Eye movement abnormalities in myotonic dystrophy.
We studied saccade and smooth pursuit eye movements in 31 patients suffering from myotonic dystrophy (MD). On the basis of mean value comparisons, saccades were slower and hypometric and smooth pursuit eye movements performed worse in MD patients than in controls. On an individual basis, saccade duration was prolonged in 67.7%, saccades were hypometric in 19.4%, saccade latency was delayed in 9.7%, and the smooth pursuit performance index was decreased in 9.7% of patients. Eye movement abnormalities did not correlate with those detectable by visual, brain-stem auditory and somatosensory evoked potentials. We attempted to classify eye movement abnormalities as myogenic or neurogenic on the basis of differences in combination of eye movement abnormalities and the occurrence of D5/D35 dissociation; the latter consists of a prolonged duration for large (35 degrees) but not for small (5 degrees) saccades. Since D5/D35 dissociation occurred in 26/33 multiple sclerosis patients with increased saccade duration, we considered it to be a neurogenic pattern attributable to a central nervous system (CNS) dysfunction. A prolonged duration without dissociation especially in combination with saccade hypometria, is interpreted as a myogenic pattern, although the lack of dissociation may also occur with CNS impairment in case of a marked increase in saccade duration. Accordingly we classified the oculomotor abnormalities detected as neurogenic in 11 MD patients and as myogenic in another 10, but in some subjects belonging to the second group concomitant CNS impairment is not to be excluded.