Anomalous Origin of the Occipital Artery from the Internal Carotid Artery

Abstract

Anomalous branches of the cervical portion of the internal carotid artery are rare. In a review of the anatomic literature we have found reports of a few cases in which the occipital artery originated from the cervical portion of the internal carotid artery (1, 2, 4–7). Adachi (1) in 1928 illustrated such an instance, discovered at postmortem examination in a 30-year-old man. In this patient, the occipital artery arose from the internal carotid artery about 2 cm distal to its origin. Hyrtl (4) mentioned a case in 1841 in which the posterior auricular and occipital arteries originated from a common trunk from the internal carotid artery. Seidel (7) in 1965 demonstrated by angiography in a patient all branches of the external carotid artery arising separately from the internal carotid artery. We are reporting an anomalous origin of the occipital artery from the internal carotid artery in three patients. In the first two, investigated for a possible subdural hematoma, an occipital artery was found to arise from the internal carotid artery 2.0 cm distal to its origin (Fig. 1). A similar anomaly was seen in Case III,3 studied for possible vascular occlusive disease (Fig. 2). A complete occlusion of the proximal portion of the internal carotid artery was demonstrated by subclavian arteriography. The distal internal carotid artery remained patent because of collateral flow from the vertebral artery by way of its muscular branches to the occipital artery and then directly into the internal carotid artery. This roentgenographic observation was confirmed at surgery, and a thromboendarterectomy on the short occluded segment of the internal carotid artery reestablished the normal flow. Discussion The occipital artery normally arises from the external carotid artery at the level of the external maxillary artery and extends superiorly to supply the muscles in the suboccipital region. An occipital artery that arises from the internal carotid artery has not, to our knowledge, been previously demonstrated by angiography. Anastomoses of the occipital artery with muscular branches of the vertebral artery, as well as direct origin of this branch from the vertebral artery are not uncommon (3). A direct collateral communication, however, between the vertebral artery and the mid-portion of the internal carotid artery through an occipital branch has not been reported previously. When the proximal internal carotid artery becomes occluded, thrombosis usually extends to the carotid siphon and surgical repair is unsatisfactory. An anomalous occipital artery in Case III, however, maintained patency of the distal internal carotid artery, permitting a surgical restoration of the vessel. The collateral flow into the distal internal carotid artery might have been overlooked had selective arteriography of the common carotid artery been performed without opacification of the vertebral artery.