Internal carotid artery hypoplasia: 1. Internal carotid artery hypoplasia is a rare congenital variant that results from abnormal development during embryogenesis. On average, one out of three patients has left internal carotid artery hypoplasia; 2. The disease is usually asymptomatic, but symptoms associated with cerebrovascular insufficiency may appear later. Diagnostic points: internal carotid artery agenesis with carotid canal absence, a point that distinguishes the disease from chronic arterial occlusion with well established collateral circulation through the posterior and anterior communicating arteries and increased blood flow in the internal carotid and communicating arteries there, radiologists often suspect the presence of intracranial saccular aneurysms. The following 3 vascular anomalies are associated with internal carotid artery hypoplasia: 1. high degree of intracranial aneurysm (24%C34%); 2. anomalous arterial collateral circulation, including continuous embryonic vessels (e.g., continuous trigeminal artery); 3. external carotid artery supplying blood across the head to the soft meningeal vessels. Differential diagnosis: internal carotid artery dysplasia: small specific vessels with small carotid canal; complete occlusion of internal carotid artery thrombosis: normal size of carotid canal. Treatment: Symptomatic treatment if necessary.