Long QT syndrome is an abnormality of the heart's electrical system (see Abnormal Heart Rhythms: Overview of Abnormal Heart Rhythms), which may cause loss of consciousness or sudden death.

• Long QT syndrome can be caused by a genetic abnormality, drug use, or a disorder.
• This syndrome causes the heart to beat unusually fast, which can lead to sudden unconsciousness.
• Stress tests and electrocardiography can help confirm the diagnosis.
• Beta-blockers and pacemakers are the best forms of treatment, but some people may benefit from surgery.

The QT is an interval between two points on an electrocardiogram (see Diagnosis of Heart and Blood Vessel Disorders: Electrocardiography). People with long QT syndrome have a prolongation of the QT interval. Long QT syndrome may affect as many as 1 of 7,000 people. In the United States, it may cause sudden death in 3,000 to 4,000 children and young adults each year. In children, this disorder is usually due to a genetic abnormality. Specific tests that evaluate for the most common genetic causes are now available. A person with the disorder may have family members who died suddenly and inexplicably. In most adults, long QT syndrome is caused by use of a drug or by a disorder.

People who have long QT syndrome are predisposed to developing an unusually fast heart rate, which often occurs during physical activity or emotional excitement. When the heart rate is too fast, the brain may not receive enough blood. The result is loss of consciousness and sometimes sudden death. Some people with long QT syndrome are also born deaf.

Doctors may recommend electrocardiography (ECG) for children or young adults who have suddenly and inexplicably lost consciousness. The procedure may be performed while the person is resting or after receiving drugs given by vein. The person also may be asked to walk on a treadmill or pedal an exercise bicycle in a procedure called exercise stress testing.

Beta-blockers are effective for most children and adults. For children and adults who do not respond to drugs, a pacemaker or a combination pacemaker-internal defibrillator may be tried. An internal defibrillator can shock the heart, reviving the person, whenever the heart develops a lethal rhythm abnormality. Occasionally, as an alternative, a nerve in the neck is cut in a procedure called cervicothoracic sympathectomy. Cutting this nerve can help prevent the fast heart rate that causes sudden death. For some children, restriction from competitive sports may be recommended.

Last full review/revision December 2008 by Gregory S. Liptak, MD, MPH