Long QT syndrome: An inherited defect in heart rhythm that predisposes to syncope without warning (sudden fainting spells), dizziness, palpitations, seizures and sudden death. The name of the syndrome comes from the QT segment in the tracing on the electrocardiogram (ECG). This segment lasts slightly longer in the syndrome than normal. The heart takes longer to recharge itself between beats. Certain conditions can trigger an abnormal cardiac rhythm. Among the known triggers are intense physical exercise, swimming, being suddenly startled or badly frightened.
The diagnosis of the long QT syndrome can be made by the electrocardiogram (ECG) in about 50% of cases. However, in about 10% of cases, the QT interval on the initial ECG looks normal and in another 40% the QT interval is not sufficiently prolonged to permit a clear-cut diagnosis.
The syndrome can also be detected by identifying a mutation in one of 5 genes (KCNQ1, KCNH2, SCN5A, KCNE1, and KCNE2) that regulate the heart's electrical system. The mutation causes a defect in ion channels that predisposes the heart to lapse into a very rapid, ineffectual rhythm called fibrillation. (In about a third of cases, there is no detectable mutation in these 5 genes.)
The mutations responsible for the long QT syndrome are inherited in an autosomal dominant manner, which means that the mutant gene is on a non-sex chromosome and that each child of an affected parent has a 1 in 2 (50%) chance of inheriting it.
Early diagnosis is essential. Drugs called beta-blockers can help to maintain a normal heart rhythm in 90% of cases. In the remaining cases, a pacemaker can be implanted to set the heart's rhythm or an automatic defibrillator can be put in that can detect and correct an abnormal heart rhythm.
Children with long QT syndrome should not engage in competitive sports. Noncompetitive activities are generally all right. However, they should stop whatever they are doing if they experience any possible symptoms of the long QT syndrome.
In the US alone, it is estimated that 4,000 children and young adults die yearly of the long QT syndrome. It is a common cause of sudden death among school athletes, as well as the cause of many unexplained drownings. The first sign may, unfortunately, be sudden cardiac death. The heart muscle abruptly goes into fibrillation, beating too fast and so ineffectively that the blood stops circulating. The heart has to be defibrillated (shocked back into a normal rhythm) within a few minutes if the person is to survive.