Channelopathies Myotonias and Periodic Paralysis

The channelopathies are disorders of ion channels that result in altered excitability of cellular membranes. At present, more is known about muscle and peripheral nerve than central nervous system channelopathies. Most of the well-characterized channelopathies are disorders of muscle membrane ion channels. A disturbance of ion channel function may result in muscle membrane hyperexcitability leading to myotonia as the dominant feature, or, alternatively, it may result in muscle membrane hypoexcitability leading to the episodic weakness seen in periodic paralysis. The muscle channelopathies can be grouped into sodium, calcium, and chloride channel disorders (.,Table.i37-1 ). This chapter focuses on the inherited channelopathies, but it is important to be aware that there are acquired, usually autoimmune, channelopathies that also result in nerve and muscle disease.

Traditionally, the myotonias have been divided into dystrophic and nondystrophic disorders. In the dystrophic myotonias, myotonia is one of several muscle symptoms, with muscle atrophy and weakness being most prominent. These diseases include dystrophia myotonica and proximal myotonic myopathy (see Chaptei.3.6. ). [1i , y In contrast, in nondystrophic myotonias the most prominent symptom is myotonia, and it is these disorders that have been shown to result from a gene lesion in a muscle channel protein. Those nondystrophic disorders are the focus of this chapter.

The periodic paralyses have been traditionally divided into those associated with a high or normal serum potassium concentration (i.e., hyperkalemic periodic paralysis) and those associated with a low serum potassium concentration (i.e., hypokalemic periodic paralysis). In fact, however, the abnormal serum potassium concentration is clearly the consequence rather than the cause of the peri

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