SUMMARY: We report a rare case of hypoplasia of the right internal carotid artery (ICA) with ipsilateral congenital Horner syndrome. The etiology and pathogenesis of hypoplasia of the ICA is not well understood. Multiple types of collateral flow have been reported to develop to maintain blood supply to the ipsilateral cerebral hemisphere. Although collateral flow may allow these patients to remain asymptomatic, we postulate that the enlarged posterior communicating artery (PcomA) in our patient caused mass effect on the cisternal segment of cranial nerve III causing intermittent mydriasis apart from Horner syndrome.
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