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Unilateral myopathy: An unusual presentation of thyrotoxicosis
Abstract
Proximal muscle weakness, sometimes associated with atrophy and usually with electromyographic abnormalities, often occurs in patients with thyrotoxicosis. It affects men more commonly than women and may occasionally overshadow the other manifestations of thyrotoxicosis. It is usually bilateral. We report on a patient with thyrotoxicosis who presented with unilateral myopathy affecting the left shoulder girdle.
Morphological and electrophysiological studies were performed on intercostal muscle biopsies from 2 thyrotoxic patients. The diseased fibers had numberous areas of subsarcolemmal glycogen accumulations and abnormal membranous projections. Both Type I and II muscle fibers were atrophied. Diseased fibers were substantially depolarized and when artificially 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.