Long QT Syndrome

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. 

As a result of this abnormal heart rate (arrhythmia), people who have LQTS are at risk for loss of consciousness (syncope) and sudden death, often at a young age. In fact, LQTS is one of the more common causes of sudden death in young people.

Each year, an estimated 3,000 to 4,000 people die from LQTS in the United States.

What Does QT Stand For?

An electrocardiogram (ECG) is a diagnostic test that shows how your heart's electrical system is working.

It does this by producing a result called a tracing, which essentially looks like jumpy lines on graph paper. This charting of electrical impulses is made of up several specific waves that represent different activities in the heart.

QT relates to the Q wave (part of the Q-R-S wave combination called the QRS complex) and the T wave.

After the heart's electrical impulse stimulates a heart cell (thus causing it to beat), recharging must occur in order. This is what makes the cell ready for the next electrical impulse.

The total length of time it takes for this recharging to occur is called the QT interval. On an ECG, it the measurement between the Q wave (discharge) and T wave (recharge).

Causes of Long QT Syndrome

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.

Congenital LQTS is present in approximately one in 2000 people. The acquired variants of LQTS are much more common and probably affect around 2% to 4% of the population.

Congenital 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. It includes:

• Antiarrhythmic drugs
• Antibiotics (especially erythromycin, clarithromycin, and azithromycin)
• Several medications used for nausea and vomitin
• Many antidepressants and antipsychotic drugs

The following can also trigger LQTS in susceptible people:

• Low blood potassium (hypokalemia)
• Low blood magnesium (hypomagnesemia)
• Liver or kidney problems

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

Long QT Syndrome Symptoms

The symptoms of LQTS only occur when a person develops an episode of dangerous ventricular tachycardia.

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

What someone might experience depends on the length of time the arrhythmia persists:

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

However, for about 1 out of every 10 people with LQTS, sudden death is the first sign of the disorder. This may happen during sleep, suddenly while swimming, or during any other episode when the person faints and does not regain consciousness.

Diagnosis

Doctors should consider LQTS in anyone who has lost consciousness or experienced cardiac arrest, which is a sudden stop in heart function.

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.

Doctors usually identify LQTS by observing an abnormally prolonged QT interval on an ECG. Sometimes a doctor may have a patient exercise on a treadmill during an ECG in order to bring out the ECG abnormalities, if present.

Genetic testing for LQTS and its variants is becoming much more commonly used than it was just a few years ago.

Family members of someone known to have LQTS should also be evaluated for the syndrome.

Treatment

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.

These medications are protective against sudden cardiac arrest (SCA) in some, and reduce adverse events in others. However, beta-blockers may not significantly reduce the overall incidence of syncope and sudden death in all patients with LQTS.

It is particularly important for people with LQTS and its variants to avoid the many drugs that cause an extended 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, so be sure that all of your prescribing doctors and your pharmacist are aware of your LQTS diagnosis.

For many people with LQTS, an 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 they're already taking beta-blockers).

For people who can't tolerate beta-blockers or who still have LQTS 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.