The Myopathy of Hyperkalemic Periodic Paralysis: An Electron Microscopic Study

HYPERKALEMIC periodic paralysis has been recognized as a distinct disorder only since 1951 when several generations of a family were described suffering from episodes of periodic paralysis without an accompanying fall in the serum potassium.¹ Gamstorp further elucidated the main clinical features of the disease and established that the serum potassium was elevated during the paralytic attack.^2,3 Later reports emphasized the common occurrence of myotonic features^4,5 and that permanent myopathic weakness involving the abdominal and proximal limb girdle muscles may occur.^6,7 Early muscle biopsies failed to reveal significant histopathological alterations.^8,9 However, more recent reports have demonstrated infrequent central vacuolation of minor proportions with some nonspecific myopathic features^7,10; these changes are similar to, but less marked, than those in hypokalemic periodic paralysis. Publications on the fine structure of skeletal muscle in this condition have shown few abnormalities¹¹ or vacuoles possibly arising from the