Diagnosis and Management of Long QT Syndrome

Electrocardiographic Criteria

The European Society of Cardiology recommends that a QTc ≥480 ms on repeated 12-lead ECGs in the absence of secondary causes establishes the diagnosis clinically 1
A QTc ≥500 ms is considered unequivocal LQTS regardless of family history or symptoms, according to the European Heart Journal 2
The American College of Cardiology states that QTc ≥460 ms with unexplained syncope is sufficient for diagnosis 1

The American College of Cardiology recommends genetic counseling and genetic testing for patients with clinically diagnosed LQTS (Class I recommendation) 3, 4
Genetic testing identifies disease-causing mutations in approximately 75% of cases, with three main genes (KCNQ1, KCNH2, SCN5A) accounting for 90% of genetically positive cases, as reported by the European Heart Journal 1

The European Heart Journal suggests that obtaining detailed family history, specifically asking about early sudden death, fainting spells, seizures, and congenital deafness, is essential for diagnosis 5, 6
Excluding secondary causes of QT prolongation, such as QT-prolonging medications, electrolyte abnormalities, and structural heart disease, is crucial for accurate diagnosis 5, 6

The American College of Cardiology identifies QTc approaching 600 ms, T-wave alternans, 2:1 AV block secondary to QT prolongation, hearing loss, and history of cardiac arrest or syncope as highest risk indicators 5, 6, 3, 4, 7
The European Heart Journal reports that LQT2 and LQT3 genotypes carry higher risk, with females with LQT2 genotype being particularly high-risk 3, 4

The European Heart Journal recommends repeating ECG after a few days to confirm the finding in neonates with QTc >440 ms 5, 6, 8
If the second ECG shows QTc ≥500 ms, the infant is very likely affected and should be treated, according to the European Heart Journal 5, 6, 7

A normal QT interval in the presence of family history does not rule out familial LQTS, as low penetrance exists in LQTS, meaning gene carriers may not display the clinical phenotype, and approximately 25-30% of genetically confirmed LQTS patients have a QTc <440 ms 9, 10
Approximately 30% of cases are due to "de novo" mutations, which means unaffected parents and no family history, highlighting the importance of considering LQTS in patients with syncope even without a family history 9

With a positive family history, the probability of LQTS becomes high even with borderline or normal ECG findings, and repeat ECGs, 24-hour Holter monitoring, and genetic testing and counseling should be considered 10, 11, 9
Family screening, including ECGs from parents and siblings, is essential for diagnosis, as it can help identify affected family members and inform management decisions 9, 10

T-wave morphology abnormalities, such as notches on the T wave in precordial leads, suggest LQTS, and mild bradycardia and T-wave alternans may also be found in LQTS patients 9, 10
Syncope is an ominous finding in suspected LQTS, and the combination of family history, syncope, and even borderline QTc warrants complete diagnostic procedures and consideration of therapy initiation 10, 12

Beta-blockers are first-line therapy and should be initiated when diagnosis probability is high based on family history, even with borderline ECG findings, and patients with suspected LQTS require continuous cardiac monitoring, avoidance of QT-prolonging medications, correction of electrolyte abnormalities, and restriction of strenuous or competitive exercise 10, 12
The American College of Cardiology recommends genetic testing and counseling for patients with clinically suspected LQTS, and identifies disease-causing mutations in approximately 75% of cases 9, 10