Long QT Syndrome
The hidden heart rhythm disorder that can cause sudden fainting — and how to protect your child.
What Is It
Understanding Long QT Syndrome
The heart’s electrical system controls the rhythm of each heartbeat. After every contraction, the heart muscle must “recharge” before it can beat again — this recovery period is called repolarisation, and it is measured on an ECG as the QT interval.
In Long QT Syndrome (LQTS), this recharging period is abnormally prolonged. A delayed repolarisation creates an electrical window of vulnerability during which a dangerous, potentially fatal arrhythmia called Torsades de Pointes (TdP) — a chaotic rapid rhythm — can be triggered. TdP causes sudden loss of consciousness (syncope) and, if it degenerates into ventricular fibrillation, sudden cardiac arrest.
Inherited vs. Acquired LQTS
| Type | Cause | Key Features |
|---|---|---|
| Inherited LQTS | Mutation in genes encoding cardiac ion channels (KCNQ1, KCNH2, SCN5A, and others) | Present from birth; affects 1:2000; family history often present; autosomal dominant in most types |
| Acquired LQTS | Drugs, electrolyte abnormalities (low potassium, magnesium, calcium), bradycardia | Reversible if the cause is removed; many common medications can prolong the QT |
The Three Main Genetic Subtypes
Most common (~35%). Arrhythmias triggered by exercise, particularly swimming. Beta-blockers are highly effective. Diving/swimming should be carefully risk-assessed.
Second most common (~30%). Arrhythmias triggered by sudden auditory stimuli (alarm clocks, phone rings, loud noises) or emotional stress. Keep the bedroom phone away from the bed. Beta-blockers effective but somewhat less so than in LQT1.
Third most common (~10%). Events occur predominantly at rest or during sleep — making it the most dangerous and the hardest to predict. Beta-blockers are LESS effective; implantable defibrillator (ICD) is more often recommended.
Symptoms & Triggers
Classic presentation: a young person collapses suddenly during sport, swimming, or with emotional stress. Before losing consciousness, they may have palpitations, dizziness, or a sense of the heart “stopping.” The episode may be mistaken for a seizure or a simple faint.
Red flag symptoms in a child with LQTS: unexplained fainting (especially during or immediately after exercise), near-drowning events, seizures without a clear neurological diagnosis, and family history of sudden unexplained death in a young person.
Diagnosis
12-lead ECG: The corrected QT interval (QTc) is measured. A QTc >460 ms in children is borderline; >480 ms is significantly prolonged; >500 ms is associated with high arrhythmic risk. Crucially, up to 50% of gene-positive patients have a normal or borderline QTc at rest — a normal ECG does not rule out LQTS.
Exercise stress test: The QT interval should shorten during exercise. Paradoxical QT prolongation during recovery is characteristic of LQT1.
Genetic testing: Identifies the causative gene mutation in ~75% of cases. A positive gene test identifies at-risk family members even if their ECG is normal. All first-degree relatives of a confirmed LQTS patient should be screened.
Treatment
Beta-blockers (nadolol or propranolol) are the cornerstone of treatment — they prevent adrenergic triggers from initiating TdP. They are highly effective in LQT1 and LQT2. Compliance is critical: missing doses significantly increases arrhythmic risk.
Lifestyle measures: Avoiding QT-prolonging medications (a list is maintained at crediblemeds.org), maintaining normal potassium levels, avoiding intense competitive sport in high-risk patients, and specific activity restrictions based on subtype (e.g., swimming supervision for LQT1).
Implantable Cardioverter-Defibrillator (ICD): Recommended after a survived cardiac arrest, for LQT3 patients, or for those with persistent symptoms despite medication. The ICD detects VF/VT and delivers a shock to restore normal rhythm.
Left cardiac sympathetic denervation (LCSD): A surgical procedure that interrupts the left-sided sympathetic nerves supplying the heart, reducing adrenergic triggers. Used in patients who cannot tolerate beta-blockers, have breakthrough events on medication, or for whom an ICD is not feasible.
Fainting (syncope) in children is common and most often benign — the most common cause is a simple vasovagal (neurocardiogenic) faint, not a heart rhythm problem. However, certain features make a cardiac cause more likely and demand urgent evaluation: fainting during or immediately after exercise, fainting triggered by swimming, fainting preceded by palpitations, fainting with no warning at all, or a family history of sudden unexplained death in a young person. An ECG is a reasonable first investigation for any child with unexplained syncope.
Yes — this is one of the most important points about LQTS. Up to 50% of individuals with a confirmed genetic mutation in an LQTS gene have a normal or borderline QT interval on their resting ECG. If LQTS is suspected clinically or there is a strong family history, genetic testing should be pursued regardless of the ECG finding. A normal ECG does not exclude LQTS.
Hundreds of medications — including many common antibiotics, antihistamines, antidepressants, antifungals, and even some motility drugs — can prolong the QT interval. A comprehensive and regularly updated list is available at crediblemeds.org. Always tell every doctor, dentist, and pharmacist that your child has LQTS before any medication is prescribed. Never self-medicate with over-the-counter antihistamines or similar drugs without checking first.
Yes, absolutely. Most inherited LQTS is autosomal dominant — meaning a first-degree relative (parent, sibling, child) has a 50% chance of carrying the same mutation. All first-degree relatives of a newly diagnosed LQTS patient should have a 12-lead ECG and ideally genetic testing. Identifying gene-positive relatives who are currently asymptomatic allows protective treatment to be started before a life-threatening event occurs.
With appropriate diagnosis and treatment, the vast majority of children with LQTS lead full and active lives. Beta-blocker therapy dramatically reduces arrhythmic risk. Some activity restrictions apply depending on the subtype and severity — your cardiologist will individualise these. Many adults with treated LQTS work, exercise, travel, and have their own children. The key is consistent medication adherence and avoiding known triggers.
Key Takeaways
- LQTS is a disorder of cardiac electrical recharging that predisposes to life-threatening arrhythmias (Torsades de Pointes).
- Up to 50% of affected individuals have a normal resting ECG — a normal ECG does not rule out LQTS.
- LQT1 events are triggered by exercise/swimming; LQT2 by auditory stimuli; LQT3 events occur at rest or during sleep.
- Beta-blockers are the cornerstone of treatment and are highly effective in LQT1 and LQT2.
- All first-degree relatives of a LQTS patient should be screened with ECG and genetic testing.
- Avoid QT-prolonging medications — check crediblemeds.org for a full and current list.
Sources
Schwartz PJ et al. Diagnosis of the Long QT Syndrome. Circulation 1993; 88(2):782–784. (Original Schwartz Score)
Allen HD et al. Moss and Adams’ Heart Disease in Infants, Children, and Adolescents, 10th Ed. Wolters Kluwer, 2021.
AHA/ACC/HRS Guidelines for Management of Patients with Ventricular Arrhythmias and Prevention of Sudden Cardiac Death, 2017.
Educational purposes only. Not a substitute for professional medical advice.
