Long QT Syndrome

Diagnosis

Tests needed to diagnose LQTS vary with each individual. To diagnose and evaluate long QT syndrome, Mayo doctors may order one or more of the following tests:

Electrocardiograms. This test studies the heart's rhythm and gauges the severity of the problem. Some people with suspected long QT syndrome have a clearly prolonged QT interval on an ECG. However others do not, making the condition more difficult to diagnose. Other testing may then be necessary.

Ambulatory ECG monitoring (Holter monitoring). This test is used to monitor the heart for rhythm irregularities during normal activity for an uninterrupted 24-hour period. During the test, electrodes attached to the chest are connected to a portable recorder that attaches to a person's belt or is carried by a shoulder strap. The recorded information can then be analyzed by computer to check for heart rhythm irregularities, such as prolonged QT intervals.

Exercise stress test. This ECG test is performed while the patient is walking on a treadmill or pedaling a stationary bike. By monitoring the heart while it is working harder, a doctor can better assess its function and limits.

Epinephrine QT stress test. This QT stress test is performed while the patient is given a medication that stimulates the heart in a way similar to exercise. The medication is given through a vein in the arm and may include epinephrine (adrenaline). In some people with long QT syndrome, fainting spells are triggered by sudden bursts of adrenaline in the body, such as are experienced during intense exercise or emotional upset.

Electroencephalogram (EEG). This test checks for neurologic causes of fainting, such as a seizure disorder. The procedure measures the waves of electrical activity the brain produces.

LQTS genetic testing. Blood tests can check for mutations in the genes currently known to cause inherited long QT syndrome. The first genetic mutations responsible for long QT were initially discovered in 1995. Over the past decade, genetic testing has been available at only a few research laboratories throughout the world, including Mayo Clinic's Sudden Death Genomics Laboratory. Mayo Clinic is a world leader in research to improve genetic testing for this disorder, playing an integral role in developing the clinical genetic test for detecting cardiac ion channel mutations. Clinical genetic testing for LQTS has been available since May 2004.