Square-wave jerks and smooth pursuit impairment as subtle early signs of brain involvement in Langerhans' cell histiocytosis.

Central nervous system (CNS) involvement in Langerhans' cell histiocytosis (LCH) has been described as a progressive neurological disorder marked by motor and cognitive decline. Detailed analysis of ocular motor abnormalities is lacking. We report on a 60-year-old male with histologically confirmed LCH who developed oscillopsia and gait ataxia over a 1-year period. Eye movements recorded with infrared oculography revealed a high rate of square-wave jerks (SWJ) with frequencies of 41 min(-1) on average and amplitudes between 1 degrees and 7 degrees , as well as marked impairment of smooth tracking of sinusoidally moving targets. Furthermore, static posturography disclosed increased body sway, with an abnormally high sway path. The initial brain MRI was unremarkable. Due to the presumed cerebellar dysfunction we performed a second MRI 1 year later that disclosed deep cerebellar lesions compatible with LCH relapse within the CNS. The abnormal high SWJ rate and the impaired smooth pursuit performance correctly heralded later involvement of the cerebellum anticipating lesion appearance in the MRI.