Permanent muscle weakness in hypokalemic periodic paralysis.

OBJECTIVE

To map the phenotypic spectrum in 55 individuals with mutations in known to cause hypokalemic periodic paralysis (HypoPP) using medical history, muscle strength testing, and muscle MRI.

METHODS

Adults with a mutation in known to cause HypoPP were included. Medical history was obtained. Muscle strength and MRI assessments were performed.

RESULTS

Fifty-five persons were included. Three patients presented with permanent muscle weakness and never attacks of paralysis. Seventeen patients presented with a mixed phenotype of periodic paralysis and permanent weakness. Thirty-one patients presented with the classical phenotype of periodic attacks of paralysis and no permanent weakness. Four participants were asymptomatic. Different phenotypes were present in 9 of 18 families. All patients with permanent weakness had abnormal replacement of muscle by fat on MRI. In addition, 20 of 35 participants with no permanent weakness had abnormal fat replacement of muscle on MRI. The most severely affected muscles were the paraspinal muscles, psoas, iliacus, the posterior muscles of the thigh and gastrocnemius, and soleus of the calf. Age was associated with permanent weakness and correlated with severity of weakness and fat replacement of muscle on MRI.

CONCLUSIONS

Our results show that phenotype in individuals with HypoPP-causing mutations in varies from asymptomatic to periodic paralysis with or without permanent muscle weakness or permanent weakness as sole presenting picture. Variable phenotypes are found within families. Muscle MRI reveals fat replacement in patients with no permanent muscle weakness, suggesting a convergence of phenotype towards a fixed myopathy with aging.