16 genes now known, most incomplete penetrance. Important in drug induced Torsades too. LQTS probably accounts for many sudden unexpected deaths, some evidence for SUDI too. Some genes associated with epilepsy. Triggers for cardiac event seem to vary from 1 gene to another eg exercise (LQT1), sudden loud noise (LQT2).
Variable expressivity too, so 1 normal ECG not adequate.
Other ECG clues are bradycardia, T wave alternans or biphasic, abnormal U waves. T wave changes with posture or exercise may also be seen. Allow at least 3 consecutive beats to measure, to allow for sinus arrhythmia.
Consensus diagnostic criteria
- LQTS risk score >=3.5 (in the absence of an alternative cause) – score is complex, but involves QTc, Torsades, T wave abnormalities, syncope, cong deafness, FH.
- Pathogenic mutation in a LQTS gene
- QTc >500 on repeated ECG
- QTc 480-499 (repeated) PLUS unexplained syncope.
Treatment is with beta blockers, avoidance of triggers.[Current Opinion in Pediatrics. 26(6):727-33, 2014 PMID: 25313972].