Update on the medial longitudinal fasciculus syndrome.

The medial longitudinal fasciculus is a key structure for conjugate horizontal eye movements by relaying signals from the abducens internuclear neurons to the medial rectus subdivision of the contralateral oculomotor nucleus. Thus, lesions involving the medial longitudinal fasciculus give rise to a typical neuro-ophthalmological sign, the internuclear ophthalmoplegia, which is characterized by impaired adduction of the ipsilesional eye and dissociated abducting nystagmus of the contralateral eye during attempted contralesional horizontal gaze. In addition, medial longitudinal fasciculus lesions may produce various other ocular motor abnormalities since the medial longitudinal fasciculus also conveys the signals for the control of the vestibulo-ocular reflex and smooth pursuit. Other possible ocular motor abnormalities include spontaneous vertical-torsional nystagmus, contraversive ocular tilt reaction, and impaired vestibulo-ocular reflex, especially for the contralesional posterior canal. Recognition of the ocular motor findings observed in the medial longitudinal fasciculus syndrome would aid in detection and localization of potentially grave lesions involving the brainstem.