Contralateral trochlear nerve paresis and ipsilateral Horner's syndrome.

Two patients had paresis of the trochlear nerve contralateral to the site of lesions in the brainstem. Both patients had ipsilateral blepharoptosis and miosis suggesting oculosympathetic paresis from involvement of the descending sympathetic tract, adjacent to the fourth cranial nerve nucleus and its fascicles, in the caudal mesencephalon. Cerebral antiography documented an arteriovenous malformation of the brainstem in Case 1. Magnetic resonance imaging disclosed a lesion of high signal intensity on T2-weighted images involving the dorsal mesencephalon in Case 2. Involvement of the superior cerebellar peduncle produced ipsilateral dysmetria and ataxia. Lesions involving the fourth cranial nerve nucleus or its fascicles, before decussation in the superior medullary velum, and adjacent sympathetic fibers may produce an ipsilateral Horner's syndrome and contralateral superior oblique muscle paresis.