Long QT Syndrome

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Long QT Syndrome

What is Long QT Syndrome?

Long QT syndrome is a heart condition that causes problems in the rhythm of the heartbeat. It is usually diagnosed by an electrocardiogram (EKG or ECG) of the heart.

Signs and Symptoms

People with long QT syndrome may have:

  • Fainting, especially during exercise
  • Sudden cardiac arrest (when the heart suddenly stops beating)
  • Seizure-like episodes
  • Near-drowning (from fainting while swimming)
  • Sudden death 
  • No symptoms at all

It is most common for symptoms to develop during the teens or 20s, but heart problems can show up anytime between infancy and later adulthood.


The genetic form of long QT syndrome is caused by a change in a gene that helps control the way the heart works. These gene changes – called mutations – are usually inherited. Only one gene has to have a mutation for a person to have long QT syndrome. People with inherited long QT syndrome don't usually have any other health problems or differences in their bodies because of this gene change. Long QT syndrome does not affect intelligence.

Long QT syndrome can sometimes be "acquired." This means that a person has a prolonged QTc interval that is temporary and not genetic. Acquired long QT syndrome can be caused by heart failure or a slow heart rhythm due to another health problem. Low potassium levels in the blood and certain drugs can also cause it. An acquired type of long QT syndrome should be ruled out before having genetic testing.

Sometimes, long QT syndrome is just one part of a disease that also causes other symptoms, like deafness, facial differences, or muscle weakness. These rare genetic conditions include Jervell and Lange-Nielsen Syndrome, Timothy Syndrome, and Anderson-Tawil syndrome.

Next: Genetics of Long QT Syndrome

Why are you considering genetic testing?

I think I might have long QT syndrome, but I haven't seen a doctor yet.

I have long QT syndrome but have not yet had genetic testing.

I have a family member who has a gene mutation that causes long QT syndrome.

I have a family history of definite or possible long QT syndrome, but no one has had genetic testing yet.

The symptoms you may feel from long QT syndrome are very similar to symptoms caused by many different heart conditions. People with symptoms of heart disease may have an electrocardiogram (ECG) to record the rhythm of the heart, an echocardiogram to look at the heart using ultrasound, or other tests.

Genetic testing shouldn't be considered until you have been diagnosed with long QT syndrome and all of the other known causes have been ruled out.

Genetic testing may be useful for you.

  • If testing finds a mutation, the results confirm that you have inherited long QT syndrome. Knowing information about your long QT syndrome mutation can help your doctor treat your condition and prevent complications. Mutation information may:
    • Impact your treatment decisions
    • Tell more about the chance for severe heart problems and which triggers that you should avoid
    • Allow for single mutation testing in your at-risk family members. This is less expensive and more accurate than complete long QT syndrome testing.
  • If testing doesn't find a mutation, the results don't rule out that you have inherited long QT syndrome. Genetic testing cannot find all of the gene mutations that cause long QT syndrome. You and your family members still need to have earlier and more frequent heart screening.

Genetic testing may be useful for you.

  • If testing finds the mutation that runs in your family, you can be certain that you have inherited a tendency to develop long QT syndrome. The symptoms of long QT syndrome can be different between people in the same family. Some people have the mutation but never get symptoms at all. So, your results don't necessarily predict your future, but they do mean that you may need heart screening and possibly medicine to prevent heart problems. Your results give you information to make decisions about using medicine or other treatments, getting regular checkups with your doctor, and avoiding activities that might trigger symptoms.
  • If testing doesn't find a mutation, you can be sure that you did not inherit the mutation that runs in your family. As long as your heart appears to be healthy, you won't need any extra heart screening or medicine.

Genetic testing may be useful for you, but you may not be the best person to test first.

The first person in the family to have genetic testing should be someone most likely to have long QT syndrome. If your relative(s) that might have long QT syndrome aren't available or willing to have genetic testing, you can have testing but the meaning of your results will be less clear.

  • When testing finds a mutation, then it is likely that you have inherited a tendency to develop long QT syndrome. You would need heart screening and you may need to take medicine to prevent heart problems.
  • When testing doesn't find a mutation, there are two possible explanations – you may not have inherited the mutation that causes long QT syndrome in your family; or the mutation that causes long QT syndrome in your family cannot be found with the test that was done. In this situation, a normal result doesn't change your medical care. You are still considered at risk to develop long QT syndrome and you need to have heart screening.
  • Testing can also find changes in the gene that haven't been seen before. It may not be clear whether they cause long QT syndrome. In this case, you should have heart screening, but you can't be clearly diagnosed with long QT syndrome.