Electromyographic distinction between paramyotonia congenita and myotonia congenita

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Abstract
We studied three patients with paramyotonia congenita and three with myotonia congenita. At room temperature, myotonic bursts in the EMG were similar in the two disorders. After repetitive maximal contractions, patients with paramyotonia showed decreasing recruitment pattern and increasing after-activity, while myotonia patients showed unchanged recruitment pattern, and after-activity tended to decrease, but these differences were not consistent. However, immersion of the hand in ice water for 10 minutes caused unambiguous differences: In paramyotonia, the myotonic discharges disappeared and the muscles went into stiff, electrically silent, contracture; in myotonia patients, the myotonic activity increased and muscle contraction patterns remained normal.

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