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T. M. Blekher, M. R. Weaver, S. Hui, J. G. Jackson, J. C. Stout, X. Beristain, J. Wojcieszek, T. M. Foroud, R. D. Yee; Visual Scanning and Strategy in the Pre-Diagnostic and Early Stages of Huntington Disease. Invest. Ophthalmol. Vis. Sci. 2008;49(13):131.
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Quantitative measures of volitional saccades such as memory-guided and anti-saccades are considered to be sensitive biomarkers in the pre-diagnostic and early stages of Huntington disease (HD), which is a genetic neurodegenerative disorder resulting from an expanded number of CAG trinucleotide repeats (CAG+) in the huntingtin gene. The goal of this study was to analyze visual scanning while individuals completed a neuropsychological test (DS; Digit Symbol Subscale of the Wechsler Adult Intelligence Survey-Revised), dissect the general strategy employed by individuals, and delineate why CAG+ participants demonstrate deficits on this test early in disease progression.
The study sample included control individuals who were non-carriers of the HD gene expansion (NC, n=23), pre-diagnostic HD individuals (PDHD, n=21), and subjects recently diagnosed with HD (HD, n=19). All participants completed a uniform clinical evaluation that included administration of the Unified Huntington’s Disease Rating Scale (UHDRS) by a movement disorder neurologist and molecular testing to determine HD gene status. A high resolution, video-based eye tracking system recorded eye movements during the completion of the DS test. Quantitative measures of the subject’s visual scanning and strategy (consistency, speed, regularity, and learning) were computed.
All participants, regardless of their gene status, employed a simple strategy when completing the DS test. The strategy relied grossly on active visual scanning. The HD and PDHD individuals demonstrated three types of significant visual scanning and strategy abnormalities: inconsistency, reduced speed, and irregularity. The abnormalities increased with advancing motor signs of HD. The performance of HD, PDHD, and NC groups improved slightly and in a parallel fashion across the duration of the DS test.
The assessment of visual scanning allowed us to identify basic components of the strategy deficit observed in HD and PDHD patients while administering a widely used neuropsychological test of cognitive function and provided insight as to the brain regions involved.
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