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Suboccipital segment of the vertebral artery: A cadaveric study
Correspondence Address: DOI: 10.4103/0028-3886.55610 PMID: 19770546
Objective: To study the course, relationships, branches and possible anomalies of the vertebral artery in the suboccipital region in adult Indian cadavers. Materials and Methods: Twenty-one suboccipital segment vertebral artery specimens from embalmed, Indian adult cadavers were dissected and studied. Dissection was performed using microsurgical instruments and was carried out from the skin up to the vertebral artery in layers. The course, relationships and the branches of the vertebral artery were studied and measurements were taken using Vernier calipers. The readings obtained were corroborated with the measurements derived from the digital images using a computer. Observations: All the vertebral arteries had a tortuous course and were covered with rich venous plexuses. None of the specimens had an anomalous course. The artery was divided into a vertical segment (Vv) between C2 and C1 vertebra and a horizontal segment (Vh) from the C1 transverse foramina to its dural entry. The mean diameter of the artery was 4.8mm. The shortest distance of Vv segment from the dural tube was 16.1mm, and the distance from the C2 ganglion was 7.2mm. The average length of the Vv segment was 15mm and the average length of the Vh segment was 35.6mm. The average of the shortest distance between the vertebral artery and the midline was 13.4mm. Conclusion: The vertebral artery has a tortuous course and is prone to accidental iatrogenic injury, which can result in devastating neurological sequelae depending on contralateral vertebral artery flow. A thorough anatomical knowledge of this segment is essential for the surgeon who intends to operate in this area. Keywords: Cadaveric study, suboccipital cavernous sinus, vertebral artery
Anatomy of the suboccipital segment of the vertebral artery (VA) has been a subject of multiple studies, and this study was performed exclusively on adult Indian cadavers. The vertebral arteries contribute to the formation of posterior circulation. Unlike its course elsewhere, the suboccipital segment has a tortuous course and is prone to iatrogenic injuries. Intraoperative injury can lead to catastrophic bleeding and compromise of cerebral blood flow, causing unpredictable neurological deficits depending on contralateral VA flow.
Twenty-one cadaveric specimens from eight full and five half-cut, embalmed adult Indian cadavers were used for the study. Using microsurgical instruments, dissection was carried out from the skin up to the VA, in layers. A midline incision was made from the EOP up to the C7 spinous process and further dissection carried out. The muscular layers were dissected, defined and reflected laterally exposing the suboccipital region. The spinous process of the axis and the posterior arch of the Atlas More Details were then visualized. The transverse process of C1 was identified along with the attachment of the superior and inferior oblique muscles. There is an area filled with loose areolar tissue caudal to the lower border of the inferior oblique muscle, between C1 and C2. Using blunt dissection in this area, the ventral ramus of C2 nerve root was identified and traced anteriorly, which coursed around the vertebral artery [Figure 1]. Rich venous plexus covered this segment of the vertebral artery. The suboccipital muscles were then removed and the entire posterior arch of the C1 was dissected subperiosteally up to the transverse processes on either side. The VA was then dissected and followed upto its dural entry point [Figure 2]. The bony landmarks were defined and digital photographs were taken. The course, relationships and the branches of the VA were studied and measurements were taken using Vernier calipers with accuracy of 0.1mm. The readings obtained were then corroborated with the measurements derived from the digital images on a computer. The measured parameters [Table 1] were analyzed statistically and compared with previous studies.
In all specimens, the VA had a tortuous course and was covered with rich venous plexuses [Figure 3]. A well-defined periosteal sheath encased the VA at the transverse foramina. None of the specimens had an anomalous course. For convenience of description and measurements, the artery was divided into two segments: Vv and Vh segment [Figure 4]. Vv segment runs between C2 and C1 and has a ' vertical course' , while the segment of the artery exiting from C1 transverse foramina up to its dural entry with a ' horizontal course' was labeled as Vh segment. Vv segment The VA exits from the transverse foramen of the axis and ascends up in a tortuous course in close relation to the atlanto-axial joint [Figure 5]. It initially takes a lateral bend, designated as Loop 1 and then straightens up (Loop 2) to enter the transverse foramen of atlas. It is densely covered by venous plexuses and is in close relation with the anterior ramus of the C2 nerve root, which runs around the artery from posterior to anterior. The mean length of this segment was 15.0mm (SD 1.53), with a range of 12.6mm to17.4mm and a median value of 15.2mm. The artery was at a mean distance of 7.2mm (SD 0.75) from the C2 cervical ganglion, with a range of 5.9mm to 8.4mm and a median value of 7.4mm. The artery was at a mean distance of 16.1mm (SD 0.98) from the lateral border of the dural tube, with a range of 14.7mm to 17.3mm and a median value of 16.2mm. The artery had a mean circumference of 15.0mm (SD 2.63) with a range of 9.2mm to 20.2mm and a median value of 16.2mm. The calculated mean diameter was 4.8mm. This segment gave a prominent radiculomuscular branch, which divided into radicular and a muscular branch [Figure 6]. A well-defined tough periosteal sheath surrounded the VA and its venous plexus in the transversary canal in all the specimens. This sheath protected the artery from damage during removal of the posterior wall of the transversary canal using a drill [Figure 7]. Vh segment After its exit from the C1 transverse foramen, the artery follows a horizontal course, immediately changing its direction posteriorly and medially. The artery then arches around the lateral mass of C1 to enter the VA groove on the C1 posterior arch (Loop 3). As the artery enters the VA groove, it changes its direction and curves anteriorly and medially to reach the dural entry point (Loop 4). There is a tough fibro-periosteal sheath that bridges the edges of the VA groove, converting it into a tunnel. In one cadaver this bridging fibrous tissue was completely ossified on the right side forming a bony tunnel [Figure 8] and was partially ossified on the left side. The C1 nerve root courses posterior and inferior to the atlantal part of the artery. The branches arising from this segment were inconsistent; there were muscular and dural branches, but the posterior inferior cerebellar artery did not arise extradurally in any of the specimens. A rich venous plexus cushions this segment of the artery. The mean length of this segment was 35.6mm (SD 2.44), with a range of 31.5mm to 40mm and a median value of 35.6mm. The artery was at a mean distance of 20.5mm (SD 3.0) from the mastoid tip, with a range of 16mm to 27.4mm and a median value of 19.6mm. The artery was at a mean distance of 13.4mm (SD 2.0) from its dural entry point to the midline, with a range of 9.5to 18.2mm and a median value of 13.0mm. The mean length of the artery that curves posterior to the lateral mass of the atlas, from its exit at the transverse foramen at C1 up to the VA tunnel on the posterior atlantal arch was 16.1mm (SD 1.23) with a range of 13.8 to 18.5mm and a median value of 16.1mm.
Suboccipital segment of the VA has a tortuous course with multiple loops unlike its other segments, possibly to accommodate the wide movements occurring at craniovertebral junction. [1],[2] However, this makes the artery prone to iatrogenic injury during surgical procedures like lateral mass fixation, posterior-circulation revascularization and various approaches to CV junction, such as midline posterior, far lateral, retromastoid-suboccipital and transcondylar approaches. [3],[4],[5],[6],[7],[8],[9],[10] Sekhar et al . reported their experience with the lateral transcondylar approach in 22 patients, out of which there was VA injury in four patients. Aggressive retraction and accidental injury were the reasons for VA injury. The arterial anatomy had been distorted either by the tumor or due to prior treatment in three cases. In one case the injury occurred at the C2 foramen during attempted transposition of the vessel. [2] The typical course of the VA carries the vessel in proximity to the ideal screw trajectory during C1-C2 transarticular screw fixation.[7] Wright and Lauryssen in their retrospective survey of the American Association of Neurological Surgeons/Congress of Neurological Surgeons section of disorders of the spine and peripheral nerves, on VA injury in C1-2 transarticular screw fixation, reported 31 cases of known VA injury and 23 cases of suspected VA injury during the surgical procedure. Additionally, the incidence of VA anomalies in the atlantoaxial region is 2.3%, therefore, a thorough understanding of the three-dimensional anatomy of this arterial segment is imperative for the surgeon in order to avoid accidental injury to the artery. Intraoperative injury to the VA can lead to catastrophic bleeding and compromise of cerebral blood flow, causing unpredictable neurological deficits depending on contralateral VA flow. [8] The VA originates from the supero-posterior aspect of the subclavian artery. It ascends and enters the transverse foramen of the C6 vertebra. This segment is designated as V1. It then ascends through the transverse foramina of C6 up to C2 in a linear fashion, this segment is designated as V2. After its exit from C2 transverse foramen, the artery adopts a serpentine and tortuous course in relation to the CVJ and becomes intracranial by piercing the dura and arachnoid at the level of the atlanto-occipital joint. This segment has been conventionally termed as V3 or the suboccipital segment. Once intracranial, the arteries on either side curve around the medulla and courses anteriorly to join each other to form the basilar trunk at the lower end of the pons. This segment is termed as V4. [2],[11] The VA is frequently asymmetric. Sometimes, the minor VA is too small to be detected by angiography and may end in the posterior inferior cerebellar artery. If this minor VA ends in the basilar trunk it is termed as 'hypoplastic' and if not connected to the basilar artery it is termed as 'atretic'. Sometimes the artery may be completely absent, in which case it is frequently replaced by a persistent congenital anastomosis, the proatlantal artery. El-Bary et al. , in their study on 14 cadaveric specimens, observed that the VA was at an average distance of 21mm from the mastoid tip, and the distance of the mastoid tip from the tip of the C1 transverse process was 19.6mm. The average diameter of the artery was 4mm on the left side and 3.9mm on the right side, and the shortest distance of the VA from the midline was 16.8mm. [12] Goel et al . in their cadaveric study on 20 specimens found the average length of the Vv segment to be 15.7mm, the distance of the Vv segment from the C2 ganglia and the dural tube was 7.5mm and 15.3mm, respectively. The average length of the Vh segment was 35.7mm. [2] Lang et al . [1] observed the paramedian distance of the VA from its dural entry point to be 11.8mm and Ebraheim et al . [13] observed the distance of the VA from the midline on the posterior aspect of the C1 arch to be 12mm. We found the average paramedian distance from the dural entry point to be 13mm, though the range was from 9.5mm to 18.2mm. Rhoton noted that the VA was partially covered by the posterior atlanto-occipital membrane and the bony groove was transformed into a bony canal that completely surrounds a short segment of the artery. Of the 50 arteries he 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. [14] Senoglu et al. in their study on 166 dry C1 specimens noted that eight had complete bilateral and 10 had unilateral osseous bridges, forming a canal on the superior surface of the posterior arch of C1. [15] In our study, we found the bridging fibrous tissue to be completely ossified on the right side forming a bony tunnel and incompletely on the left side in one cadaver. Though anatomical variations may potentially increase the rate of intraoperative complications, this osseous bridge is more likely to reduce the risk of injury as it constitutes an additional barrier protecting the artery. There is a well-defined periosteal sheath which surrounds the vertebral artery and its venous plexus in the transversary canal. This was found to be a tough and well-defined layer in our study. Transposition of the VA requires removal of the posterior wall of the transversary canal. Bruneau et al. advocated the use of kerrison punch for bone removal. The same may be performed using a drill with diamond burr as the artery is well protected by this periosteal sheath within the canal; however, caution has to be exercised to prevent accidental slippage and injury to the adjacent segment of the vessel. [16] The venous compartment in the suboccipital region has been termed as suboccipital cavernous sinus, as its structure is strikingly similar to the cavernous sinus. It is bordered proximally by the lateral ring, distally by the distal (dural) ring, inferiorly by the posterior arch of the atlas, ventrally by the dura and the capsule of the atlanto-occipital condylar joint and dorsally by the posterior atlanto-occipital membrane. It is surrounded by a fibrous membrane and contains the Vh, the muscular artery of the V3h, the posterior meningeal artery, the periarterial autonomic neural plexus and the C1 nerve. It communicates with the contralateral sinus via the internal vertebral venous plexus, the occipital sinus via the marginal sinus, the jugular bulb and vein via the anterior, lateral and posterior condylar veins. [17] Brisk venous bleed encountered during surgery in this region must caution the proximity to the VA.
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8]
[Table 1]
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