3. Value of the exercise compound muscle action potential study and provocative tests in possible periodic paralysis

Introduction: Pathogenic mutations can be identified in up to 70% of patients with periodic paralysis (PP). However, a substantial proportion of individuals with typical PP symptoms do not carry the known mutations. And, there are many patients with atypical symptoms referred for evaluation. Objectives: To evaluate whether the sensitivity of the exercise compound muscle action potential (CMAP) study is improved after provocative tests for the diagnosis of PP and to determine whether these tests are of value in assessing patients with possible periodic paralysis (P3). Methods: Six patients with genetically or clinically definite PP, 11 patients with P3, and 21 healthy volunteers were studied. The exercise CMAP study (McManis protocol) and serum potassium were performed before and after bicycle exercise and after oral glucose and potassium loading. Abnormal decrement is defined as >40% decline in CMAP amplitude or >50% in CMAP area. Healthy volunteers only participated in the bicycle exercise. Results: Of 6 definite PP participants, 4 (67%) had abnormal decrement on the pre-bicycle study compared to 4 (36%) in the P3 participants. Although percent decrement was similar in each group, time-to-nadir appeared to be longer in the P3 group (52.6 versus 40 min). Of 9 participants who had normal baseline studies, glucose loading provoked abnormal decrement in 1 definite PP, and potassium loading provoked hyperkalemia and abnormal decrement in 1 possible PP. All healthy volunteers had normal exercise CMAP studies. Conclusion: Abnormal exercise CMAP study occurs in both definite and possible PP. Provocative testing may improve the diagnostic accuracy in a mutation-negative possible PP.