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Quantitative oculomotor and nonmotor assessments in late-onset GM2 gangliosidosis
Christopher D Stephen1, David Balkwill2, Peter James2
1From the Ataxia Unit (C.D.S., J.D.S.), Center for Rare Neurological Diseases (C.D.S., P.J., E.H., F.E.), Cognitive Behavioral Neurology Unit (J.D.S.), Laboratory for Neuroanatomy and Cerebellar Neurobiology (C.D.S., J.D.S.), and Sleep Division, Department of Neurology (K.S.), Massachusetts General Hospital, and Jenks Vestibular Physiology Laboratory (D.B., R.L.), Massachusetts Eye and Ear Infirmary, Harvard Medical School, Boston. cstephen@mgh.harvard.edu.
Objective:
A cross-sectional study was performed to evaluate whether quantitative oculomotor measures correlate with disease severity in late-onset GM2 gangliosidosis (LOGG) and assess cognition and sleep as potential early nonmotor features.
Methods:
Ten patients with LOGG underwent quantitative oculomotor recordings, including measurements of the angular vestibulo-ocular reflex (VOR), with results compared to age- and sex-matched controls. Disease severity was assessed by ataxia rating scales. Cognitive/neuropsychiatric features were assessed by the cerebellar cognitive affective syndrome (CCAS) scale, Cerebellar Neuropsychiatric Rating Scale, and sleep quality evaluated using subjective sleep scales.
Results:
Oculomotor abnormalities were found in all participants, including 3/10 with clinically normal eye movements. Abnormalities involved impaired saccadic accuracy (5/10), abnormal vertical (8/10) and horizontal (4/10) pursuit, reduced optokinetic nystagmus (OKN) responses (7/10), low VOR gain (10/10), and impaired VOR cancellation (2/10). Compared to controls, the LOGG group showed significant differences in saccade, VOR, OKN, and visually enhanced VOR gains. Severity of saccadic dysmetria, OKN, and VOR fixation-suppression impairments correlated with ataxia scales (p < 0.05). Nine out of ten patients with LOGG had evidence of the CCAS (5/10 definite, 2/10 probable, 2/10 possible). Excessive daytime sleepiness was present in 4/10 and 8/10 had poor subjective sleep quality.
Conclusions:
Cerebellar oculomotor abnormalities were present in all patients with LOGG, including those with normal clinical oculomotor examinations. Saccade accuracy (dorsal cerebellar vermis localization), fixation suppression, and OKN gain (cerebellar flocculus/paraflocculus localization) correlated with disease severity, suggesting that quantitative oculomotor measurements could be used to track disease progression. We found evidence of the CCAS, suggesting that cerebellar dysfunction may explain the cognitive disorder in LOGG. Sleep impairments were prevalent and require further study.
