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|Year : 2015 | Volume
| Issue : 3 | Page : 315-317
Decoding the V3 segment of the vertebral artery...
R Girish Menon, G Lakshmi Prasad
Department of Neurosurgery, Kasturba Medical College, Manipal, Karnataka, India
|Date of Web Publication||5-Jun-2015|
R Girish Menon
Department of Neurosurgery, Kasturba Medical College, Manipal, Karnataka
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Menon R G, Prasad G L. Decoding the V3 segment of the vertebral artery... Neurol India 2015;63:315-7
The management philosophy and strategies for craniovertebral junction (CVJ) anomalies have undergone a paradigm shift in the last decade. The advent of high resolution 3-dimensional (3D) computed tomography (CT) reconstruction and dynamic imaging has widened the spectra of bony CVJ anomalies. The concepts on etiopathogenesis of these anomalies have been challenged and newer treatment modalities have evolved. The utility of anterior approach, performed more frequently earlier, has waned in recent years. The availability of better hardware and an improved understanding of the concepts and biomechanics of the atlanto-axial joint complex has made posterior instrumentation surgery the standard of care for CVJ instability. Screw rod constructs (SRC) and Trans articular fixations (TAF) have emerged as safe and effective treatment options for craniocervical instability ensuring over 95% fusion rates. 
The emphasis has now shifted and is focused on reducing the operative complications. The commonly encountered complications are instrumentation failure, vertebral artery injury (VAI), dural tears, and wound infection. Iatrogenic VAI is rare, but a potentially fatal complication with an incidence risk ranging from 1.3% for SRC to 4% for TAF  The incidence of VAI can be higher if associated with vertebral artery (VA) anomalies which are frequent associations with CVJ anomalies. To achieve optimum screw position and minimize VAI a thorough knowledge of atlantoaxial anatomy especially with respect to the course of VA is essential. This effort by the authors aims at sorting out the missing links in this much needed area.
The anatomy of the V3 and V4 segment of the VA is highly variable and cadaveric studies focusing on this part of VA anatomy are limited. , A 3D understanding of the anatomy is an essential prerequisite for any kind of surgery in the craniovertebral region. A routine noncontrast CT of the cervical spine may provide some useful information regarding the course of VA. A narrow pedicle and isthmus with reduced sagittal diameters or an asymmetry in the size of the transverse foramina are indirect markers that may give a clue. However, complete and detailed analysis can only be possible by angiography. CT angiography has replaced conventional angiography because of its improved resolution, facilities for 3D reconstruction, ability to draw the VA accurately in relation to the CVJ osseous anatomy, and stereophonic analysis from a voluntary direction.  Furthermore, it is noninvasive and can be acquired in a short time. [Figure 1] and [Figure 2] depict VA anomalies diagnosed by CT angiography in 2 cases of developmental atlanto-axial dislocation (AAD).
|Figure 1: A 30-year-old female with atlanto-axial dislocation. Three-dimensional computed tomographic angiography demonstrating the abnormal course of bilateral vertebral arteries; (a) Both the VAs are seen to travel laterally from the C2 foramen and enter the cranial cavity through an osseous foramen in the condyle. A fenestration of a short segment of the left VA can be seen (arrow in b). Furthermore, note can be made of the right sided VA dominance in this patient|
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|Figure 2: A 16-year-old male with atlanto-axial dislocation. Three-dimensional CT angiography demonstrating the abnormal course of both vertebral arteries (a). The left VA, after its exit from the C2 transverse foramen, follows a lateral course and then enters the cranium through a foramen in the occipital condyle. This can be clearly appreciated in the cranial reconstruction view; (b) On the right side, there is a persistent first intersegmental artery as the right VA courses posteromedially from C2 and enters the cranium by passing under the C1 arch|
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What makes this approach more challenging is the high incidence of VA anomalies in CVJ developmental defects which ranges from 10% to 60%, and the fact that these anomalies are most often present unilaterally. ,, The failure of resegmentation of the embryonic structures has been strongly associated with VA anomalies at the CVJ. Preoperative knowledge of the existence of such anomalies is mandatory as VA can be injured anywhere, either in the stage of soft tissue dissection or during hardware placement, and occasionally during anterior decompression procedures if there is excessive medial deviation of the vessel. The type of anomalies can be broadly divided into morphological anomalies, variations of anatomical course, and others. Anomalies in the first category include aplasia or hypoplasia of VA with change in dominance, while those in the second category include abnormal medial deviation, high riding VA, VA coursing and entering the cranial cavity between C1 and C2, and VA coursing laterally from C1 foramen into an osseous canal in the occipital condyle. Rhoton noted that the VA in some cases 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 in some cases.  Anomalies in the last category include fenestrations or duplications, persistent first intersegmental artery, and others. Majority of these have been considered to be remnants of the first intersegmental artery at the embryonic stage. Wang et al. have classified VA anomalies into four types, based on the course of the artery in the CVJ region detected on 3D CT angiography.  They observed that the risk of VA injury during C1 lateral mass placement is highest with Type-II and Type-III anomalies. Salunke et al. have described four types of variations-the first intersegmental artery, fenestration of VA, anomalous posterior inferior cerebellar artery (PICA) crossing the C1-2 joint, and a medial loop of VA.  Ulm et al. identified four sites along the V 3 segment that are anatomically most likely to be injured during surgical approaches to the CVJ-the vertical portion of V 3 , the horizontal segment of V 3 , point of entry into the foramen magnum, and the transition between the V 2 and V 3 segments after exiting the C-2 vertebral foramen. 
Surgical approach needs to be tailored to the preoperatively defined vascular anatomy. Risk of VAI is greater with TAF and preoperative imaging helps in avoiding that approach in nearly 20% of cases. A medial loop of the VA, as detected on preoperative imaging, warrants a transpedicular or C-2 lateral mass screws instead of pars interarticularis screws. Preoperative imaging also helps to determine the anomalous side as it is preferable to dissect and distract the normal side first. An important anatomical variant which is revealed by a preoperative angiogram is the one where the PICA and anterior spinal artery arise from the V3 segment, which is normally devoid of any branches. Injury to these vessels during dissection can lead to grave consequences. Surgeons also need to be aware that the exact relationship of the VA to the osseous CVJ structures is dynamic in nature with possibility of the changes in the location of the artery during the neck movement. 
Sardhara et al., in this paper have succeeded in analyzing a larger number of patients and comparing VA variations with a set of controls, thereby, adding significantly to the available fund of knowledge.  The one drawback of this paper would be the user friendliness of the proposed classification. Molinari et al. in their systematic review of published literature on VA anomaly and injury in patients with degenerative cervical spinal conditions propose a comprehensive and user-friendly system for the purpose of preoperative planning. This study, limited to subaxial cervical spine surgery, has helped in significantly reducing operative complications following cervical spine surgery by grading VA anatomy with respect to intraforaminal midline migration, extraforaminal abnormal entrance to the cervical spine, and arterial side dominance. A similar meta-analysis of the published reports on VA anomalies in the CVJ would be an appropriate sequelae to this article.
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[Figure 1], [Figure 2]