Primary position upbeat nystagmus localizing value.

Primary position upbeat nystagmus has been associated predominantly with lesions of the midbrain, midline cerebellum, and lower brainstem. However, the precise localization still remains unclear. We report one case of primary position upbeat nystagmus in which magnetic resonance imaging showed probable bilateral lesions of the prepositus hypoglossi nuclei. Morphological and physiological studies have shown that this nucleus plays important roles in vertical eye movements. We conclude that the dysfunction of bilateral prepositus hypoglossi nuclei causes the upbeat nystagmus in our patient.