Walsh & Hoyt: Ocular Motor Syndromes Caused by Lesions in the Medulla

Many structures within the medulla are important in the control of eye movements: the vestibular nuclei, perihypoglossal nuclei, and inferior olive and its outflow pathway through the inferior cerebellar peduncle. The perihypoglossal nuclei consist of the nucleus prepositus hypoglossi (NPH), which lies in the floor of the fourth ventricle, the intercalatus nucleus, and ventrally the nucleus of Roller. These nuclei are interconnected with other ocular motor structures in the brain stem and cerebellum. The NPH and the adjacent medial vestibular nuclei (MVN) are critical for holding horizontal positions of gaze (the neural integrator). These structures also participate in vertical gaze-holding, although more rostral structures, especially the interstitial nucleus of Cajal (INC), also contribute. With lesions in the paramedian structures of the medulla, nystagmuscommonly upbeatis the most common finding. Upbeat nystagmus may also reflect involvement of a ventral tegmental pathway for the upward vestibulo-ocular reflex (VOR) producing a downward vestibular bias and a consequent upbeat nystagmus. Wernickes disease commonly affects the region of NPH and MVN, which may account for the horizontal gaze-evoked nystagmus and spontaneous vertical nystagmus and loss of vestibular responses that occur with this disease.