Long QT syndrome with nocturnal cardiac events caused by a KCNH2 missense mutation (G604S).

An 8-year-old boy suffered from an unconsciousness attack and torsade de pointes arrhythmia during sleep or at rest. His electrocardiogram showed prolonged QT intervals, but the T wave morphology was atypical for type 1, 2 or 3 congenital long-QT syndrome (LQTS). Intravenous epinephrine slightly prolonged the QT interval whereas mexiletine infusion shortened the QT interval. Although these clinical characteristics might suggest type 3 LQTS, a genetic analysis identified the G604S-KCNH2 mutation (type 2 LQTS). Because mismatches between the genotype and phenotype of LQTS are possible, genetic analysis of LQTS is important to identify the most appropriate therapeutic option and risk stratification.