Long QT Syndrome

A Common Cause of Sudden Death in Young People

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Long QT Syndrome (LQTS) is a disorder of the heart's electrical system that can lead to a potentially fatal type of ventricular tachycardia known as torsades de pointes

LQTS can be either an inherited disorder or acquired after birth, but evidence suggests that even people with the acquired type of LQTS have a genetic predisposition to the condition.

Either way, people who have LQTS are at risk for syncope (loss of consciousness) and sudden death, often at a young age.

Overview

People with LQTS have prolonged QT intervals on their ECGs. The QT interval represents repolarization, or "recharge," of a cardiac cell. After the heart's electrical impulse stimulates a heart cell (thus causing it to beat), recharging must occur in order for the cell to be ready for the next electrical impulse. The QT interval (which is measured from the beginning of the QRS complex to the end of the T wave on the ECG) is the total length of time it takes to discharge, then recharge a cardiac cell. In LQTS, the QT interval is prolonged. Abnormalities in the QT interval are responsible for the arrhythmias associated with LQTS.

Causes

Congenital LQTS. Several genes have been identified that affect the QT interval, so several varieties of LQTS exist. Some families have a very high incidence of LQTS. Because so many genes can affect the QT interval, many variations in LQTS have been identified. Some of these ("classic" LQTS) are associated with a high incidence of dangerous arrhythmias and sudden death, which often occur in several family members. Other forms of congenital LQTS can be much less dangerous. 

Acquired LQTS. People with the acquired form of LQTS have normal baseline ECGs, including normal QT intervals. However, their QT intervals may become prolonged, and they may become at risk for dangerous arrhythmias, when they are exposed to certain drugs, or if they develop certain metabolic or electrolyte abnormalities. 

A long list of drugs can trigger LQTS in these individuals include antiarrhythmic drugs, antibiotics (especially erythromycin, clarithromycin and azithromycin), several medications used for nausea and vomiting, and many antidepressant and antipsychotic drugs. 

In addition, hypokalemia (low blood potassium level), hypomagnesemia (low blood magnesium level), and liver or kidney problems can also trigger LQTS in susceptible people. 

Many experts now believe that many (if not most) people with acquired LQTS also have an underlying genetic predisposition to LQTS, that only becomes manifest when they are “stressed” with a “triggering” drug or electrolyte or metabolic problem.

Prevalence

Congenital LQTS is present in approximately one of 5000 people. LQTS is one of the more common causes of sudden death in young people, resulting in between 2000 and 3000 deaths per year. The acquired variants of LQTS are much more common, and probably affect up to 2 - 3% of the population.

Symptoms

The symptoms of LQTS occur only when the patient develops an episode of dangerous ventricular tachycardia, and the degree of symptoms depends on the length of time the arrhythmia persists. If it lasts only momentarily, a few seconds of extreme dizziness may be the only symptom. If it persists for more than 10 seconds or so, syncope occurs. And if it lasts for more than a few minutes, the victim usually never regains consciousness.

In those with some varieties of LQTS, episodes are often triggered by sudden bursts of adrenaline; such as might occur during physical exertion, when severely startled, or when becoming extremely angry.

Fortunately, most people with LQTS variants never experience life-threatening symptoms.

Diagnosis

Doctors should think of LQTS in anyone who has had syncope or cardiac arrest, and in family members of a person with known LQTS. Any young person with syncope that occurs during exercise, or in any other circumstance in which a surge of adrenaline levels was likely to have been present, should have LQTS specifically ruled out.

The diagnosis of LQTS is usually made by observing an abnormally prolonged QT interval on the ECG. Sometimes a treadmill test is necessary to bring out the ECG abnormalities. Genetic testing for LQTS and its variants is becoming much more commonly used than it was just a few years ago.

Treatments

Many patients with overt LQTS are treated with beta blockers. Beta blockers blunt the surges of adrenaline that trigger episodes of arrhythmias in these patients. Unfortunately, it has not yet been proven that beta-blockers significantly reduce the overall incidence of syncope and sudden death in patients with LQTS.

It is particularly important for people with LQTS and its variants to avoid the many drugs that cause a prolongation of the QT interval. In these people, such drugs are very likely to provoke episodes of ventricular tachycardia. Drugs that prolong the QT interval are unfortunately common. CredibleMeds maintains an updated list of drugs that often prolong the QT interval.

For many people with LQTS, the implantable defibrillator is the best treatment. This device should be used in patients who have survived cardiac arrests, and probably in patients who have had syncope due to LQTS especially if syncope occurs while already taking beta blockers.

For people who can't tolerate beta blockers or who still have events while on therapy, left cardiac sympathetic denervation surgery may be performed.

A Word From Verywell

LQTS is a disorder of the heart’s electrical system that can produce sudden, potentially life-threatening cardiac arrhythmias. The classic form of LQTS is inherited, but even the “acquired” variants tend to have an underlying genetic component. 

Fatal outcomes with LQTS can almost always be prevented, as long as those who are at risk for dangerous arrhythmias can be identified.

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