Spontaneous activity in electromyography: mechanisms and practical interest.

Spontaneous activities are a major semeiologic sign in electromyography. Th e present article deals with the different aspects recorded in practice in normal and pathological cases. There are two types of spontaneous activitie s, those related to motor unit hyperactivity (fasciculations and myokymia) and those related to the hyperactivity of one or more muscle fibers: fibril lations. positive sharp waves. myotonic discharges and complex repetitive d ischarges. In the first case the lesion is located in the axone and in the second in the membrane of the muscle fibers: All theories related to the ce lls' abnormalities share a common feature: spontaneous activites result fro m abnormal firing of the membrane action potential of muscular fibers, This functional abnormality may results from different types of lesions within the cells' membrane and determines the aspects of spontaneous activities. I mpaired function of muscular cells' membranes can be produced by denervatio n or lesion of the membrane structure itself. The latter can be multiple an d linked with the membrane proteins (such as laminine or dystrophin as in A IDS diseases) or with ion channel disturbances. Multiple membrane cell alte rations may produce the same kind of spontaneous activity: for instance, my otonic discharges have the same morphology in Thomsen and Steinert's diseas e despite their different mechanisms and fibrillations seen in denervations and myopathies. The practical consequences are discussed and a new classif ication of these spontaneous activities Is proposed. (C) 2000 Editions scie ntifiques et medicates Elsevier SAS.