Long QT Syndrome (LQTS)

What long QT syndrome is

Each heartbeat is triggered by an electrical signal. On an ECG, the QT interval measures the time from the start of the ventricles' electrical activation to the end of their recovery (repolarization). This recovery depends on ion channels that move potassium and sodium in and out of heart cells.

In long QT syndrome, faulty or blocked channels slow recovery, so the QT interval is longer than normal. During that vulnerable period, an extra beat can set off torsades de pointes, a fast, twisting ventricular rhythm. If it stops by itself, the person faints and wakes up. If it degenerates into ventricular fibrillation, it causes cardiac arrest and death without immediate CPR and defibrillation.

QT length varies with heart rate, so doctors use a corrected value, the QTc. Values above about 450 ms in men and 460 ms in women are borderline to prolonged. A QTc of 500 ms or more carries a clearly higher risk.

Types and causes

Inherited (congenital) long QT

About 1 in 2,000 people carry a long QT gene variant. Most are inherited from one parent (Romano-Ward syndrome). The three main types have different triggers:

A rare recessive form, Jervell and Lange-Nielsen syndrome, causes very long QT intervals plus deafness from birth.

Acquired long QT

• Medicines: some antiarrhythmics (sotalol, amiodarone, dofetilide), macrolide antibiotics (azithromycin, erythromycin), fluoroquinolones, some antifungals, antipsychotics (haloperidol), some antidepressants (citalopram), antinausea drugs (ondansetron), methadone and others. The CredibleMeds website keeps an updated list.
• Low blood salts: low potassium, low magnesium and low calcium, often from vomiting, diarrhea or diuretics.
• Other: a very slow heart rate, heart failure, anorexia nervosa, hypothyroidism, and brain bleeds.

Many people with acquired long QT have a mild hidden genetic tendency that shows only when a drug is added.

Symptoms

• Fainting (syncope), typically sudden, without warning, and linked to the triggers above
• Seizure-like jerking during a faint, which is often misdiagnosed as epilepsy
• Palpitations or a brief racing heartbeat
• Cardiac arrest or sudden cardiac death, sometimes as the first sign
• Unexplained drowning in a strong swimmer

Many people with a long QT gene never have symptoms.

Diagnosis

1. 12-lead ECG: measures the QTc and T wave shape, which hints at the type. Several ECGs may be needed, since the QT varies.
2. History: fainting circumstances, medicines, and family history of fainting, seizures or sudden death under 40.
3. Holter or exercise testing: shows how the QT behaves during exercise and recovery.
4. Genetic testing: finds a causal variant in most people with a clear diagnosis, and allows testing of relatives.
5. Blood tests: potassium, magnesium, calcium and thyroid function.
6. Family screening: ECGs for parents, siblings and children.

Doctors combine findings with a scoring system (the Schwartz score). Seeing a cardiologist who specializes in inherited rhythm disorders is recommended.

Treatment

• Beta-blockers (nadolol or propranolol are preferred) are first-line for most people with inherited LQTS, including many without symptoms. They blunt adrenaline-triggered events, especially in LQT1. Do not stop them suddenly.
• Avoid QT-prolonging drugs. Check every new prescription and over-the-counter medicine with your doctor or pharmacist.
• Keep potassium and magnesium normal. Replace fluids and salts promptly during vomiting, diarrhea or heavy sweating.
• Implantable cardioverter-defibrillator (ICD): for people who have survived a cardiac arrest, or who keep fainting despite beta-blockers, and some very high-risk patients.
• Left cardiac sympathetic denervation: surgery to cut nerves that drive arrhythmias, when beta-blockers are not enough or not tolerated.
• Mexiletine for some people with LQT3.
• Lifestyle: most people can now exercise with a specialist's guidance. Those with LQT1 should avoid swimming alone. People with LQT2 may move alarms and phones out of the bedroom.

For acquired long QT, the offending drug is stopped and salt levels corrected. Torsades in the hospital is treated with intravenous magnesium.

Outlook

With treatment, most people with long QT syndrome live normal lifespans. The risk of a serious event is highest in those who have already fainted, have a QTc over 500 ms, or have had a cardiac arrest. Family members who share the condition can be protected once identified, and everyone in the household should learn CPR and know where the nearest AED is.

Frequently asked questions

What are the symptoms of long QT syndrome?

Many people have none. When symptoms occur, the main ones are sudden fainting, often triggered by exercise, swimming, a loud noise or strong emotion, seizure-like jerking during a faint, palpitations, and in some cases cardiac arrest. An ECG is needed to diagnose it.

What causes a prolonged QT interval?

It can be inherited, through variants in genes for heart ion channels, or acquired from medicines such as certain antibiotics, antipsychotics, antidepressants and antiarrhythmics, low potassium or magnesium, a very slow heart rate, and other conditions. Sometimes a drug unmasks a hidden inherited tendency.

Is long QT syndrome serious?

It can be, because it can cause fainting and sudden cardiac arrest. But with diagnosis and treatment, most people live normal lives. Beta-blockers, avoiding trigger drugs and keeping potassium and magnesium levels normal greatly lower the risk. High-risk people receive an implanted defibrillator.

Can you exercise with long QT syndrome?

Most people can, after an assessment by a specialist in inherited arrhythmias. Activity advice depends on the genetic type, symptoms and QTc. People with LQT1 should avoid swimming alone. An AED should be available at sports events, and teammates or coaches should know about the condition.

Can long QT syndrome be cured?

Inherited long QT cannot be cured, because it is genetic, but it can be controlled very effectively. Acquired long QT often resolves once the causative medicine is stopped or blood salts are corrected, though the person may remain sensitive to QT-prolonging drugs in the future.

Is long QT syndrome inherited?

Often, yes. Most inherited cases follow a dominant pattern, so each child of an affected parent has a 50% chance of carrying the variant. Close relatives of anyone diagnosed should have an ECG, and genetic testing when a variant has been found in the family.