Hyperthyroid myopathy. Intracellular electrophysiological measurements in biopsied human intercostal muscle.

Morphological and electrophysiological studies were performed on intercostal muscle biopsies from 2 thyrotoxic patients. The diseased fibers had numerous areas of subsarcolemmal glyogen accumulations and abnormal membranous projections. Both Type I and Type II muscle fibers were atrophied. Diseased fibers were substantially depolarized and when artifically hyperpolarized showed earlier inactivation of the sodium conductance as a function of membrane potential, and a critical depolarization potential more depolarized than in normal fibers. When stimulated at 20 pulses/sec, or faster, the diseased fibers could not generate normal action potentials due to membrane depolarization and the appearance of a marked after-hyperpolarization. Muscle weakness associated with hyperthyroidism is attributed to the reduced membrane excitability.