PARAMYOTONIA-CONGENITA - ABNORMAL SHORT EXERCISE TEST, AND IMPROVEMENT AFTER MEXILETINE THERAPY

The diagnosis of paramyotonia congenita (PC) can be aided by demonstra ting a decrease in compound motor action potential amplitude after exe rcise and a decrement on repetitive stimulation, following cold exposu re. We report a patient with PC who presented with complaints of cold- induced hand and jaw stiffening, in the absence of any episodes of wea kness. Treatment with mexiletine led to resolution of the abnormalitie s exhibited during a short exercise test and repetitive stimulation fo llowing ice bath immersion. Molecular genetic analysis revealed a miss ense mutation (cytosine to thymidine) on chromosome 17 in the alpha-su bunit of the skeletal muscle sodium channel gene that results in the r eplacement of threonine with methionine. This case demonstrates that, despite the absence of weakness, the short exercise test following col d exposure can be used to confirm the diagnosis of PC in patients with out episodic weakness. Furthermore, improvement of the electrophysiolo gic abnormalities with mexiletine was documented, corresponding with c linical improvement. (C) 1994 John Wiley and Sons, Inc.