Recent advances in neuromuscular diseases have also widened the diagnostic spectrum of myotonic disorders. Treatment, prognosis, and genetic aspects are different in the various syndromes and mandate a correct diagnosis. The combination of neurologic examination, standard EMG, exercise test, cold exposure, potassium loading, eye examination, and pedigree analysis allows correct classification of nearly all patients with myotonic disorders. In this review emphasis is placed on clinical features and electrophysiologic evaluation.

Patients with different myotonic syndromes were evaluated for the presence of transient decrease of muscle fiber excitability. The evoked compound muscle action potential (CMAP) was recorded from the hypothenar eminence; single, supramaximal stimuli were applied to the ulnar nerve at the wrist before and after various periods of voluntary, isometric exercise of the hand muscles. A variable decline of the CMAP after exercise was present in all subjects with myotonia. The results were similar to those obtained with rapid repetitive nerve stimulation.

Seven patients with paramyotonia congenita (PC) from two families were studied. Voluntary exercise of the hand muscles was performed at different hand temperatures, both before and after treatment with tocainide. All patients developed stiffness, prolonged weakness, and small compound muscle action potentials (CMAPs) following exercise; the temperature at which this occurred was individually different. Two patients with PC and associated episodes of generalized weakness underwent potassium loading. A prolonged exercise test was performed both immediately before and 90 minutes after K-loading. Exercise-induced weakness and CMAP-decline occurred only with high serum K levels. Thiazide treatment in these two patients was ineffective. All seven patients responded well to tocainide. Treatment response and side effects were dose-dependent. Good clinical improvement has been maintained in all patients for more than 6 months, with relatively small doses of tocainide (400-1200 mg/day).

Forty patients with polymyositis or dermatomyositis underwent detailed electromyographic evaluation. The paraspinal muscles of all patients were examined, as were several extremity muscles. The distribution of fibrillation potentials (FPs) in different muscles is discussed. FPs were most frequent in paraspinal muscles. We conclude that, for any patient suspected of having a myopathy, electromyographic examination should include the paraspinal muscles.

Five diabetic patients developed upper back or abdominal pain associated with substantial weight loss in three. Electrophysiological evidence of associated thoracoabdominal somatic neuropathy was found in all cases. Signs of a generalized neuropathy were present in two patients, and four had asymmetrical proximal leg weakness. At least two have improved spontaneously. The syndrome is closely related to diabetic amyotrophy; it is probably more common than is recognized.