Far-Lateral Approach for Vertebral and Posterior Inferior Cerebellar Artery Aneurysms

Muscular Layers

The muscular layers encountered during the far-lateral exposure can be organized by their relative depth: (1) superficial; (2) middle ( Figs. 53.1 and 53.2 ); (3) deep ( Fig. 53.3 ). It is crucial to have an in-depth understanding of the muscular anatomy because these muscles function as important landmarks to locate the occipital artery (OA) for revascularization or the VA, to avoid its injury. However, dissecting the muscles layer by layer during the approach is unnecessary and is not recommended, as the healing and recovery is promoted by leaving the muscle flap in one piece and reapproximating its attachment below the superior nuchal line during closure. This technique minimizes dead space and prevents pseudo-meningocele formation. The superficial layer consists of the trapezius and sternocleidomastoid muscles. The middle layer consists of the splenius capitis (origin: nuchal line and lower cervical and T1-T3 spinous processes; insertion: mastoid process and inferior to the lateral half of the superior nuchal line), longissimus capitis (origin: transverse process of upper thoracic spine; insertion: mastoid process), and the semispinalis capitis (origin: articular process of C4-C6 and transverse process of C7-T7; insertion: bony area between the inferior and superior nuchal lines). The deep muscular layer is formed by the superior oblique (origin: C1 transverse process; insertion: bony area between inferior and superior nuchal lines lateral to semispinalis), inferior oblique (origin: C2 spinous process; insertion: C1 transverse process), and rectus capitis posterior major (origin: C2 spinous process; insertion: inferior nuchal line).

It is important to note the course of the OA ( Figs. 53.2, 53.3, and 53.4 ), as it may be used in revascularization after trapping or excision of the aneurysm or affected vessel segment. Therefore, understanding its location and course is needed to avoid inadvertent injury during exposure. The OA originates from the external carotid artery at the mastoid tip. It is divided into three segments: (1) The digastric segment extends from its point of origin until it reaches the occipital groove; in a cadaveric study by Guangfu Di et al., the OA passes above the longissimus capitis in 83.33% of cases and below it in 16.6% of cases. (2) The suboccipital segment begins from the occipital groove until the artery reaches the superior nuchal line; dissection of this segment frees the artery for use as a donor vessel in anastomotic constructs. (3) The terminal segment courses superior to the superior nuchal line to the vertex, adjacent to the greater occipital nerve, where it irrigates the superficial scalp layers.

Atlantal Segment of the Vertebral Artery (V3) and the Suboccipital Triangle

The suboccipital triangle is formed by three of the four deep muscles, with its superolateral border formed by the superior oblique, the inferolateral border by the inferior oblique, and the superomedial border by the rectus capitis posterior major. The triangle contains the V3 segment of the VA, the C1 nerve root (runs antero-inferior to V3), and a rich venous plexus surrounding the VA that drains into either the internal jugular vein or the vertebral venous plexus. ^, The V3 segment can be divided into five parts: (1) foraminal segment, where the V3 ascends within the C1 transverse foramen until it turns posteriorly; (2) sagittal segment, from the end of the first segment until it shifts to a horizontal position on the C1 posterior neural arch; (3) transverse segment, coursing on the J-groove or sulcus arteriosus of the C1 posterior arch medial to the atlanto-occipital joint; (4) medial condylar segment, continuing from the third segment until it ascends to enter the dura; and (5) dural segment, surrounded by the dural cuff.

The V3 has multiple branches, the first of which is the radiculo-muscular branch that exits at the C1 transverse foramen (enters the dura of the exiting C2 nerve root, with another branch supplying the muscles). The second is the muscular branch, and the third is the posterior meningeal branch.

There are three anomalies a neurosurgeon should consider prior to exposing the V3 segment.

A periosteal calcified bridge/ponticulus posticus, due to ossification of the posterior atlanto-occipital ligament, around the vessel can lead to iatrogenic injury to the V3 if unrecognized. Its prevalence has been reported between 6.2% and 26.2% and can occur as a complete bony covering (3% prevalence) or incomplete (12% prevalence). ^, The posterior spinal artery (PSA) normally originates from the intradural V4 segment of the VA; however, an extradural origin of this important vessel is common, reported to have a prevalence of up to 41%, with inadvertent injury leading to posterior spinal syndrome.

The third anomaly is an extradural PICA arising from the V3 segment, where it can be injured during dissection of the C1 posterior arch or transposition of the V3 segment. In addition, a V3 dissecting aneurysm may obstruct the blood flow within the extradural PICA origin. The prevalence of such an anomaly is 5% to 20%, with its origin at the posterior or lateral wall of the V3, coursing parallel to the V3. Of note, the prevalence of an extradural PICA origin is higher in the nondominant VA (22.5%), compared with the dominant VA (6.25%) or codominant configuration (3.2%). The extradural PICA retains its supply of the lateral medulla, inferior vermis, inferior cerebellar hemisphere, and tonsils, and therefore an injury may lead to the classic Wallenberg syndrome, as well as tonsillar edema and herniation. ^, It can be difficult to visualize whether a patient has an extradural PICA origin intraoperatively, given the rich venous plexus surrounding the V3. Therefore, careful study of the preoperative digital subtraction angiogram and the computed tomography angiogram (CTA), which offers a higher spatial resolution compared with magnetic resonance angiogram (MRA), is crucial.

Occipital Condyle, Hypoglossal Canal, and the Jugular Tubercle

The occipital condyles (OCs) are bony projections originating from the lateral and anterior aspect of the foramen magnum, where they articulate with the superior articular facet of C1. The hypoglossal nerve (HN) courses in the superior limit of the OC, with an anterolateral (45 degrees) direction. The hypoglossal canal (HC) is bordered by the JT superiorly, the OC inferiorly, the jugular foramen (JF) superolaterally, and the sigmoid sinus laterally ( Fig. 53.4 ). The contents of the HC include the meningeal branch of the ascending pharyngeal artery, the HN, and a venous plexus draining into the basilar venous system and marginal sinus. ^, Drilling the OC (trans-condylar approach) creates a larger surgical corridor to access the ventral foramen magnum; however, the maximal extent of drilling that still avoids creating cranio-cervical instability is unknown, with various authors quoting one-third up to three-fourths of the OC. Drilling the condyle achieves an exposure angle of 47 ± 2 degrees, as opposed to 88 ± 2 degrees without OC drilling—almost double the transverse view and surgical freedom compared with no OC drilling. HN injury due to the heat of the drill may occur in the presence of insufficient irrigation or lack of identification of the cortical bony covering of the canal; venous bleeding from the OC venous plexus is a warning sign that HN injury is imminent.

The JTs are two bony prominences originating from the condylar and basilar part of the occipital bone, and they are located medial to the inferior half of the JF and superior to the HC. ^, The lower cranial nerves (9, 10, and 11) course posterior to the JT into the JF, while the vertebrobasilar junction (VBJ) is located at the level of the JT in 40% of patients, above the JT in 42%, and below the JT in 18%. In a VA or PICA aneurysm, the JT may obstruct a complete view, and therefore drilling of the JT (supracondylar transtubercular approach) may be necessary to achieve safe clipping. ^,

Intradural Vertebral Artery (V4) and Posterior Inferior Cerebellar Artery

Upon piercing the dura, the V4 segment travels along the lateral aspect of the medulla into the premedullary cistern. The VBJ is located anterior to the medulla in 72%, at the ponto-medullary junction in 9%, and anterior to the pons in 19%. Major branches of the V4 are divided based on their branching origin. Laterally, the origin of PICA (first branch of the V4) is found, on average, 16 mm proximal to the VBJ, as well as perforator branches that supply the lateral medulla, inferior cerebellar peduncle, and the anterior surface of the inferior cerebellar hemisphere. ^, The anterior spinal artery (ASA) that supplies the medial medulla arises medially from the VA and is typically found distal to the PICA origin.

The PICA originates above the level of the olives in 3% of patients, at the olives in 92%, and below the olives in 5%. It is divided into five segments. (1) The anterior medullary segment/p1, from the PICA origin coursing through the HN rootlets, or looping above or below them, terminating at the preolivary sulcus. (2) The lateral medullary segment/p2 segment passes above or through the 9th, 10th, and 11th cranial nerves, terminating at the postolivary sulcus. (3) The tonsillo-medullary segment/p3, which begins from postolivary sulcus, coursing around the medulla, and loops inferiorly (caudal loop) and then rostrally, terminates at the medial inferior half of the tonsils. (4) The telovelotonsillar segment/p4 starts at the middle inferior half of the tonsils, then ascends between the tonsils and the inferior medullary velum and tela choroidea superiorly in a cranial loop before coursing posteriorly and inferiorly between the cerebellar hemispheres and the tonsils. (5) The cortical segment/p5 then divides into multiple branches supplying the cerebellar hemispheres.

The PICA branches are divided into three main components: (1) perforators; (2) choroidal branches (mainly arising from p4 to supply the choroid plexus and tela choroidea); and (3) cortical branches supplying the inferior cerebellar hemispheres, inferior vermis, and tonsils. The perforators have been categorized as direct, short circumflex (<90 degrees of the brainstem circumference) and long circumflex (>90 degrees of the brainstem circumference) by Lister et al. They noted that perforators arise from the p1 to 3 segments, where p3 has the most and p1 has the least perforators. In addition, the short circumflex perforators were the most common type, while the direct type is the least common. ^, Careful inspection and dissection of these perforators are essential for choosing an appropriate clip construct as well as freeing the PICA for performing a bypass or reimplantation as needed.