Thyrotoxic myopathy: a clinical and quantitative analytic electromyographic study.

The clinical entity of chronic thyrotoxic myopathy was considered rare by Waldenström in 1945. Forty eight consecutive patients with thyrotoxicosis were studied clinically and by electromyography for muscle involvement. Quantitative electromyographic techniques were used to analyse both the electromyographic activity of the muscle at a standard tension and also the single motor potential obtained in isolation at minimal effort. The interference pattern was described in simple numerical terms such as the number of potential changes/s and the mean amplitude. All cases that were thyrotoxic (both clinically and on thyroid function tests) showed the changes of myopathy on electromyography, though only 68% of the cases showed clinical myopathy consisting of muscle weakness and/or atrophy.