An analysis of myotonia in paramyotonia congenital

SYNOPSIS In two subjects with paramyotonia congenita myotonic delay in muscle relaxation, recorded electromyographically and with a displacement transducer, was found to increase with repeated forceful contractions. Myotonia was elicited readily in warm temperatures, was initially aggravated by cooling, but was invariably lost as muscle fatigue developed. The EMG evidence of myotonia usually subsided before complete muscle relaxation had occurred, suggesting that a defect of the contractile mechanism was present over and above any defect at membrane level. The non-dystrophic forms of myotonia may be distinguished one from the other on the basis of heredity and the patterns of myotonia and of weakness. Paramyotonia congenita is said to be characterized by 'paradoxical ' myotonia which is accentuated by repetitive muscle contraction, and by extreme sensitivity to cooling, which aggravates the myotonia and the muscle weak-ness. But paradoxical myotonia has not been a uniform finding in all patients with otherwise classical paramyotonia, and it has even been reported to be present in some but absent in other members of the same family. While these discrepancies may arise in part from differing degrees of severity in different patients examined under different circumstances, this explanation is unsatisfactory if the paradoxical nature of the myotonia is to be used as a feature distinguishing paramyotonia congenita from the dominant and recessive forms of myotonia congenita. This paper analyses aspects of myotonia in two subjects with paramyotonia congenita with particular reference to the responses to repeated muscle contraction and to muscle cooling. Muscular weakness has been analysed in a companion paper (Burke et al., 1974b). METHODS Data were obtained from 13 experimental sessions in a 25 year old subject with classical paramyotonia congenita and were confirmed in a further experi