A posterior inferior cerebellar artery of C2 transverse foramen level origin that entered the spinal canal via the C1/2 intervertebral space demonstrated by computed tomography angiography

To describe a case of a posterior inferior cerebellar artery (PICA) of C2 transverse foramen level vertebral artery (VA) origin that entered the spinal canal via the C1/2 intervertebral space. A 48-year-old man with posterior neck pain underwent computed tomography (CT) angiography and selective left vertebral angiography. Arterial dissection was found at the distal V2 segment of the left VA on subtracted CT angiography. The left PICA arising from the VA at the level of C2 transverse foramen was identified on CT angiography with bone imaging. This PICA of extracranial origin entered the spinal canal via the C1/2 intervertebral space, just like a PICA of C1/2 level origin. The origins of PICAs show several variations. PICAs originating at the extracranial C1/2 level VA are relatively rare, with a reported prevalence of approximately 1%. Our patient had a left PICA arising from the VA at the level of the C2 transverse foramen. No similar cases have been reported in the relevant English-language literature. We speculated that the proximal short segment of the PICA arising from the C1/2 level VA regressed incidentally and that the distal segment of the PICA was supplied by the muscular branch of the VA arising from the level of the C2 transverse foramen. We reported the first case of PICA arising from the C2 transverse foramen level VA. CT angiography with bone imaging is useful for identifying a PICA arising from the extracranial VA.


Introduction
The posterior inferior cerebellar artery (PICA) usually arises from the V4 segment of the vertebral artery (VA). It sometimes arises at the level of the foramen magnum, which is regarded as an extradural origin. Its computed tomography (CT) angiographic prevalence was reported to be 20.8% [8]. A PICA can also arise from the mid V3 segment of the VA and enter into the spinal canal, resulting in a PICA of extracranial C1/2 level origin. Its magnetic resonance (MR) angiographic prevalence was reported to be 1.1% [12].
We herein report a case involving a PICA arising from the extracranial C2 transverse foramen level VA that entered into the spinal canal at the C1/2 intervertebral space, which was diagnosed by CT angiography. To our knowledge, no similar cases have been reported in the relevant Englishlanguage literature.

Case report
A 48-year-old man with posterior neck pain underwent emergency cranial MR imaging with cervical and intracranial MR angiography. MR imaging showed no abnormality. MR angiography showed a left extracranial origin PICA, and slightly narrowing of the distal V2 segment of the left VA.
On the next day, CT angiography was performed using a 192-slice CT scanner (Somatom Force, Siemens Healthineers, Erlangen, Germany). A total of 60 mL of contrast media with 370 mg/mL iodine concentration was injected via the right antecubital vein at a flow rate of 4 mL/s, followed by a 20 mL saline chaser. The scanning parameters were as follows: 120 kV, 220 mA, section thickness: 0.6 mm, reconstruction interval: 0.4 mm, and scan revolution time: 0.5 s. The CT angiography data were obtained in a caudocranial direction.
The distal V2 segment of the left VA showed slightly irregular narrowing, suggesting arterial dissection on the subtracted CT angiography. A large artery arose from the proximal V3 segment of the left VA and entered into the spinal canal. This anomalous artery supplied the left PICA territory, indicating a left PICA of extracranial origin ( Fig. 1). After creating CT angiography with bone imaging, it was identified that the anomalous artery arose from the left VA at the level of the C2 transverse foramen and entered into the spinal canal via the C1/2 intervertebral space (Fig. 2).
Selective left vertebral angiography was performed 7 days after the onset. The distal V2 segment showed no definite abnormality, which was suggestive of remodeling and healing of dissection. A large muscular branch arose from the proximal segment of the left PICA of extracranial origin (Fig. 3). The patient was treated conservatively and showed a good clinical course.

Discussion
PICAs arising from the carotid system show several variations, such as PICA supplied by the jugular branch or hypoglossal branch of the ascending pharyngeal artery [9,10], variant of the persistent hypoglossal artery [13] and PICAtype persistent trigeminal artery variant. The PICA is sometimes absent, and duplication of the PICA is not rare. PICAs show three types of extradural origin [3]; (1) an extradural origin just outside the dura at the level of the foramen magnum; (2) an extradural origin that is located more laterally, above the transverse foramen of the atlas; and (3) an extracranial (cervical) origin at the point at which the VA ascends between the transverse foramen of C2 and C1. As mentioned in the Introduction, (1) and (2) are frequently seen. According to Pekcevik et al. [8], their CT angiographic prevalence was reported to be 20.8%. The remaining type (3), PICA of C1/2 level origin, is relatively rare, with a reported MR angiographic prevalence of 1.1% [12], and a reported CT angiographic prevalence of 1.0% [4].
According to Padget [7], at 5-6 weeks' gestation, the VAs develop from plexiform anastomosis between the seven embryonic cervical intersegmental arteries. The seventh intersegmental arteries enlarge and become the subclavian arteries. The VAs arise from the enlarging subclavian arteries and connect with the longitudinal neural arteries that develop to the basilar artery via the first intersegmental artery.
A VA entering via the C1/2 intervertebral space instead of the foramen magnum is called a C2 segmental type VA [6] or persistent first intersegmental artery (FIA) [12]. The latter name seems inadequate to describe this VA variant, because the FIA normally persists, forming a normal course with the VA at the level of the foramen magnum [11]. If there is a coexisting normal course of the VA and it fuses to each other, then a large arterial ring (fenestration) is formed at the C1/2 level. If a C2 segmental type VA fuses with a PICA instead of a VA with a normal course, a C1/2 level origin PICA is formed (Fig. 4a) [5,10].
The present case involved a PICA that entered into the spinal canal at the C1/2 level, just like a C1/2 level origin PICA. However, the PICA of our patient arose from the mid-C2 vertebral level (transverse foramen level), slightly proximal to the usual point (Fig. 4b). No similar case has been reported in the relevant English-language literature. Kim [4] referred to C1/2 level origin PICA as C2 origin PICA, which may have been an inadequate name. Figure 3 showed a large muscular branch arising from the proximal PICA. Therefore, we speculated that the proximal short segment of the C1/2 level origin PICA regressed incidentally and that the distal segment of the PICA was supplied by the muscular branch of the VA arising from the level of the C2 transverse foramen.
PICAs of extracranial origin have clinical significance during craniovertebral junction surgery. Aneurysms are rarely seen at the intradural segment of the PICA [1]. Bow hunter's syndrome is a transient and symptomatic vertebrobasilar insufficiency that occurs during head rotation, which results in dizziness and fainting. Compression of the dominant side of the C2 segmental type VA at the C1/2 level is commonly considered to be the cause. Enomoto et al. [2] reported a case of bow hunter's syndrome caused by compression of a C1/2 level origin PICA.