Tuesday, 23 February 2016
Myotonic Muscle Disorders And Periodic Paralysis
Hypokalemic periodic paralysis is not a myotonic disorder. However, the clinical features of periodic attacks of flaccid weakness and the development of fixed proximal weakness later in life resemble the sodiumchannel disorders discussed earlier. This is an autosomal dominant inherited disorder associated with a defect in the α subunit of a voltage-sensitive muscle calcium channel (CACNA1S) gene on chromosome 1q (hypokalemic periodic paralysis type 1). More recently, mutations were identified in the α subunit of the sodium channel gene (SCN4A) on chromosome 17q, and the term hypokalemic periodic paralysis type 2 was used to designate this small group of patients, who are clinically indistinguishable from those with hypokalemic periodic paralysis type 1. Both types result from missense mutations in the voltage-sensor domains of their respective channel. This similarity suggests a common functional defect produced by these voltage-sensor mutations, and may explain why different mutations on two different channels result in hypokalemic periodic paralysis.
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