Bilateral trochlear nerve paresis in hydrocephalus

Three patients with nonneoplastic hydrocephalus had bilateral paresis of the trochlear nerves. Associated signs, including paresis of upgaze, light-near dissociation of the pupils, and convergence-retraction nystagmus, suggested rostral involvement of the mesencephalon. Trochlear nerve paresis and accompanying signs improved after revision of ventricular shunts in two patients. Bilateral trochlear nerve paresis may be a localizing sign of involvement of the superior medullary velum (the anatomic site of trochlear nerve decussation) by a dilated sylvian aqueduct and/or downward pressure from an enlarged III ventricle.