Electrodiagnostic Findings and Clinical Characteristics of Familial Hyperkalemic Periodic Paralysis With a SCN4A Met1592Val Mutation

Lee, Jae Yeong; Kim, Jee Young; Kim, Jung Goo; Kang, Ji Hoon; Choi, Jay Chol; Lee, Jung Seok; Kang, Sa Yoon

Abstract

BACKGROUND
Hyperkalemic periodic paralysis (HYPP) is characterized by episodic flaccid paralysis of skeletal muscles that is exacerbated by the consumption of potassium-containing foods, fasting, or rest following exercise. HYPP is largely diagnosed based on clinical features and electrodiagnostic findings.
METHODS
Seven patients from three families were assessed by interviews and clinical examinations. Standardized protocols comprising short and long exercise tests were applied to 15 unaffected control subjects and the 7 patients with familial HYPP.
RESULTS
Exercise of short duration induced an immediate increase in the amplitude of the compound motor action potential (CMAP) in the patients, and this was significantly larger and lasted longer than that observed in controls within 50 seconds (p<0.05). A long exercise test induced a large increase in the CMAP amplitude in patients immediately after exercise completion, which decreased to normal values with 1 minute. In contrast, controls showed a decreased CMAP amplitude immediately after exercise, which subsequently also returned to the normal value. Precipitants of attacks were vigorous exercise and hunger in all patients, and cold and potassium-rich foods in five patients. All patients experienced clinical myotonia at the eyelids or lips.
CONCLUSIONS
We conclude that exercise tests may be helpful in confirming abnormal excitability of muscle membranes in patients with HYPP. We have described the clinical and electromyographic characteristics in familial HYPP with the Met1592Val mutation in the SCN4A gene.