Definition - one of a group of diseases that cause problems with the tone and contraction of skeletal muscles; two forms, hyperkalemic and hypokalemic

Cause - Cause - in hyperkalemic periodic paralysis, high levels of potassium in the blood interact with genetically caused abnormalities in sodium channels (pores that allow the passage of sodium molecules) in muscle cells; in hypokalemic periodic paralysis, low levels of potassium in the blood interact with genetically caused abnormalities in calcium channels (pores that allow the passage of calcium molecules) in muscle cells, or, less often, with genetic abnormalities in sodium or potassium channels

Onset - hyperkalemic, childhood; hypokalemic, early childhood to adulthood

Symptoms - in both forms, episodic attacks of muscle weakness; muscle stiffness (myotonia) can occur in hyperkalemic form

Progression - in hyperkalemic, frequency of attacks declines after middle age; in hypokalemic, number of attacks varies but severe attacks cause nearly full-blown paralysis; permanent muscle damage can occur

Inheritance - in both forms, autosomal dominant, or produced by a defective gene contributed by one parent

Facts About Myopathies