Permanent weakness and myopathy in hypokalemic periodic paralysis

Hypokalemic periodic paralysis (HypoPP) is a rare disease that consists of attacks of flaccid paralysis that often occur at night or early in the morning. Some patients with HypoPP may develop permanent muscle weakness and permanent myopathy. Four phenotypes of disease have been described: 1) no symptoms; 2) periodic paralysis and absence of permanent weakness; 3) periodic paralysis and permanent paralysis; 4) permanent weakness and absence of periodic paralysis. Vacuolar alterations are present in the muscles of all patients affected by HypoPP. In general, vacuoles are heterogeneous in size and content. Vacuoles are present in both fiber types and are not exclusive to any muscle fiber type, age, or phenotype. Generally, vacuoles contain glycogen but may also contain autophagosomes, fibrillar proteins, central nuclei, and myofiber invaginations. Magnetic resonance shows that fat accumulation of various extension is present in the muscle of patients with HypoPP. Water accumulation may also be present whereas atrophy is limited. Fat accumulation tends to increase with time together with a decrease of muscle strength. Permanent weakness and myopathy impairs significantly the quality of life. No treatment is currently available. The aim of this review is to define the characteristics and outcome of permanent weakness and myopathy in patients with HypoPP.