Bilateral internal carotid artery agenesis with artery compression of the brain parenchyma.

Bilateral internal carotid artery (ICA) agenesis is an extremely rare congenital anomaly. The most common type of collateral circulation is developed through the circle of Willis via the basilar and posterior communicating arteries. Both anterior circulations are usually supplied by enlarged posterior communicating arteries (PCOMs). We present an unusual case of bilateral ICA agenesis, which is associated with dolichoectatic left PCOM and left posterior cerebral artery (PCA) compression of left basal ganglia and thalamus. These complex cerebral and vascular anomalies can be noninvasively revealed and evaluated by CT and MR.