Types of Long QT Syndrome (LQTS)

 

🫀 Long QT Syndrome (LQTS) — A Complete, Clinically Focused Guide

Long QT Syndrome (LQTS) is an inherited or acquired cardiac channelopathy characterized by delayed myocardial repolarization, manifesting as QT interval prolongation on ECG and predisposing to life-threatening arrhythmias like Torsades de Pointes.

Understanding the genetic subtype, typical triggers, and ECG signature helps guide diagnosis, counseling, and risk-stratification.

Here is a clinical deep dive based on the three major genotypes.

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🔹 LQTS Type I (LQT1)

Gene: KCNQ1

Mutation Type: Loss of function

Ion Channel: Slow delayed rectifier potassium current (IKs)

🧠 Pathophysiology

KCNQ1 mutations impair outward K⁺ flow during repolarization → prolonged action potential and delayed relaxation, especially during sympathetic surge.

⚡ Typical Triggers

Physical exertion, especially swimming

Emotional stress

Sudden adrenergic activation

Swimming is the most classic trigger due to cold-shock and sympathetic surge—LQT1 is essentially the “exercise-triggered” subtype.

📉 ECG Clue

Broad-based T waves (wide, smooth, prolonged)

This reflects IKs failure → slow ventricular repolarization.

🧷 Clinical Notes

Most common genotype (~40–45%).

Best response to beta-blockers.

Avoid strenuous/competitive swimming unless adequately controlled.

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🔹 LQTS Type II (LQT2)

Gene: KCNH2 (HERG)

Mutation Type: Loss of function

Ion Channel: Rapid delayed rectifier potassium current (IKr)

🧠 Pathophysiology

KCNH2 mutations reduce IKr, a key current in phase 3 repolarization → marked QT prolongation, especially when startled.

⚡ Typical Triggers

Auditory stimuli (alarm clock, loud noises, doorbell)

Sudden fright

Post-partum period (very high-risk in women)

Auditory startle is uniquely characteristic of LQT2.

📉 ECG Clue

Bifid (‘notched’) T waves

This appearance reflects repolarization inhomogeneity and reduced IKr function.

🧷 Clinical Notes

Second most common subtype.

Avoid drugs that block HERG channels → high risk of acquired QT prolongation.

Post-partum women must be closely monitored.

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🔹 LQTS Type III (LQT3)

Gene: SCN5A

Mutation Type: Gain of function

Ion Channel: Persistent inward sodium current (INa)

🧠 Pathophysiology

SCN5A mutations lead to persistent Na⁺ entry during repolarization → markedly prolonged plateau phase (phase 2).

⚡ Typical Triggers

Sleep

Rest

Bradycardia-associated events

Unlike LQT1 and LQT2, LQT3 arrhythmias occur in low-adrenergic states.

📉 ECG Clue

Prolonged ST segment (long isoelectric plateau)

This reflects Na⁺ channel–mediated prolongation of the action potential plateau.

🧷 Clinical Notes

Less common but potentially more lethal.

Beta-blockers less effective compared to LQT1/2.

Mexiletine (Na⁺ channel blocker) may shorten QT.

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📊 Side-by-Side Summary

Feature LQTS I LQTS II LQTS III

Gene KCNQ1 KCNH2 SCN5A

Mutation Loss of function Loss of function Gain of function

Ion Channel IKs ↓ IKr ↓ Late INa ↑

Triggers Exercise, swimming Auditory stimuli, postpartum Sleep, rest

ECG Pattern Broad T wave Bifid T wave Long ST segment

Best Therapy Beta-blockers Beta-blockers, avoid triggers Na⁺ blockers (e.g., mexiletine)

Risk Period Exercise Post-partum Night-time

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🩺 Clinical Pearls for Practice

Swimming = LQT1

Auditory alarm = LQT2

Sleep event = LQT3

QT prolongation is not enough—T-wave morphology gives strong clues.

Avoid QT-prolonging drugs in all LQTS patients, but especially in LQT2.

Beta-blockers are lifesaving, especially nadolol and propranolol.

Consider ICD in high-risk or therapy-refractory cases.