How Is Long QT Syndrome Diagnosed?

Doctors' Considerations

Doctors called cardiologists diagnose and treat long QT syndrome (LQTS). Cardiologists are doctors who specialize in treating problems and diseases of the heart. To diagnose LQTS, doctors consider:

• EKG (electrocardiogram) results
• Medical history and physical exam
• Genetic test results

EKG

An EKG is a simple test that detects and records the electrical activity of the heart. This test may reveal a long QT interval and other signs that make it more likely that you have LQTS. Often your doctor first discovers you have a long QT interval when an EKG is done for another suspected heart problem.

Not all people with LQTS will always have a long QT interval on an EKG. The QT interval may change from time to time, sometimes being long and other times normal. Because of this, your doctor may want you to have several EKG tests over a period of days or weeks. Or your doctor will have you wear a device called a Holter monitor.

A Holter monitor, also called an ambulatory EKG, records the electrical activity of your heart for a full 24-hour period. For this test, you wear patches called electrodes on your chest that are connected by wires to a small, portable recorder. The recorder may be clipped to a belt, kept in a pocket, or hung around your neck. During the 24 hours, you do your usual daily activities. You then return the recorder to the doctor to read the results. The purpose of a Holter monitor EKG is to detect heart problems that may occur for only a few minutes out of the day.

Some people will only have a long QT interval on an EKG recording while they exercise. For this reason, your doctor may recommend that you have an exercise stress test.

For this test, a machine records your EKG while you exercise. If you're unable to exercise, a medicine may be injected through an intravenous line into your bloodstream to make your heart work harder and beat faster as if you were exercising.

Medical History and Physical Exam

Your doctor will ask whether you have had any symptoms of an irregular heartbeat. These include:

• Unexplained fainting
• A fluttering feeling in your chest, which is the result of your heart beating too fast
• Loud gasping during sleep

Your doctor will ask what over-the-counter, prescription, or recreational drugs you take.

Your doctor also will want to know whether anyone in your family was diagnosed with or shows signs of LQTS. Signs of LQTS include unexplained fainting or seizures, drowning, sudden cardiac arrest, or sudden death.

Your doctor will examine you and will look for the presence of conditions that may lower blood levels of potassium or sodium. These conditions include the eating disorder anorexia nervosa, excess vomiting or diarrhea, and certain thyroid disorders.

Genetic Tests

Genetic blood tests can detect some forms of inherited LQTS. If your doctor suspects that you have LQTS, he or she may recommend genetic testing. Genetic blood tests are usually also suggested for family members of people with LQTS. But about one-third of LQTS cases aren't detected with genetic testing. Even if you do have LQTS, the tests may not show it.

Also, as many as one-third of the people who test positive for LQTS don't have any signs or symptoms of the disorder. These people may have silent LQTS. Fewer than 1 out of 10 of these people will faint or suddenly die from an irregular heartbeat.

But even if you have silent LQTS, you may be more at risk than most people for developing an irregular heartbeat while taking medicines that affect potassium ion channels or blood levels of potassium.

Types of Inherited LQTS

If you have inherited LQTS, it may be helpful to know which of the seven inherited types of the syndrome you have. That way, you and your doctor can better target the treatment and lifestyle measures you need to take. To figure out your LQTS type, your doctor will consider:

• Genetic test results
• The types of situations that trigger an irregular heartbeat
• How well you respond to medicine