Many people who have long QT syndrome don't have any signs or symptoms. You might be aware of your condition only because of:

• Results of an electrocardiogram (ECG) done for an unrelated reason
• A family history of long QT syndrome
• Genetic testing results

For people who do experience signs and symptoms of long QT syndrome, the most common include:

• Fainting. This is the most common sign of long QT syndrome. Long QT syndrome-triggered fainting spells (syncope) are caused by the heart temporarily beating in an erratic way. These fainting spells might happen when you're excited, angry, scared or during exercise.

You may lose consciousness without warning, for instance from being startled by a ringing telephone. If you have a normal fainting spell, you usually will have a warning sign first, such as lightheadedness, heart palpitations, irregular heartbeat, weakness or blurred vision. However, a fainting spell from long QT syndrome can occur with little to no warning.

• Seizures. If the heart continues to beat erratically, the brain will eventually not get enough oxygen, which can cause seizures.
• Sudden death. Generally, the heart returns to its normal rhythm. If this doesn't happen by itself, or if an external defibrillator isn't used in time to convert the rhythm back to normal, sudden death will occur.

Signs and symptoms of inherited or congenital long QT syndrome might start as a fetus, during the first weeks to months after birth, as late as older age, or never at all. Most people who experience signs or symptoms from long QT syndrome have their first episode by age 40.

Signs and symptoms of long QT syndrome might occur during sleep or arousal from sleep.

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

Normally 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 that 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.

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.

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, they may be 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 found 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 minor genes contribute a small percent of long QT syndrome cases.

About 20 percent of people who definitely have congenital long QT syndrome have a negative genetic test result. On the other hand, among families with genetically established long QT syndrome, between 10 percent and 37 percent of the relatives with a positive long QT syndrome genetic test have a normal QT interval.

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 parent.
• Jervell and Lange-Nielsen syndrome. This rare form usually occurs earlier and is more severe. In this syndrome, children inherit genetic variants from both parents. They have long QT syndrome and also are born deaf.

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, electrolyte abnormalities such as low body potassium (hypokalemia) or medical conditions. More than 100 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)
• Some anti-nausea medications

People who develop drug-induced long QT syndrome might also have some subtle genetic defects in their hearts. These defects make them more likely to have disruptions in their heart rhythm from certain medications.

People who may have a higher risk of inherited or acquired long QT syndrome may include:

• Children, teenagers and young adults with unexplained fainting, unexplained near drownings or other accidents, unexplained seizures, or a history of cardiac arrest
• Family members of children, teenagers and young adults with unexplained fainting, unexplained near drownings or other accidents, unexplained seizures, or a history of cardiac arrest
• First-degree relatives of people with known long QT syndrome
• People taking medications known to cause prolonged QT intervals
• People with low potassium, magnesium or calcium blood levels — such as those with the eating disorder anorexia nervosa

Inherited long QT syndrome often goes undiagnosed or is misdiagnosed as a seizure disorder, such as epilepsy. However, long QT syndrome might be responsible for some otherwise unexplained deaths in children and young adults. For example, an unexplained drowning of a young person might be the first clue to inherited long QT syndrome in a family.

Most of the time, prolonged QT intervals in people with long QT syndrome never cause problems. However, physical or emotional stress might "trip up" a heart that is sensitive to prolonged QT intervals. This can cause the heart's rhythm to spin out of control, triggering life-threatening, irregular heart rhythms (arrhythmias) including:

• Torsades de pointes — 'twisting of the points.' In this arrhythmia, your heart's two lower chambers (ventricles) beat fast and chaotically, making the waves on an ECG monitor look twisted. Less blood is pumped out from your heart, so less blood reaches your brain, causing you to faint suddenly and, often, without any warning.

If a torsades de pointes episode is short — lasting less than one minute — your heart can correct itself and you regain consciousness on your own. However, if a torsades de pointes episode lasts longer, it can result in a sudden fainting spell followed by a full-body seizure. If the dangerous rhythm does not correct itself, then a life-threatening arrhythmia called ventricular fibrillation follows.

• Ventricular fibrillation. This condition causes the ventricles to beat so fast that your heart quivers and ceases pumping blood. Unless your heart is shocked back into a normal rhythm by a defibrillator, ventricular fibrillation can lead to brain damage and sudden death.

It's now known that long QT syndrome might explain some cases of sudden death in young people who otherwise appear healthy.