A 13-year-old girl is repeatedly awakened by her bedside alarm clock and immediately collapses with convulsive syncope. Her resting ECG shows a QTc of 510 ms with distinctly bifid, notched T waves in the precordial leads. Her father died suddenly in his sleep. The underlying genetic defect most likely involves:

• A Gain of function mutation in KCNQ1 encoding IKs
• B Loss of function mutation in KCNJ2 encoding IKir
• C Gain of function mutation in SCN5A encoding INa
• D Loss of function mutation in KCNH2 (HERG) encoding IKr ✓

Explanation

Auditory-triggered events, bifid or notched T waves, and a QTc near 500 ms define type 2 long QT syndrome, caused by loss of function mutations in KCNH2 (HERG) that reduce the rapid delayed rectifier potassium current IKr. Gain of function KCNQ1 mutations produce type 1 with exercise and swimming triggers and broad-based T waves, while gain of function SCN5A mutations produce type 3 with events during sleep and a long isoelectric ST segment.

Reference: Nelson Textbook of Pediatrics, 21st ed.

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Written and medically reviewed by the StethoPrep medical team.