Type II proatlantal artery (occipital subtype) with bilateral absence of the vertebral arteries

Persistence of the proatlantal artery is a rare anomalous communication between the carotid and vertebrobasilar system. Two types of proatlantal arteries have been described according to embryological considerations, origin, and anatomic course. We describe a very rare case of Type II proatlantal artery associated to bilateral absence of cervical vertebral arteries and identify two variants of this anastomotic channel based on its anatomic course. A 43-year-old man was admitted to the hospital suffering from vertigo and perioral paresthesias. Computed tomography angiography revealed bilateral absence of both cervical vertebral arteries and the presence of a large anastomotic vessel arising from the left occipital artery and supplying the distal left vertebral artery (VA) and basilar artery system. Digital subtraction angiography of the supra-aortic vessels revealed a large anastomotic vessel connecting the left external carotid artery (ECA) and the distal left VA at C1 level (Figs. 1 and 2). The large anastomotic artery arises from the posterior wall of the proximal ECA at C4 level. It courses upward, and after passing medial to the mastoid at the occipitocervical junction, becomes the distal part of the left VA that supplies the basilar system and gives rise to a musculocutaneous branch that supplies structures of the posterior portion of the vault. Primitive embryonic anastomotic vessels between the carotid and basilar arterial systems occasionally persist into adult life. There are four transient anastomoses between the posterior vascular plexus and the anterior carotid artery in early fetal life. By the 7–12 mm embryonic stage, these connections regress, with the proatlantal artery being the last to do so. Failure of involution of one of these connections causes four different types of anomalous arteries known as ‘‘persistent fetal anastomoses.’’ These arteries, cephalic to caudal, are the trigeminal, otic, hypoglossal, and proatlantal artery (PA) (Patel et al., 2003). The life span of the first three arteries is *1 week and they regress as the posterior communicating arteries develop (Gumus et al., 2004). The most commonly found persistent fetal anastomotic vessel is the trigeminal artery, followed by the hypoglossal, proatlantal, and otic artery. Two types of PA have been described. The Type I PA (proatlantal intersegmental artery) arises from the internal carotid artery (ICA) and ascends to the level of the occipito-atlantal space without passing through the transverse foramen (TF) of any vertebra. The Type II PA (C1 intersegmental artery, second segmental artery) arises from the ECA and joins the VA before entering the foramen magnum (Lasjaunias et al., 1978; Lui et al., 1987; Kolbinger et al., 1993; Basekim et al., 2004; Gumus et al., 2004). Fifty-seven percent of the described PA’s are Type I, 38% are Type II, and 5% arise from the common carotid artery (Kolbinger et al., 1993). The persistent fetal anastomoses are usually large and associated with hypoplasia or aplasia of the VA’s. When the PA is large, the VA’s are usually hypoplastic and the ipsilateral VA may be absent (Basekim et al., 2004). In our case, both VA’s were absent. Lui et al. (1987) described the persistence of both PA’s with absence of VA’s. Recently, Arráez-Aybar et al. (2011) described a case of persistence of a Type II PA associated to an arteriovenous malformation fed by the posterior cerebral artery. In their case, the PA arise from ECA (PA Type II), at the level of C4 vertebra, anteromedial to the ICA, laying on it during its course then turning in cranial-lateral-posterior direction exactly as in our case. However, the upper cervical course differed markedly, leading us to identify two different variants of the PA Type II: occipital and spinal. In its normal suboccipital course, the VA exits from the TF of the axis and ascends up in a tortuous course in close relation to the atlanto-axial joint. It initially takes a lateral bend and then straightens up to enter the TF of atlas. After its exit from the C1 TF, 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. As the artery enters the VA groove, it changes its direction and curves anteriorly and medially to reach the dural entry point (Muralimohan et al., 2009). Arráez-Aybar