Agenesis of the internal carotid artery associated with ipsilateral Horner syndrome in a child.

Agenesis of the internal carotid artery is a rare, usually asymptomatic congenital anomaly. Patients may remain asymptomatic because a network of collaterals develops in response to agenesis of the internal carotid artery, which is usually diagnosed as an incidental finding after magnetic resonance imaging. The collateral circulation is prone to developing aneurysms and subarachnoid hemorrhages. We report on a 16-year-old boy with a 1-year history of chronic daily headache. He manifested left-sided miosis and ptosis, with no evidence of anhidrosis, flushing, or pain. He was diagnosed with agenesis of the internal carotid artery with established collateral circulation. The association of Horner syndrome with agenesis of the internal carotid artery is not well reported. Agenesis of the internal carotid artery should be further evaluated to delineate the underlying mechanism, using computed tomography to examine the integrity of the bony carotid canal. Hypoplasia or agenesis of the carotid canal will confirm the congenital cause of the condition. If the canal is intact, dissection of the internal carotid artery should be considered.