Long QT syndrome is a disorder that causes electrical dysfunction in the heart. It is characterized by an abnormally long heartbeat interval on the electrocardiogram (ECG or EKG). An ECG depicts heartbeats as waves. Each heartbeat is divided into several phases and each one is assigned a letter: P, Q, R, S and T. Long QT is an abnormally long interval between points Q and T.
What is an electrocardiogram?
It is a test that records the heart’s electrical activity over a given period of time. It identifies the speed at which the heart is beating (heart rate), whether it is beating normally (heart rhythm) and the effectiveness of the heart muscle. An electrocardiogram is also called an ECG or EKG.
There are two types of long QT syndrome:
- Congenital long QT syndrome, a genetically transmitted condition
- Acquired long QT syndrome, caused by certain medications or electrolyte disturbances (e.g. low potassium levels in the body)
Useful information
Electrolyte balance refers to the equilibrium the body must maintain between its water content and the minerals it needs. Extreme vomiting, diarrhea and eating disorders can result in electrolyte imbalance.
Consequences
Long QT syndrome leads to severe ventricular rhythm disorders with palpitations, loss of consciousness and even cardiac arrest (sudden cardiac death). These events are triggered by physical activity, emotional stress, auditory stress and sometimes by medication that prolongs the QT interval.
Causes
People with long QT syndrome have a structurally normal heart. The dysfunction is “purely electrical”.
Acquired long QT syndrome is caused by:
- certain types of medication such as antidepressants, antipsychotics, antibiotics and diuretics;
- electrolyte disturbances. Extreme vomiting, diarrhea and eating disorders can result in electrolyte imbalance. This refers to an imbalance in the equilibrium the body must maintain between its water content and the minerals it needs.
Congenital long QT can also have a genetic origin. In 95% of cases, it can be transmitted to children if one of the parents carries the syndrome, since it follows an autosomal dominant pattern. The abnormal gene can be transmitted to children with a 50% risk of each child, both girls and boys, developing this disorder.
The mutation of certain genes is responsible for disrupting the electrical activity of heart cells. To date, around a dozen genes have been identified as responsible for the syndrome. The five main genes are: KCNQ1 (LQT1), HERG (LQT2), SCN5A (LQT3), KCNE1 (LQT4) and KCNE2 (LQT5). They are associated with various known forms of the disorder: long QT syndromes numbered 1 to 5, LQT1 to LQT5.
Symptoms
Certain patients may not exhibit any symptoms. In this case, the syndrome is only identified based on the abnormally long QT interval observed on an ECG. It can therefore be discovered by chance.
When symptoms do occur, they are often intermittent and infrequent. These symptoms are caused by heart rhythm disorders, such as faintness with or without loss of consciousness. Some episodes of loss of consciousness are associated with seizures. These events are triggered by physical activity, emotional stress, auditory stress and certain types of medication, but this varies from person to person.
Useful information
Restrictions on certain sports activities, such as swimming, vary from patient to patient. The cardiologist will discuss these restrictions with the patient.
Diagnosis
The diagnosis is based on analysis of an electrocardiogram, along with the measurement of the QT interval and QTc, a heart-rate corrected QT interval. The QTc interval is considered abnormally long when it is over 470ms in men and over 480ms in women.
The T-wave appearance, which is sometimes abnormal depending on the mutated gene, as well as the presence of symptoms and family history, can help in the diagnosis
Treatment
All patients should take beta-blockers, whether or not they have symptoms, since they significantly reduce the risk of faintness with loss of consciousness (syncope) and sudden cardiac death.
Implanting a cardioverter-defibrillator may be suggested in cases of recurrent syncope despite beta-blocker treatment, or after surviving sudden cardiac death. It is surgically implanted under the skin and is connected to the heart with a lead.
Implantable cardioverter-defibrillators continuously monitor the heart rhythm and automatically deliver an electric shock if an abnormal rhythm is detected. The shock helps “resynchronize” the heartbeat, allowing for effective heart contractions and restoring a regular rhythm.
Important information
People with long QT syndrome should be provided with a list of medications that are contraindicated (medications that prolong the QT interval). If another medication is prescribed, it is important to first check whether it presents a risk.
Reviewed and approved by Frédéric Sacher, Professor, MD, PhD, Cardiologist at Bordeaux University Hospital (CHU)
