Changes of forearm EMG and cerebral evoked potentials following sudden muscle stretch in patients with Huntington's disease

Abstract

Various investigators have reported that the late-reflex electromyographic (EMG) activity following muscle stretch is decreased in Huntington's disease. To explore the basis of this decreased activity, we recorded the late EMG responses together with associated cerebral responses following muscle stretch in patients with Huntington's disease. Five patients and seven controls voluntarily participated in two sets of experiments in which they grasped a handle attached to a torque motor and maintained the wrist isometrically against a constant flexor force of 2.3 newtons (N). The force was changed unpredictably (first set of experiments) or predictably (second set) to 10.4 N, causing a stretch of wrist extensors or flexors. Rectified surface EMG from the extensor and flexor carpi radialis muscles was integrated for the M2 and M3 components of the late responses. Cerebral responses were recorded from F3, F4, C3, C4, and Cz and averaged separately depending upon condition. The late muscle responses to unpredictable muscle stretch were decreased or absent in patients with Huntington's disease. The cerebral responses recorded at Cz differed markedly between patients and controls, beginning approximately 15 ms prior to the onset of the late M2 muscle response. Although the initial positivity was similar in amplitude, all other cerebral components were markedly reduced in the patient group. Both controls and patients showed a markedly attenuated cerebral response when the muscle stretch was predictable. The electrocerebral response to muscle stretch is thus altered prior to the onset of M2 in patients with Huntington's disease, suggesting that the long-latency reflex involves transcerebral pathways that are affected in this disease.