Congenital hypoplasia of the internal carotid artery (ICA) is a rare congenital anomaly. It is usually found incidentally during radiological examination for other reasons. Most patients are usually asymptomatic but a few of them may present with headache, Horner’s syndrome, or transient ischemic attack (TIA). We presented herein an asymptomatic case of left carotid artery hypoplasia found on magnetic resonance imaging (MRI) during routine checkup. MRI with magnetic resonance angiography (MRA) showed severe hypoplasia of the left ICA from carotid bifurcation to petrous segment with collateral flow from the enlarged posterior communicating artery (PCOM) to supply the left middle cerebral artery (MCA). Recognition of this anomaly is very important when planning carotid endarterectomy or transsphenoidal hypophyseal surgery, even in case of acute brain attack from thromboembolic causes.

Keywords: Carotid artery, Internal, Congenital anomalies, Magnetic resonance imaging, Agenesis, Hypoplasia