Bilateral Occipital Arteries of Internal Carotid Origin: Report of a Case and Review of the Literature

The present report describes a patient with bilateral occipital arteries of internal carotid origin, which is an extremely rare variation, and left vertebral artery ostial stenosis diagnosed by selective catheterization and digital subtraction angiography.


Discussion
Occipital artery arises from the posterior aspect of the external carotid artery, runs on the medial surface of the posterior belly of the digastric muscle and ends in the posterior part of the scalp [1]. However, occipital arteries of internal carotid origin have been reported as rare variants in the literature beginning from the anatomical studies in nineteenth century [2-13].
The first radiologically diagnosed case by angiography was reported by Siedel in 1965 [12]. The variability in origin of the vessels that normally arise from external carotid artery may be explained in embriyologic terms [9]. Lasjaunias and colleagues reported that occipital artery is formed from proatlantal artery [14]. In early gestation, the proatlantal artery anastomoses with primitive internal carotid, external carotid and vertebral arteries.
Anastomotic vessel between the primitive internal carotid artery usually regresses. Persistence of this anastomotic vessel and regression of the most proximal segment of the proatlantal artery result in the formation of an occipital artery of internal carotid origin. The incidence of anomalously originating occipital artery from the internal carotid artery is reported as 0.14% and 0.2% in two different studies performed by MRA and 3D CTA, respectively [2, 3].
To our knowledge, there are only two reported cases in the literature with bilateral occipital arteries of internal carotid origin [3,5]. High grade stenosis of the left vertebral artery resulting in collateral flow to the vertebral artery via the muscular branches of the occipital artery of internal carotid artery origin is the unique feature of this third reported case.
Anomaly of internal carotid artery originating occipital artery is commonly asymtomatic [2], however, rarely it may have clinical importance in patients with carotid bifurcation and/or proximal vertebral artery obstructive lesions. Newton et al. reported an asymptomatic case of proximal internal carotid artery occlusion [13]. Collaterals between the vertebral artery and midportion of the internal carotid artery through an anomalously originating occipital artery provided the blood flow in this case. Anomalous occipital artery can be a source of collateral flow from the proximal internal carotid artery to the distal vertebral artery in stenosis of the proximal vertebral artery as presented in this case. In the endovascular treatment of patients with carotid artery stenosis, occipital artery with anomalous origin from the internal carotid artery must be regarded if there is collateral flow with an accompanying vertebral artery stenosis. Stenting of the carotid bifurcation stenosis may diminish blood flow to the posterior circulation via occipital artery to vertebral artery anastomosis and may result in ischemia. Redistribution of the atherosclerotic plaque material during baloon dilatation and/or stenting of carotid bifurcation may obstruct external carotid artery orifice or an anomalous branch of internal carotid artery such as occipital artery. Anomalously branching occipital arteries may also cause misdiagnosis during radiologic examinations. In a reported case, whose occipital artery arising from the stump of internal carotid artery simulated continuation of the internal carotid artery and was misdiagnosed as a stenotic segment in Doppler ultrasonography [15]. Selective angiography actually revealed the occlusion of the internal carotid artery. Accompanying atherosclerotic plaques of the carotid arteries and vertebral artery severe stenosis may effect the clinical importance and endovascular treatment risk in such an extremely rare variation.

Competing Interests
The authors declare that they have no competing interests.