Marcus Gunn phenomenon

Marcus Gunn phenomenon consists of a synkenesis between the masticatory musculature and the levator muscle of the contralateral eyelid. It is seen in patients with congenital ptosis. The phenomenon is normally unilateral, but bilateral cases are not unknown. Its exact etiology is not known. We report a case of bilateral Marcus Gunn phenomenon. The ptosis is secondary to a conflict between the oculomotor nerves and the posterior cerebral arteries. Our physiopathological hypothesis is that the motor neurons of V, possibly via “en passant” connections within the superior colliculus network, activate the nucleus of III – the homolateral portion of the central caudal nucleus in particular – causing reinforcement of its tonic action. This reinforcement leads to elevation of the contralateral eyelid, despite the paresis brought on by partial compression of the oculomotor nerve fibers.