Electromyographic (EMG) study in thyrotoxic periodic paralysis.

Thyrotoxic periodic paralysis (TPP) is produced by a different metabolic disorder than familial periodic paralysis. In the familial disorder, a myopathic cause is well known, but although electromyography can be used to demonstrate the site of the lesion in TPP, such studies are rare in the medical literature and the results are conflicting. In this report, EMG studies were carried out on eight Chinese patients with thyrotoxic periodic paralysis during the attack and the results compared with studies repeated during a remission. Most cases showed a myopathic pattern during an attack of paralysis which disappeared during remission. The myopathic changes noted were a decrease in duration of muscle action potentials, an increase in polyphasic potentials, a satisfactory interference pattern with reduced amplitude and a reduced amplitude of the evoked muscle action potential on nerve stimulation. Peripheral nerve function was normal in the cases studied. It is concluded that the weakness in TPP is myopathic and that the peripheral nerve function during paralysis is normal.