Long QT Syndrome

Long QT Syndrome (LQTS) is a disorder of ventricular myocardial repolarization caused by dysfunction of ion channels that govern the cardiac action potential plateau and terminal repolarization phases. The net effect of all LQTS-causing mutations is a prolongation of the action potential duration (APD), which manifests on the surface ECG as a prolonged QT interval. This prolonged APD creates a vulnerable window during which early afterdepolarizations (EADs) can arise from reactivation of L-type calcium channels or late sodium current during phase 2–3 of the action potential, serving as the trigger for the signature arrhythmia of LQTS: torsades de pointes (TdP).

The congenital forms of LQTS follow autosomal dominant inheritance (Romano-Ward syndrome) or, rarely, autosomal recessive inheritance with associated sensorineural deafness (Jervell and Lange-Nielsen syndrome, caused by homozygous or compound heterozygous KCNQ1 or KCNE1 mutations). Over 17 genetic subtypes have been described, but three major subtypes account for approximately 90% of genotype-positive cases:

LQT1 — KCNQ1 (I_Ks Loss of Function)

LQT1 is the most common subtype, accounting for ~35–40% of genotyped LQTS patients. It is caused by loss-of-function mutations in KCNQ1 (previously KvLQT1), which encodes the alpha subunit of the slow delayed rectifier potassium channel conducting I_Ks. The I_Ks current is responsible for rate-dependent shortening of the action potential — it activates slowly during sympathetic stimulation and at faster heart rates to accelerate repolarization. Loss of I_Ks impairs the heart's ability to shorten the QT interval during exercise or adrenergic stimulation, explaining why LQT1 patients characteristically experience arrhythmic events during physical exertion (especially swimming) and emotional stress. The KCNE1 gene (encoding minK, the beta subunit of the I_Ks channel) is implicated in LQT5, producing a similar phenotype.

LQT2 — KCNH2/hERG (I_Kr Loss of Function)

LQT2 is the second most common subtype (~30–35% of genotyped cases), caused by loss-of-function mutations in KCNH2 (also known as hERG, human ether-a-go-go-related gene), which encodes the alpha subunit of the rapid delayed rectifier potassium channel conducting I_Kr. The I_Kr current is the dominant repolarizing current during phase 3 of the action potential, and its loss results in marked APD prolongation. LQT2 patients are vulnerable to arrhythmic events triggered by sudden auditory stimuli (alarm clocks, phone ringing) and emotional stress, particularly during the postpartum period. The hERG channel is also the primary target of drug-induced LQTS (see below). KCNE2 (encoding MiRP1, the I_Kr beta subunit) mutations cause LQT6.

LQT3 — SCN5A (I_Na Gain of Function)

LQT3 accounts for ~5–10% of genotyped cases and is caused by gain-of-function mutations in SCN5A, the gene encoding the cardiac sodium channel Na_v1.5. Unlike the loss-of-function SCN5A mutations seen in Brugada Syndrome, LQT3 mutations cause a persistent or late inward sodium current (I_Na,late) during the plateau phase that delays repolarization. This sustained inward current prolongs the action potential predominantly at slow heart rates, explaining why LQT3 events characteristically occur during sleep and rest. LQT3 patients have the highest per-event lethality among the three major subtypes, with cardiac events more likely to be fatal. The late sodium current in LQT3 is the therapeutic target of mexiletine and ranolazine.

Other Subtypes (LQT4–LQT17)

Rare subtypes include LQT4 (ANK2, ankyrin-B dysfunction affecting multiple ion channel trafficking), LQT7 (KCNJ2, Andersen-Tawil syndrome with periodic paralysis and U waves), LQT8 (CACNA1C, Timothy syndrome with syndactyly and multi-organ involvement), LQT9–LQT17 involving caveolin-3, SCN4B, AKAP9, SNTA1, KCNJ5, CALM1, CALM2, CALM3, and TRDN. These collectively account for <5% of genotyped cases and often present with additional extra-cardiac features.

Acquired LQTS

Drug-induced LQTS is far more common than the congenital form and is the leading cause of drug withdrawals from the market. The vast majority of culprit drugs block the hERG/I_Kr channel, reflecting the channel's unique structural vulnerability — two aromatic residues (Y652 and F656) in the inner vestibule create a high-affinity binding site for diverse drug classes. Common offenders include Class IA and III antiarrhythmics (sotalol, dofetilide, ibutilide, amiodarone), antibiotics (fluoroquinolones, macrolides, azole antifungals), antipsychotics (haloperidol, droperidol, ziprasidone), antiemetics (ondansetron, domperidone), and methadone. Electrolyte derangements — hypokalemia, hypomagnesemia, and hypocalcemia — potentiate QT prolongation by reducing repolarization reserve. Bradycardia is an independent risk factor, as slow rates allow greater accumulation of inward current during the prolonged plateau. The concept of repolarization reserve explains why drug-induced LQTS often occurs in patients harboring subclinical genetic variants (reduced baseline repolarization reserve) who are then exposed to an additional I_Kr-blocking insult.

Torsades de Pointes Mechanism

The arrhythmogenic mechanism of TdP involves a trigger (EADs generating triggered activity, typically arising from Purkinje fibers or midmyocardial M cells) acting on a substrate of heterogeneous repolarization. The transmural dispersion of repolarization (TDR) — the difference in APD between epicardial, endocardial, and midmyocardial (M cell) layers — is amplified in LQTS because M cells have the greatest prolongation of APD. This creates a vulnerable window for unidirectional block and reentry, producing the characteristic undulating QRS axis ("twisting of the points") as the reentrant wavefront rotates around the ventricular wall. The "short-long-short" initiation sequence (a PVC followed by a compensatory pause followed by another PVC) is classic and reflects pause-dependent EAD generation.

The ECG is the cornerstone of LQTS diagnosis, and careful analysis of both the QT interval and T-wave morphology can provide critical genotype-specific information. The hallmark finding is prolongation of the corrected QT interval (QTc), but the diagnosis requires integration of clinical context, family history, and genetic data, as the QTc alone has significant overlap between affected individuals and the normal population.

QTc Measurement and Correction Formulas

The QT interval is measured from the onset of the QRS complex to the end of the T wave, ideally in leads II or V5 where the T wave is most clearly delineated. The end of the T wave is defined by the tangent method: a tangent line is drawn along the steepest portion of the descending limb of the T wave, and the intersection of this tangent with the baseline defines the T-wave offset. This method is more reproducible than simply identifying where the T wave "appears to end." When a U wave is present, the nadir between the T and U wave should be used as the T-wave offset, and the U wave should not be included in the QT measurement — unless the T and U waves are fused, in which case the composite T-U complex is measured.

Because the QT interval is heart-rate dependent (it shortens at faster rates), correction formulas are applied to normalize the QT to a heart rate of 60 bpm. The most commonly used formula is the Bazett formula: QTc = QT / √RR (where RR is in seconds). However, Bazett's formula overcorrects at fast heart rates (>100 bpm) and undercorrects at slow heart rates (<60 bpm), which can lead to both false-positive and false-negative diagnoses. The Fridericia formula (QTc = QT / RR^1/3) provides better accuracy across a wider range of heart rates and is increasingly preferred, particularly in pharmacologic QT studies and drug safety evaluations. Other formulas include Hodges (linear correction: QTc = QT + 1.75 × [HR − 60]) and Framingham (QTc = QT + 0.154 × [1 − RR]). In clinical practice, it is advisable to measure the QT at heart rates between 50–90 bpm where correction formulas are most accurate, and to average measurements over 3–5 consecutive beats.

QTc Thresholds for Diagnosis

Diagnostic QTc thresholds are sex-specific because women have inherently longer QTc intervals than men (attributed to the effects of testosterone on I_Kr). A QTc >470 ms in women and >460 ms in men is considered prolonged and raises clinical suspicion for LQTS. However, these thresholds must be interpreted in context: up to 25–40% of genotype-positive LQTS patients have a QTc within the normal range on any given ECG (concealed LQTS), while a small percentage of the normal population has a QTc exceeding these thresholds. A QTc ≥500 ms is associated with a significantly increased risk of TdP and SCD, and is a major risk factor in the Schwartz score and in management algorithms for ICD implantation. A QTc ≥600 ms is almost always pathologic and typically indicates a severe phenotype or compound/homozygous mutations (e.g., Jervell and Lange-Nielsen syndrome).

T-Wave Morphology by Genotype

Perhaps the most clinically useful ECG clue in LQTS is the genotype-specific T-wave morphology, which can suggest the underlying genetic subtype before genetic testing results are available. These patterns reflect the different phases of the action potential affected by each mutation:

LQT1: characteristically shows a broad-based T wave with a smooth, gradually rising and falling contour. The T wave has a normal amplitude but is widened at the base, reflecting the homogeneous prolongation of repolarization caused by I_Ks loss. The onset of the T wave is not delayed — the ST segment is relatively normal in duration — but the T wave itself is temporally stretched. This pattern is most evident in the precordial leads.

LQT2: displays low-amplitude, notched, or bifid T waves with a characteristic "split" appearance. The T wave may have two distinct humps (sometimes described as a T-U complex), or a notch on the downslope. This pattern reflects the heterogeneous prolongation of repolarization caused by I_Kr loss, with differential effects on M cells versus epicardial and endocardial layers creating a multiphasic T wave. The notched morphology is most prominent in leads V2–V4 and is highly specific for LQT2 — its presence has a positive predictive value exceeding 90% for KCNH2 mutations.

LQT3: shows a late-onset, peaked T wave preceded by a long isoelectric ST segment. The prolongation is concentrated in the ST segment rather than the T wave itself, reflecting the persistent late sodium current that extends the plateau phase (phase 2) of the action potential. The T wave, when it finally arrives, tends to be narrow and peaked. This "long ST–narrow T" pattern is distinctive and can be identified even at a glance.

T-Wave Alternans

Macroscopic T-wave alternans (TWA) — beat-to-beat alternation of T-wave amplitude, morphology, or polarity — is an ominous finding in LQTS that indicates extreme repolarization instability and imminent risk of TdP. It reflects alternating long and short APDs in different myocardial layers and is often a pre-arrhythmic harbinger. Microvolt TWA detected by ambulatory monitoring or exercise testing may have prognostic significance, but macroscopic TWA visible on the surface ECG demands immediate clinical action (IV magnesium, temporary pacing, isoproterenol in acquired LQTS).

Unlike many other arrhythmic conditions where the electrophysiology study (EPS) plays a central diagnostic and therapeutic role, the EPS has a limited and largely supplementary role in Long QT Syndrome. The diagnosis of LQTS is established through clinical criteria, ECG analysis, and genetic testing, and the management strategy is guided by genotype, symptom severity, and QTc duration rather than by invasive electrophysiologic findings.

Programmed Ventricular Stimulation

Programmed ventricular stimulation (PVS) has been studied as a potential risk stratification tool in LQTS, but its predictive value for spontaneous arrhythmic events is poor. The arrhythmia mechanism in LQTS — triggered activity from EADs rather than scar-based reentry — is fundamentally different from the mechanisms amenable to provocation by extrastimuli. TdP in LQTS is typically initiated by a pause-dependent sequence and requires a specific autonomic and metabolic milieu that is not reproduced by programmed stimulation. Studies have shown that VF or polymorphic VT can be induced in up to 50–60% of LQTS patients during aggressive stimulation protocols (triple extrastimuli), but inducibility does not reliably distinguish patients who will have spontaneous events from those who will not. Both false-positive and false-negative rates are unacceptably high, and PVS is not recommended as a routine risk stratification tool in LQTS (in contrast to its debated role in Brugada Syndrome).

QT Dispersion

QT dispersion — the difference between the longest and shortest QT intervals across the 12-lead ECG — has been proposed as a surrogate marker of spatial heterogeneity of repolarization. LQTS patients tend to have increased QT dispersion (often >80–100 ms) compared to normal subjects, reflecting the transmural and interventricular differences in APD. However, QT dispersion has poor reproducibility, is influenced by T-wave amplitude and measurement technique, and has not been validated as an independent predictor of arrhythmic risk in prospective studies. It remains a research tool rather than a clinical decision-making parameter.

Genetic Testing as the Primary Diagnostic Tool

Comprehensive genetic testing has supplanted the EPS as the most important diagnostic investigation in suspected LQTS. Current guidelines recommend genetic testing for all patients with a clinical diagnosis of LQTS (based on QTc prolongation and/or clinical features) and for asymptomatic first-degree relatives of genotype-positive probands. Multigene panels typically include the three major genes (KCNQ1, KCNH2, SCN5A) along with minor LQTS genes (KCNE1, KCNE2, ANK2, KCNJ2, CACNA1C, and others). The yield of genetic testing is approximately 75–80% in patients with a strong clinical phenotype (QTc ≥480 ms plus symptoms) but drops to ~50% in those with borderline QTc prolongation. Variants of uncertain significance (VUS) remain a significant challenge, particularly in minor genes, and require careful clinical-genetic correlation.

Schwartz Score for Clinical Diagnosis

The Schwartz diagnostic score provides a structured clinical framework for diagnosing LQTS by integrating ECG findings, clinical history, and family history into a points-based system. Key scoring elements include: QTc ≥480 ms (3 points), QTc 460–479 ms (2 points), QTc 450–459 ms in males (1 point); torsades de pointes (2 points); T-wave alternans (1 point); notched T wave in 3 leads (1 point); low heart rate for age (0.5 points); syncope with stress (2 points); syncope without stress (1 point); congenital deafness (0.5 points); family member with definite LQTS (1 point); unexplained SCD in family member <30 years (0.5 points). A score ≤1 indicates low probability, 2–3 intermediate probability, and ≥3.5 high probability of LQTS. The Schwartz score remains the recommended clinical diagnostic tool in the 2022 ESC Guidelines and complements genetic testing.

Epinephrine QT Stress Testing

The epinephrine QT stress test (Shimizu protocol or Mayo protocol) is a provocative test that can unmask concealed LQTS and help differentiate subtypes. Low-dose epinephrine (0.05–0.1 mcg/kg/min) is infused while the QTc is monitored continuously. In LQT1 patients, epinephrine produces a paradoxical prolongation of the QTc interval — the QTc increases by ≥30 ms from baseline during steady-state infusion. This occurs because the adrenergically stimulated I_Ks current, which would normally shorten repolarization, is dysfunctional. Normal individuals and LQT2/LQT3 patients typically show QTc shortening or no significant change with epinephrine. The sensitivity and specificity of the epinephrine test for LQT1 are approximately 92% and 86%, respectively, making it a valuable adjunct when genetic testing is inconclusive or pending. Exercise stress testing (treadmill) can also reveal abnormal QTc responses: failure of the QTc to shorten during peak exercise or paradoxical QTc prolongation during recovery is suggestive of LQTS, particularly LQT1 and LQT2.

Management of LQTS is guided by genotype, symptom history, QTc duration, and individual risk factors. The goals of therapy are to prevent triggered activity by suppressing adrenergic triggers, to shorten the QT interval where possible, and to provide a safety net against sudden cardiac death in high-risk patients. A genotype-informed approach to management has become standard, as the three major subtypes respond differently to pharmacologic and lifestyle interventions.

Beta-Blockers as First-Line Therapy

Beta-blockers are the cornerstone of LQTS management and are recommended for all symptomatic patients and high-risk asymptomatic patients (QTc ≥470 ms, LQT1, or LQT2). They reduce cardiac events by blunting adrenergic stimulation, decreasing heart rate, and suppressing EAD-triggered activity. Not all beta-blockers are equal in LQTS: nadolol is the preferred agent due to its long half-life, consistent serum levels, and superior efficacy demonstrated in retrospective analyses. In the landmark Moss et al. study, beta-blocker therapy reduced the risk of cardiac events by approximately 64% in LQT1 and LQT2 patients. Propranolol is an alternative with additional sodium channel blocking properties that may be beneficial in LQT3. Metoprolol should be avoided — a study by Chockalingam et al. demonstrated that metoprolol was associated with a significantly higher rate of breakthrough cardiac events compared to nadolol and propranolol, possibly due to its shorter half-life and selective beta-1 activity.

Beta-blocker efficacy is genotype-dependent: LQT1 patients derive the greatest benefit because their arrhythmic events are primarily adrenergically mediated (exercise, emotional stress). LQT2 patients have a moderate response, with events partially related to adrenergic triggers (startle, emotional stress) but also occurring during transitions from rest to arousal. LQT3 patients have the least benefit from beta-blockers alone, as their events occur predominantly during rest and bradycardia — slowing the heart rate further may theoretically be detrimental. However, beta-blockers are still generally recommended for LQT3 patients unless contraindicated, often in combination with sodium channel blockers.

Genotype-Specific Trigger Avoidance

Lifestyle modification and trigger avoidance are essential components of LQTS management and are tailored to the underlying genotype:

LQT1: Avoid competitive swimming and strenuous exertion. Swimming is a uniquely high-risk activity for LQT1 patients — the combination of exercise, cold water immersion (which augments sympathetic drive), and the face immersion reflex (which enhances vagal tone) creates a "perfect storm" for arrhythmogenesis. Supervised, non-competitive recreational swimming with a buddy may be considered on an individualized basis. Intense competitive sports are generally contraindicated, though recent guidelines have adopted a more permissive shared decision-making approach for genotype-positive, phenotype-negative athletes on adequate beta-blocker therapy.

LQT2: Avoid sudden auditory stimuli and emotional stress. Patients should be advised to silence alarm clocks, put phones on vibrate mode, and avoid being startled awake. Doorbell and telephone sounds should be modified to less jarring tones. The postpartum period (first 9 months) carries elevated risk, particularly in women who discontinue beta-blockers during pregnancy. Potassium supplementation to maintain serum K⁺ ≥4.0 mEq/L is especially important in LQT2 because hypokalemia directly reduces I_Kr conductance.

LQT3: Avoid situations that promote bradycardia and prolonged rest. Unlike LQT1 and LQT2, LQT3 events tend to occur during sleep. Some experts recommend avoiding deep sleep without a safety net (e.g., event monitors, bed partner awareness). Fever should be treated aggressively with antipyretics as it may unmask or worsen the phenotype.

Drug Avoidance

All LQTS patients — regardless of genotype — must avoid QT-prolonging medications. The comprehensive list is maintained at CredibleMeds.org and includes: Class IA and III antiarrhythmics (sotalol, dofetilide, procainamide), fluoroquinolone and macrolide antibiotics, azole antifungals, antipsychotics (haloperidol, ziprasidone, thioridazine), certain antiemetics (ondansetron at high doses, droperidol), methadone, and numerous other agents. Patients should carry a wallet card or smartphone app listing prohibited medications and should inform all healthcare providers of their diagnosis before any new prescription.

ICD Indications

Class I indications for ICD implantation in LQTS include: (1) survivors of cardiac arrest, and (2) patients with recurrent syncope despite adequate beta-blocker therapy. Class IIa indications include patients with a QTc ≥500 ms in the setting of LQT2 or LQT3 genotype (higher per-event lethality), and patients with features suggesting high risk (T-wave alternans, QTc ≥500 ms, LQT3, Jervell and Lange-Nielsen syndrome). The decision to implant an ICD in LQTS must balance arrhythmic risk against the long-term burden of device complications — most LQTS patients are young, and inappropriate shocks (often triggered by T-wave oversensing in the setting of prolonged repolarization), lead failures, and infections accumulate over decades. Subcutaneous ICDs (S-ICDs) are an attractive alternative in LQTS as they avoid transvenous lead complications, but T-wave oversensing screening must be performed rigorously before implantation.

Left Cardiac Sympathetic Denervation (LCSD)

LCSD (also called left stellectomy) involves surgical excision of the lower third to half of the left stellate ganglion and the T2–T4 left thoracic sympathetic ganglia. This procedure reduces left-sided cardiac sympathetic input, raises the ventricular fibrillation threshold, and shortens the QT interval by an average of 40 ms. LCSD is indicated for (1) patients with recurrent syncope or ICD shocks despite maximal beta-blocker therapy, (2) patients who cannot tolerate beta-blockers, and (3) patients who refuse or are not candidates for ICD implantation. The procedure can be performed via a minimally invasive video-assisted thoracoscopic surgery (VATS) approach. Long-term studies by Schwartz et al. have demonstrated a ~91% reduction in cardiac events after LCSD. Common side effects include transient left-sided Horner syndrome (ptosis, miosis, anhidrosis) and compensatory right-sided hyperhidrosis.

Mexiletine for LQT3

Mexiletine, a Class IB sodium channel blocker, directly targets the pathophysiologic mechanism of LQT3 by blocking the persistent late sodium current (I_Na,late). Clinical studies have demonstrated that mexiletine shortens the QTc by 50–80 ms in LQT3 patients and reduces the incidence of arrhythmic events. It is recommended as adjunctive therapy to beta-blockers in LQT3 patients with a QTc ≥500 ms or recurrent symptoms. Ranolazine is an alternative late sodium current blocker with a more favorable side-effect profile, though clinical experience in LQTS is more limited. For LQT2, potassium supplementation and potassium-sparing agents (spironolactone, amiloride) can increase serum potassium and enhance I_Kr conductance.

Risk Stratification

Key risk factors for cardiac events in LQTS include: QTc ≥500 ms (the single strongest predictor, associated with a 2–3-fold increase in risk), prior cardiac arrest or syncope, LQT3 genotype (highest per-event lethality), male sex before puberty (LQT1) and female sex after puberty (LQT2), Jervell and Lange-Nielsen syndrome, T-wave alternans, compound or multiple mutations, and family history of SCD. The 1-2-3 rule provides a pragmatic framework: LQT1 patients have the highest event rate but lowest per-event mortality; LQT2 patients have intermediate event rate and mortality; LQT3 patients have the lowest event rate but highest per-event mortality. Individualized risk assessment integrating genotype, QTc, sex, age, symptom history, and mutation-specific data guides the intensity of therapy.