Congenital absence of the left internal carotid artery.

The absence of the internal carotid artery (ICA) is a rare congenital anomaly, occurring in <0.01% of the population. Aplasia of the ICA may be harmless; however, the significance of ICA aplasia may be associated with conditions of clinical importance, such as in the setting of surgery, thromboembolic disease, and detection of cerebral aneurysms, and therefore should prompt further evaluation to rule out abnormalities. We present a case of left ICA aplasia diagnosed after work-up of neurological events. The incidence of intracranial aneurysm in association with aplasia has been reported as 25-43% compared to 2-4% in the general population. Mechanisms to explain the association between aplasia and intracranial aneurysms include embryological development or hemodynamic derangement. Recognition of this anomaly becomes important in thromboembolic disease as emboli in one cerebral hemisphere may be explained by atherosclerotic disease in the contralateral common carotid artery or vertebrobasilar system. Of significance, planning endarterectomy denotes consideration as both cerebral hemispheres may be dependent upon the atheromatous carotid. Recognizing this anomaly is important and may help prevent false diagnosis of carotid dissections or high-grade carotid stenosis.