The Long Exercise Test in The Diagnosis of Hypokalemic Periodic Paralysis

Objectives: Hypokalemic periodic paralysis (HPP) is characterized by intermittent episodes of muscle weakness or paralysis. The attacks may occur in different time intervals and may last for a few hours to several days. Although, the serum potassium level is found to be low during the attack periods potassium levels are usually normal between attacks. In this study, we aimed to investigate the role of exercise test (ET) responses by using electromyography (EMG) in the diagnosis of HPP.Methods: In this study, 5 patients with clinically observed hypokalemic periodic paralysis were evaluated. All of them were idiopathic. All of them underwent electrophysiological exercise testing in inter-attack period. The supra-maximal compound muscle action potential (CMAP) at the abductor digiti minimi (ADM) muscle was recorded by stimulating the ulnar nerve at the wrist for this purpose. The patient performed to right hand fifth finger maximal voluntary muscle contraction for 5 minutes (with relaxation for 5 seconds every 20 seconds). The ADM muscle was relaxed completely after the 5 minutes of exercise. CMAP was then recorded immediately thereafter every 2 minutes for 30 minutes or until the amplitude of the elicited CMAP no longer declined. The percentages of decrement were calculated.Results: Compound muscle action potential CMAP amplitude decrements were measured 48%, 55%, 54%, 41%, 19% respectively from the abductor digiti minimi muscle of all participants. As a result of exercise testing in 4 of 5 patients within attack free period findings were compatible with HPP.Conclusion: The exercise test is a useful diagnostic test for HPP, but in the absence of recent weakness negative results must be viewed with caution. It is a non-invasive functional test that can provide insights into abnormalities of muscle excitability. CMAP amplitude changes after exercise are helpful in diagnosis.