Long latency EMG responses in early diagnosis of Huntington's chorea.

In healthy subjects, 2 EMG responses of the thenar muscles can be distinguished, elicited by electrical stimulation of the median nerve during an isometric contraction: an early spinal response (M1) and a long latency response (LLR) (M2); earlier studies have shown that in patients with Huntington's chorea (HC) this late EMG response is missing. LLR were studied in the nine subjects at risk out of three families with definite HC. In 6 of them, LLR was clearly asymmetrical or absent on one or both sides, while normal LLRs were seen in the rest of the subjects studied; LLR abnormalities found in clinically free members of HC families may assist in early diagnosis of individuals who may later develop symptoms and may help in genetic counselling.