Muscle weakness after rest in myotonic disorders: an electrophysiological study

SYNOPSIS 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. Alterations in the amplitude of both mechanical and electrical muscle responses evoked by repetitive nerve stimulation have been noted in myotonic disorders (Lambert and Beckett, 1950; McComas et al., 1971; Ricker et al., 1973). Kirby and Kraft (1973) have suggested that these changes might be related to the transient weak-ness which may occur in patients with myotonia. However, when observed, these changes have usually been transitory, inconstant, or evident in only a proportion of subjects studied. A study of the effect of repetitive nerve stimulation upon the evoked muscle action potential (MAP) was therefore undertaken in patients with myo-tonia congenita and dystrophia myotonica in an attempt to define the extent and degree of these changes and the site of the defect responsible for them. METHODS Four patients with myotonia congenita (three being siblings) and two with dystrophia myotonica were studied (Table). Those with myotonia congenita had noticed muscle stiffness throughout their lives. Those with dystrophia myotonica had notice