Correlation between muscle strength and electromyography with blood potassium level after exercise test in patients with hypokalemic periodic paralysis

Abstract

Objective To assess the correlations between muscle strength and amplitude of compound muscle action potential(CMAP)with blood potassium level in hypokalemic periodic paralysis after long exercise test(ET).Methods ET of abductor digiti minimi(ADM)was performed on 78 patients with hypokalemic periodic paralysis.Ulnar and median CMAP amplitude,blood potassium level,muscle strength of ADM,palmar interossei muscle and abductor pollicis brevis were measured before and 120 minutes after exercise.The correlations of muscle strength,CMAP amplitude and blood potassium level were analyzed.Results Ulnar CMAP amplitude was(4.6 ±2.7)mV after ET and(9.6 ±3.2)mV before ET(t =16.047,P =0.000)in 78 patients with hypokalemic periodic paralysis,respectively.Median CMAP amplitude was(10.9 ± 4.2)mV after ET and(11.2 ± 3.9)mV before ET(t =0.673,P =0.822),respectively.After ET,muscle strength of ADM decreased in 76 patients,score on MRC was less than Ⅲ in ADM but V in palmar interossei muscle and abductor pollicis brevis in 41 patients,the blood potassium level was tested in 10 of them,which was(3.8 ±0.3)mmol/L before ET and(3.9 ±0.4)mmol/L after ET(t =0.395,P =0.702).Conclusion In patients with hypokalemic periodic paralysis,blood potassium level is not the key factor affecting muscle strength and CMAP amplitude after ET. Key words: Hypokalemic periodic paralysis; Exercise test; Action potentials; Electromyography; Potassium