Long QT syndrome is a heart rhythm disorder that can cause serious irregular heart rhythms (arrhythmias).

In a normal heart, your heart circulates blood throughout your body during each heartbeat. Your heart's chambers contract and relax to pump blood. These actions are controlled by electrical impulses created in the sinus node — a group of cells in the upper right chamber of your heart. These impulses travel through your heart and cause it to beat.

After each heartbeat, your heart's electrical system recharges itself in preparation for the next heartbeat. This process is known as repolarization.

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

Prolonged QT interval

An electrocardiogram (ECG) measures electrical impulses as they travel through your heart. Patches with wires attached to your skin measure these impulses, which are displayed on a monitor or printed on paper as waves of electrical activity.

An ECG measures electrical impulses as five distinct waves. Doctors label these five waves using the letters P, Q, R, S and T. The waves labeled Q through T show electrical activity in your heart's lower chambers (ventricles).

The space between the start of the Q wave and the end of the T wave (QT interval) corresponds to the time it takes for your heart to contract and then refill with blood before beginning the next contraction.

By measuring the QT interval, doctors can measure whether the QT interval occurs in a normal amount of time. If it takes longer than normal to occur, it's called a prolonged QT interval. The upper limit of a normal QT interval takes into account age, sex, and regularity and speed of the heart rate.

Long QT syndrome results from abnormalities in the heart's electrical recharging system. However, the heart's structure is normal. Abnormalities in your heart's electrical system might be inherited or acquired due to an underlying medical condition or a medication.

Inherited long QT syndrome

At least 17 genes associated with long QT syndrome have been discovered so far, and hundreds of mutations within these genes have been identified. Mutations in three of these genes account for about 75 percent of long QT syndrome cases, while mutations in the other 14 minor genes contribute less than 5 percent of long QT syndrome cases. About 20 percent of people who definitely have long QT syndrome have a negative genetic test result.

Doctors have described two forms of inherited long QT syndrome:

  • Romano-Ward syndrome. This more common form occurs in people who inherit only a single genetic variant from one of their parents.
  • Jervell and Lange-Nielsen syndrome. Signs and symptoms of this rare form usually occur earlier and are more severe than in Romano-Ward syndrome. It's seen in children who have long QT syndrome and are also born deaf because they inherited genetic variants from each parent.

Additionally, scientists have been investigating a possible link between sudden infant death syndrome (SIDS) and long QT syndrome and have discovered that approximately five to 10 percent of babies affected by SIDS had a genetic defect or mutation for long QT syndrome.

Acquired long QT syndrome

Acquired long QT syndrome can be caused by certain medications or medical conditions. More than 75 medications — many of them common — can lengthen the QT interval in otherwise healthy people and cause a form of acquired long QT syndrome known as drug-induced long QT syndrome.

Medications that can lengthen the QT interval and upset heart rhythm include:

  • Certain antibiotics
  • Certain antidepressant and antipsychotic medications
  • Some antihistamines
  • Diuretics
  • Medications used to maintain normal heart rhythms (antiarrhythmic medications)
  • Cholesterol-lowering medications
  • Certain diabetes medications

People who develop drug-induced long QT syndrome might also have some subtle genetic defects in their hearts, making them more susceptible to disruptions in heart rhythm from taking drugs that can cause prolonged QT intervals.

Oct. 27, 2015