Long QT syndrome is an inherited condition that affects the heart’s electrical system. The heart muscle itself is usually normal in structure. What is different is the way the heart “recharges” between beats.

Every heartbeat is triggered by an electrical signal. After each beat, the heart cells need a brief moment to reset before the next beat. On an electrocardiogram (the heart tracing done with stickers on the chest), this reset period is measured as the “QT interval” – the distance between two points on the tracing named Q and T. In long QT syndrome, this reset takes longer than it should. You can think of it like a camera flash that needs extra time to recharge; if the next signal arrives before it is ready, the heart can fall into a chaotic, fast rhythm. This dangerous rhythm can cause sudden fainting, seizure-like episodes, or, rarely, cardiac arrest.

The reset depends on tiny gates, called ion channels, in the walls of heart cells. These gates let potassium, sodium, and calcium move in and out in a carefully timed way. Long QT syndrome happens when a change (a pathogenic variant) in a gene that builds one of these gates makes it work less well — or, in some types, work too much.

Three genes account for most cases where a genetic cause is found:
KCNQ1 (type 1) Makes a potassium gate. Accounts for about 30–35% of genetically confirmed cases.
KCNH2 (type 2) Makes a different potassium gate. Accounts for about 25–30% of cases.
SCN5A (type 3) Makes a sodium gate. Accounts for about 5–10% of cases.

An international expert review in 2020 found strong evidence for only these three genes in typical long QT syndrome. A few other genes (CALM1, CALM2, CALM3, TRDN, and CACNA1C) cause rarer, often more severe forms that usually appear in infancy or early childhood. Long QT syndrome affects about 1 in every 2,000 to 2,500 people worldwide.