Electromyographic changes in thyrotoxicosis

OBJECTIVE: To document electromyographic (EMG) changes in thyrotoxic patients, and to categorize the type of myopathic process in thyrotoxicosis.

METHODS: This case control study was designed to show the EMG changes in thyrotoxic patients and to compare these findings with that of normal aged matched controls to show the significance of these changes in thyrotoxic patients. Student’s test was applied on the results and P value was extracted. Subjects in this study were chosen according to certain criteria depending mainly on their blood level of thyroid hormone (triiodothyronine and thyroxine) and thyrotropin. All of them are thyrotoxic patients, with age range between 15-45 years. There were 25 patients, 15 female and 10 male. Another 25 subjects of the same age and sex were chosen as normal controls. Patients with features of myopathy or neuropathy from diseases other than thyrotoxicosis were excluded carefully from studied patients and the normal controls.

RESULTS: The EMG findings in thyrotoxic patients were as follows: No spontaneous activities in the proximal muscles (deltoid and in rectus femoris muscles). The amplitude of the motor unit action potentials ranged between 200-800 microv with a mean of 488.8+/-159.3 microv in the deltoid muscle, while the amplitude of the action potential in rectus femoris muscle in thyrotoxic patients ranged between 350-900 microv. In abductor pollicis brevis muscle the action potential amplitude in thyrotoxic patients ranged between 500-2150 microv, there was significant difference between thyrotoxic patients and normal controls. The duration of the motor unit potential in thyrotoxic patients ranged between 7-11.5 msec with a mean of 8.51+/-1.24 msec in the deltoid muscle, slightly higher figures in rectus femoris muscle, indicating significant difference in the duration of action potential between patients and normal controls. The other parameters of the EMG study indicate a myopathic process involving proximal muscles in 76% of thyrotoxic patients and a neuropathic process involving distal muscles in 28% of thyrotoxic patients.

CONCLUSION: Thyrotoxicosis involves proximal muscles more than distal muscles. The myopathic process in thyrotoxicosis can be observed clearly in the EMG study of the proximal muscles. The EMG findings in thyrotoxic myopathy includes short duration polyphasic potentials with early recruitment full interference pattern. The distal muscles in thyrotoxic patients may show EMG findings of a rather neuropathic process.