Hypoplasia of the internal carotid artery--report of an unusual case.

We present a rare case of unilateral internal carotid artery (ICA) hypoplasia associated with arterial anomalies in the circle of Willis. The ipsilateral middle cerebral artery was supplied via anomalous arteries from the posterior cerebral artery and the ICA. The ipsilateral common carotid artery also originated from the anomalous brachiocephalic trunk. The etiology of the hypoplastic ICA is uncertain, but the associated multiple vascular anomalies support the congenital origin.