Changes in amplitude of the evoked muscle action potential (MAP) have been observed in four patients with myotonia congenita and two with dystrophia myotonica. A fade in the response occurred in every case with stimulus frequencies of 10 per second or less, provided that the muscle was in a rested state and that long enough stimulus trains were used. Intramuscular stimulation and recording techniques show that the myotonic muscle fibre is the site of this defect. The MAP fade is thought to represent the transient weakness from which such patients may suffer, particularly after rest. Since neither this weakness nor the fade was related to the severity of the myotonia, nor were they significantly influenced by cooling or hydantoins, they may well be due to a separate defect in the myotonic muscle from that which causes the hyperexcitability of the fibre membrane.
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