Like woodworm spreading through a ship's timbers, most slowly progressive diseases begin long before clinical symptoms appear. Although modern techniques can often detect preclinical changes — and, potentially, can identify how and where a disease begins — there is one main problem. Patients rarely complain of being ill before they experience symptoms, so it can be tricky to find a suitable population to test. Reporting in this issue, however, Smith and colleagues have neatly circumvented these difficulties in their study of the genetically inherited movement disorder Huntington's disease (Smith, M. A., Brandt, J. & Shadmehr, R. Nature 403, 544–549; 2000).
Smith et al. have identified asymptomatic carriers of the Huntington's disease gene, and predict that these people will develop this movement disorder within the next five to ten years. The authors tested subjects for subtle deficits in the control of skilled arm movements, and, extraordinarily, found changes in their ability to point accurately at targets up to seven years before the predicted onset of clinical symptoms. Given that the pathology so early in the disease is probably limited to a relatively small population of neurons within the basal ganglia (an area of the brain beneath the cerebral cortex that is involved in controlling movement), this observation gives an unexpected insight into the function of these neuronal structures in normal movement.
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