Silent periods, long-latency reflexes and cortical MEPs in Huntington's disease and at-risk relatives.

Short-latency (R1) and long-latency (R2) components of the stretch reflex, the electromyographic silent period (S-X interval) and motor evoked potentials (MEPs) elicited by magnetic stimulation of the cortex were measured in Huntington's disease (HD), at-risk first degree relatives and a normal control group. R1 and MEPs as well as central motor delay were normal in all 3 groups. R2 was absent in 7/9 patients with HD and 3/13 at-risk subjects. The S-X interval was reduced (less than 84 msec) in 7/9 patients and 5/13 at-risk subjects. Combined studies were abnormal in 88.9% of patients and 54% of at-risk subjects.