Within the past decade, investigators have uncovered the genetic basis for multiple long QT syndromes (LQTSs). These include LQT1, in which mutations in the gene KCNQ1 lead to abnormal IKs, a slow delayed-rectifier potassium current; LQT2, in which mutations in the gene KCNH2 lead to abnormal IKr, a rapid delayed-rectifier potassium current; and LQT3, in which mutations in the gene SCN5A lead to abnormal INa, a sodium channel current.1,2 After genetic testing was made available, it became clear that clinical evaluation based on the resting electrocardiogram was inadequate to detect many cases of LQTS.3 In fact, there is a considerable overlap between QT intervals of unaffected individuals and those with LQTS.4 Thus, in many instances, genetic testing adds important information beyond clinical evaluation in determining which patients are at risk both of personal cardiac events and of transmitting LQTS to their offspring.
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