Skew Deviation and Spontaneous Nystagmus Due to Posterior Fossa Lesions

This is a 50-year-old woman who reported the abrupt onset of imbalance, right upper extremity incoordination and binocular vertical diplopia several months prior to her presentation to our clinic. On examination, she had a left hypertropia that was fairly comitant (measuring 5 prism diopters) associated with a right head tilt and ocular counterroll towards the right ear (excyloduction OD, incycloduction OS) - taken together, the left hypertropia was interpreted as a skew deviation, part of a complete ocular tilt reaction given the counterroll and head tilt. There was also spontaneous upbeat-torsional nystagmus (upper pole beating toward the right ear), and the upbeat component increased in upgaze. There was also decreased temperature sensation of the left face, left arm and leg. There was a mild right hemiparesis and ataxia of the right upper extremity that was out of proportion to her mild weakness. An ocular tilt reaction/skew deviation is caused by a lesion involving the utricle-ocular motor pathways, and with a left hypertropia, the pathology may occur 1) caudal to the decussation of these fibers: the right labyrinth, 8th nerve (although typically very subtle and short-lived with peripheral lesions) or right lateral medulla (caudal to the decussation of these fibers) - in her case, there was nothing to suggest a peripheral vestibular disorder, and no other signs to localize to the right lateral medulla, or 2) rostral to the decussation of these fibers: left medial longitudinal fasciculus (MLF), left interstitial nucleus of Cajal (INC) - in her case, there was no left INO and torsional nystagmus associated with left MLF or INC pathology should be ipsiversive (towards the left ear). Therefore, her skew and the pattern of her nystagmus didn't fit well with any one of these specific structures (with the appropriate laterality). MRI demonstrated multifocal enhancing brainstem lesions of unclear origin, so it was thought that her skew and nystagmus was probably due to separate lesions. She was chronically immunosuppressed for inflammatory myositis that had been diagnosed years prior, so work-up for infectious, inflammatory, and neoplastic (lymphoma) disorders was initiated.