How Is Long QT Syndrome Diagnosed?

Cardiologists diagnose and treat long QT syndrome (LQTS). Cardiologists are doctors who specialize in treating heart diseases and conditions. 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 heart's electrical activity. This test may reveal a long QT interval and other signs that make it more likely that you have LQTS. Often, doctors first discover a long QT interval when an EKG is done for another suspected heart problem.

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

A Holter monitor records the electrical signals of your heart for a full 24- or 48-hour period. You wear small patches called electrodes on your chest that are connected by wires to a small, portable recorder. The recorder can be clipped to a belt, kept in a pocket, or hung around your neck.

While you wear the monitor, you do your usual daily activities. You also keep a notebook, noting any symptoms you have and the time they occurred. You then return both the recorder and the notebook to your doctor to read the results. Your doctor can see how your heart was beating at the time you had symptoms.

The purpose of a Holter monitor 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 a stress test.

For this test, an EKG is done while you exercise. If you're unable to exercise, medicine may be injected into your bloodstream to make your heart work hard and beat fast, as if you were exercising.

Medical History and Physical Exam

Your doctor will ask whether you've had any symptoms of an abnormal heartbeat. Symptoms may 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 may ask what over-the-counter, prescription, or recreational drugs you take. He or she also may want to know whether anyone in your family has been 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 look for signs of conditions that may lower blood levels of potassium or sodium. These conditions include the eating disorders anorexia nervosa and bulimia, 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 usually are suggested for family members of people who have LQTS as well.

However, genetic tests can't detect about one-third of LQTS cases. So, even if you 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. Less than 10 percent of these people will faint or suddenly die from an abnormal heartbeat.

Even if you have silent LQTS, you may be at higher risk than most people for developing an abnormal 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. This will help you and your doctor plan your treatment and decide which lifestyle changes you should make.

To find out what type of LQTS you have, your doctor will consider:

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