Torsional nystagmus due to medullary pilocytic astrocytoma

This is a 30-year-old woman who experienced headaches which led to an MRI and the diagnosis of a right medullary pilocytic astrocytoma, confirmed pathologically. Examination was performed a year after the initial diagnosis, and several months prior to this exam oscillopsia was experienced for the first time. MRI demonstrated that the cystic portion had increased in size and it was thought that her newly appreciated nystagmus and resultant oscillopsia were related to this interval change. On exam, she had spontaneous torsional nystagmus, which was unidirectional in right and left gaze, with the top poles beating towards the right ear in each position. Saccades, smooth pursuit and the vestibulo-ocular reflex were unremarkable. Pure torsional nystagmus is almost always central in origin. An acute destructive lesion (e.g., vestibular neuritis) causing unilateral vestibular loss will cause contralesional horizontal-torsional nystagmus related to unopposed semicircular canal (SCC) afferents contralaterally. For pure torsional nystagmus to result from a peripheral lesion, certain SCCs must be spared (e.g., both horizontal SCCs, which will cancel out so that a horizontal jerk component is not seen; both vertical SCCs on one side) while others must be strategically damaged (e.g., both vertical SCCs on the opposite side). In contrast, damaging the central fibers originating from both vertical SCCs on one side due to a medullary lesion is a much more common cause of torsional nystagmus. Pure torsional nystagmus may also result from unilateral interstitial nucleus of Cajal (ipsiversive nystagmus) or rostral interstitial medial longitudinal fasciculus (contraversive) injury, while vertical-torsional nystagmus may result from medullary (direction can be less predictable depending on how caudal or rostral the injury occurs), medial longitudinal fasciculus (ipsiversive), or superior cerebellar peduncle localizations, among others