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Background and Purpose Ocular manifestation is one of the frequent signs of an acute attackin multiple sclerosis (MS), although primary position upbeat nystagmus (PPUN) is rare. The purpose of this study is to determine the incidence of PPUN in MS and to determine the lesions that are responsible for this sign. Methods The medical records of 120 MS patients with acute brain lesions were reviewed overa consecutive period of 9 years; of these, 6 patients were found to have PPUN. Other ocular motor abnormalities were analyzed in combination with upbeat nystagmus, video-oculographic findings, and lesions detected on brain MRI. Results Lesions in the pontine tegmentum involving the medial longitudinal fasciculus (MLF)and ventral tegmental tract (VTT) were the most common, being observed in three of the six patients with PPUN. One patient exhibited caudal medullary lesions bilaterally affecting the paramedian portion of the posterior tegmentum, and two patients exhibited multiple lesions involving the pons with the cerebral peduncle or medulla. In five patients, other ocular motor dysfunctions, such as gaze-evoked nystagmus (n=3) and internuclear ophthalmoplegia (n=1), were found in combination with upbeat nystagmus. Conclusions PPUN is an infrequent, ocular manifestation noted during an acute attack of MS, and was observed in 5% of the present cases. Brainstem lesions in these cases primarily involved the pontine tegmentum and the caudal medulla. These findings support the theory that upbeat nystagmus is attributable to damage to the upward vestibulo-ocular reflex pathway related to the vestibular nucleus, VTT, and interconnecting pathways.
