Muscle Cramps in Chronic Thyrotoxic Myopathy

NEUROMUSCULAR disorders in the course of hyperthyroidism are recognized¹⁻³and presently include exophthalmic ophthalmoplegia,⁴thyrotoxic myopathy,⁵⁻¹⁰myasthenia gravis,³,¹¹,¹²and periodic paralysis.¹³,¹⁴Chronic thyrotoxic myopathy is most common but the mechanism of muscular dysfunction is obscure. The clinical features of chronic thyrotoxic myopathy are due to progressive weakness, more pronounced in proximal muscles. Disability may be mild or severe. Muscular wasting is frequently seen, and muscle cramps also occur occasionally.¹⁵ McArdle¹⁶described a patient with a lifelong history of cramps after muscular exercise and postulated that this disorder was due to a defect in muscle glycogen break-down. Subsequently, Schmid et al,¹⁷Mommaerts et al,¹⁸and Pearson et al¹⁹found that muscle phosphorylase was lacking in similar patients. Since then, absence of muscle phosphorylase in additional cases has been confirmed.²⁰,²¹Recently, Tarui et al,²²and Layzer et al,²³described