural tunnel, ascends in front of the rootlets of the hypoglossal nerve to reach the front of the medulla oblongata where it unites near the junction of pons and medulla with its mate to form the basilar artery (Fig. 7.2, N and O). Before reaching the lower border of pons, the vertebral artery gives off the PICA, which courses backward around the lateral surface of the medulla and between the rootlets of glossopharyngeal, vagus, and accessory nerves. The anterior, lateral, and tonsillomedullary PICA segments and the intradural segment of the glossopharyngeal, vagus, and accessory nerves, which may be exposed in this approach, are described in greater detail in this issue in the chapters on the cerebellar arteries and cerebellopontine angle (Figs. 7.2, N and O, and 7.3-7.5) (11). 

Discussion

The basic far-lateral approach without drilling of the occipital condyle may be all that is required to reach some lesions located along the anterolateral margin of the foramen magnum. However, it also provides a route through which the transcondylar, supracondylar, and paracondylar approaches and several modifications of these approaches can be completed. The transcondylar exposures can be categorized into several variants. An atlanto-occipital transarticular approach, in which the adjacent posterior parts of the occipital condyle and the superior articular facet of C1 are removed to facilitate completion of a circular dural incision, permitting the vertebral artery with the surrounding cuff of dura to be mobilized. A more extensive removal of the articular surfaces and condyles can be done to gain access to extradural lesions situated along the anterior and lateral margins of the foramen magnum. Another variant, the occipital transcondylar variant, is directed above the atlanto-occipital joint through the occipital condyle and below the hypoglossal canal to access the lower clivus and the area in front of the medulla. The supracondylar approach directed above the occipital condyle can also be varied, depending on the pathology to be exposed. The supracondylar exposure can be directed above the occipital condyle to the hypoglossal canal or both above and below the hypoglossal canal to the lateral side of the clivus. In the transtubercular variant of the supracondylar approach, the prominence of the jugular tubercle that blocks access to the area in front of the glossopharyngeal, vagus, and accessory nerves is removed extradurally to increase visualization of the area in front of the brainstem and to expose the origin of a PICA that arises from the distal part of the vertebral artery near the midline. The paracondylar approach also has several variants. In the transjugular variant, the exposure is directed lateral to the condyle through the jugular process of the occipital bone to the posterior surface of the jugular bulb. The approach can also be extended lateral to the jugular foramen into the posterior aspect of the mastoid to access the mastoid segment of the facial nerve and the stylomastoid foramen. 

Many suboccipital operations are completed without requiring that each individual muscle be identified. However, identification of selective muscles is an essential part of completing the transcondylar, supracondylar, and paracondylar approaches. Muscles that are especially significant in identifying the neural, vascular, and osseous structures involved in these exposures are the three muscles forming the suboccipital triangle and the levator scapulae, rectus capitis lateralis, and the posterior belly of the digastric. Identification of the individual muscles is also helpful in exposing and preserving the occipital artery if it is needed for a bypass procedure and in preserving the peripheral branches of the upper cervical nerves. The levator scapulae muscle provides an excellent landmark for localizing the vertebral artery as it ascends between the transverse foramina of the atlas and axis where the artery is located medial to the upper attachments of the muscle. The main risk in this area is related to a tortuous vertebral artery that loops posteriorly as it ascends between the transverse processes of the axis and atlas, making it vulnerable to injury when one expects the artery to be passing straight upward from the lower to the upper transverse foramen. The artery is also susceptible to damage as it passes behind the superior articular facet of the atlas. The artery normally hugs the posterior surface of the superior articular facet of the atlas, extending upward only to the level of the atlanto-occipital joint. However, if the artery elongates and becomes tortuous it can loop upward behind the occipital condyle, even resting against the occipital bone behind the condyle. It also can loop backward and bulge posteriorly between the lips of the suboccipital triangle, which it can damage if one expects it to be found in the depth of the suboccipital triangle. 

In obliterating and coagulating the venous plexus around the vertebral artery, there is the risk that some of the branches of the vertebral artery, which arise in an extradural location or even a hypoplastic vertebral artery, might be occluded or divided. The posterior spinal artery, and uncommonly the PICA, may arise extradurally in the region of the portion of the vertebral venous plexus, which may need to be partially excised or obliterated to gain access to the vertebral artery. 

The far-lateral approach, in which the exposure is carried up to, but does not include, the posterior margin of the occipital condyle, may be selected for lesions located along the lateral or anterolateral aspect of the foramen magnum. It is frequently necessary to remove a small portion of both the occipital condyle and the superior articular facet of the atlas if there is a need to complete a circumferential dural incision around the site where the artery penetrates the dura, so that the artery can be displaced for access to lesions located ventral to the artery and in front of the cervicomedullary junction. For lesions requiring a greater anterior and superior exposure, the posterior third of the occipital condyle can be removed without entering the hypoglossal canal. It is possible to drill the cancellous bone of the occipital condyle to expose the lateral clivus and hypoglossal canal while preserving some of the cortical bone of the condyle and the articular surface so that the joint is not disrupted (Figs. 7.3 and 7.5). The cortical surface around the hypoglossal canal can be preserved if there is no need to expose the nerve within the canal. 

Another key aspect of this approach is the condyle drilling, which requires an understanding of the relationship of the hypoglossal canal to the occipital condyle (Figs. 7.3–7.6). The maximum extent of the upper portion of occipital condyle that could be drilled without exposing the hypoglossal canal is the posterior third of its long axis. The occipital condyle sometimes can be covered by a hypertrophic superior articular facet of C1 that protrudes into the foramen magnum, making it easy to overlook the upper medial portion of the occipital condyle. In exposing lesions located along the anterior portion of the cervical cord, the inferior portion of the occipital condyle and the superior facet of C1 can be removed after retracting the vertebral artery inferior and medially. In drilling the upper posterior portion of the condyle, the posterior condylar vein may be a source of bleeding, which could be mistaken for bleeding from the venous plexus in the hypoglossal canal. After exposing the hypoglossal canal, the jugular tubercle, which is located just above and anterior to the canal, is identified. The drilling can be extended to a supracondylar location above the hypoglossal canal for removal of all or part of the jugular tubercle, so that the dura covering the tubercle can be pushed forward to gain access to the front of the medulla and the pontomedullary junction. Removal of the jugular tubercle may yield better visualization of the intradural segment of vertebral artery and the origin of the PICA, especially if the PICA originates from the upper part of the vertebral artery. The supracondylar approach, in which the jugular tubercle is removed and the hypoglossal canal is exposed or opened, provides a route for reaching extradural lesions located in the lower lateral part of the clivus in front of the hypoglossal canal. The extradural removal of the jugular tubercle should be performed with caution because of the risk of injuring the glossopharyngeal, vagus, and the accessory nerves that hug and often course in a shallow groove at the site where they cross the tubercle. 

The paracondylar exposure, which accesses the posterior margin of the jugular foramen and the jugular bulb, can be completed without drilling the occipital condyle (8, 20). An excellent landmark for identifying the jugular process is the rectus capitis lateralis, which extends upward from the transverse process of the atlas to attach to the jugular process just behind the jugular bulb. The muscle is located medial to the site where the occipital artery enters the retromastoid area by passing between the rectus capitis lateralis and posterior belly of the digastric. The jugular foramen and jugular bulb is accessed by drilling the jugular process at the posterior margin of the foramen. Drilling lateral to the jugular bulb from this posterior exposure risks damaging the facial nerve in the facial canal at and just above the stylomastoid foramen. The posterior belly of the digastric muscle, which attaches along the digastric groove just posterior to the stylomastoid foramen, provides a useful landmark for identifying the facial nerve. A limited or more extensive mastoidectomy may be completed, depending on the length of the mastoid segment of the facial nerve to be exposed and the extent to which the bone on the lateral aspect of the jugular bulb must be removed. A wider exposure of the jugular foramen is obtained by a retrolabyrinthine transtemporal approach, in which a more extensive mastoidectomy is completed and the mastoid and the tympanic segments of the facial nerve are exposed so that the facial nerve can be transposed forward to provide access to both the lateral and the posterior margin of the jugular foramen. 

Several controversies concern the positioning of the patient and the type of skin incision (20). The modified park bench position that we use offers the main advantage of avoiding air embolism (2, 3, 10, 14). The sitting position recommended by others is associated with a less distended venous plexus, but the rich net of veins around the cervical muscles, vertebral artery, and bone in the region offers the risk of air embolism (4, 13). A straight scalp incision has been recommended as being easier to open and close (10, 13). However, the thick cervical muscular mass and need for extensive retraction create a deep surgical field and the lateral position of the incision makes it difficult to complete a wide removal of the posterior C1 arch and C2 lamina, which is especially important if the lesion extends through the foramen magnum. We prefer an inverted horseshoe incision, with the medial limb extended so that a wide C1 to C2 laminectomy can be completed, and a lateral limb extended below the C1 transverse process so that the muscles attaching to the transverse processes are visualized (2, 5, 17, 18, 20). A musculofascial cuff is left attached along the superior nuchal line for closure. The flap on the upper part of the occipital squama can be reflected as a single layer, however it is helpful to identify the muscles forming the suboccipital triangle as an aid to exposing the vertebral artery. Anatomically, muscle dissection layer by layer offers the best preservation of the muscular landmarks. However, reflection of the superficial muscles individually carries a greater risk of flap dehiscence. Elevating the muscles attached to the upper part of the occipital squama with the scalp minimizes this problem and allows identification of important deep muscular landmarks, such as the suboccipital triangle and levator scapulae for localizing the vertebral artery and the rectus capitis lateralis for localizing the posterior portion of the jugular bulb. 

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

REFERENCES

References

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