Causes

Long QT syndrome is a heart rhythm disorder caused by abnormalities in the heart's electrical recharging system. The heart's structure is normal.

Normally, the heart sends blood out to the body during each heartbeat. The heart's chambers contract and relax to pump the blood. This coordinated action is controlled by your heart's electrical system. Electrical signals (impulses) travel from the top to the bottom of your heart and trigger it to contract and beat. After each heartbeat, the system recharges itself to prepare for the next heartbeat.

In long QT syndrome, your heart's electrical system takes longer than normal to recharge between beats. This delay, which often can be seen on an electrocardiogram (ECG), is called a prolonged QT interval.

Long QT syndrome is often grouped into two main categories, depending on the cause.

• If you are born with the condition, it's called congenital long QT syndrome. Some forms of long QT syndrome result from a genetic mutation that is passed down through families (inherited).
• If an underlying medical condition or medication causes it, it's called acquired long QT syndrome. This type of long QT syndrome is usually reversible when the underlying cause is identified.

Causes of congenital long QT syndrome

More than 17 genes have been linked to long QT syndrome so far, and researchers have identified hundreds of mutations within these genes.

Doctors have described two forms of congenital long QT syndrome:

• Romano-Ward syndrome (autosomal dominant form). This more common form occurs in people who inherit only a single gene variant from one parent.
• Jervell and Lange-Nielsen syndrome (autosomal recessive form). This rare form usually occurs earlier and is more severe. In this syndrome, children receive the faulty gene variants from both parents. The children are born with long QT syndrome and deafness.

Causes of acquired long QT syndrome

More than 100 medications — many of them common — can lengthen the QT interval in otherwise healthy people and cause acquired long QT syndrome.

Medications that can cause acquired long QT syndrome include:

• Certain common antibiotics, such as erythromycin (Eryc, Erythrocin, others), azithromycin (Zithromax, Zmax) and others
• Certain antifungal medications taken by mouth used to treat yeast infections
• Diuretics that cause an electrolyte imbalance (low potassium, most commonly)
• Heart rhythm drugs (especially anti-arrhythmic medications that lengthen the QT interval)
• Certain antidepressant and antipsychotic medications
• Some anti-nausea medications

If your condition is caused by a medication, it may be called drug-induced long QT syndrome. Always tell your doctor about all the medications you take, including those you buy without a prescription.

Other causes of acquired long QT syndrome include:

• Low potassium level (hypokalemia)
• Low calcium level (hypocalcemia)
• Low magnesium level (hypomagnesemia)
• COVID-19 infection