Familial periodic paralysis with hypokalaemia. Study of a muscle biopsy in the myopathic stage of the disorder.

A 51 year-old male patient was affected by a dominantly inherited periodic paralysis. With large potassium supplements and ageing, the number and severity of attacks became considerably reduced. Increasing weakness and atrophy of the lower extremities were documented by clinical examination and by computer-assisted tomography of the muscles. A biopsy was taken in the vastus lateralis muscle without any attempt to induce a hypokalaemic paralytic attack. Light microscopy showed multiple intra- and extracellular vacuoles, rimmed vacuoles, myonecrosis, fatty degeneration and endomysial fibrosis. The endomysial nerve bundles were normal. Both fiber types were vacuolated. Quantitative studies revealed abnormal variability coefficients and increased atrophy factors for all types. Electron microscopy showed dilatations of the tubular system and of the sarcoplasmic reticulum communicating with large vacuoles limited by a single membrane. Other vacuoles were covered by a basement membrane and could contain collagen fibers or capillaries. Accumulation of myeloid bodies and of 10-13 nm filaments were also noted in the sarcoplasm. Cytoplasmic bodies were present. No tubular aggregates could be found. The nerve bundles were normal. These findings were in part similar to the ones reported by Gérard et al. (1978) in the son's biopsy during an induced paralytic attack. Significant findings in our case are the sequence of events leading to muscle fibre destruction, still detectable at an advanced stage of the disease. Myopathic changes represent a delayed but severe complication of the disorder.