Long QT syndrome (LQTS)
Long QT syndrome (LQTS) is the most common and best understood type of channelopathy. It occurs in about 1 in 2,000 people. In 70% of people with LQTS, gene testing can identify the ion channels involved. In most cases, two of the potassium channels that regulate the movement of potassium ions from the inside to the outside of the cell are affected. In a small proportion of people with LQTS, a sodium channel that regulates the flow of sodium ions from the outside to the inside of cells is affected.
In people with potassium channel associated LQTS, the channels do not behave as efficiently as normal. They let potassium ions out of the cell too slowly. If the sodium channel is affected, too many sodium ions are allowed into the cell. This results in an electrical disturbance in the cells of the heart called prolonged repolarisation. This can be seen on an ECG recording as a lengthening of the time period known as the QT interval. This is where the name long QT syndrome comes from.
Long QT syndrome types include the Romano-Ward syndrome (the commonest form) and Jervell and Lange-Nielsen syndrome (a rare form associated with deafness).
Jervell and Lange-Nielsen syndrome: Jervell and Lange-Nielsen syndrome is a condition that causes profound hearing loss from birth, in addition to causing prolonged QT interval. Jervell and Lange-Nielsen syndrome is uncommon; it affects an estimated 1.6 – 6 per 1 million people worldwide. This condition is inherited in an autosomal recessive pattern, which means that both parents must be carrying the culprit gene in order to have an affected child.
Romano-Ward syndrome: Romano-Ward syndrome is the most common form of congenital long QT syndrome, which purely affects cardiac ion channels to produce prolonged QT interval. It does not affect any other systems unlike Jervell and Lange-Nielsen syndrome. It is inherited in an autosomal dominant fashion which means that one parent with the culprit gene is sufficient to produce an affected child.
The above syndromes are only a proportion of long QT syndromes. There are now 13 recognised types of long QT, numbered LQT1 to LQT13.
LQTS varies greatly in severity. Symptoms vary according to the type of channel involved, whether the person is male or female, their age, and the length of the QT interval on the ECG. Males are more likely to have symptoms before puberty, while females are more likely to have them in adolescence and early adulthood. Relatives from the same family who have inherited the same mutation may have very different experiences. For example, some may have a normal QT interval and not have any symptoms; some may have a very abnormal QT interval but no symptoms; and some may have a very abnormal QT interval and have many symptoms that put them at risk.
The most common symptom of LQTS is blackouts. Sometimes palpitations due to extra or ectopic heartbeats can be a problem.
There are usually no physical signs of LQTS. However, certain rare forms of LQTS may be associated with muscle weakness, minor abnormalities of the skull, chin, fingers and toes; or reduced hearing.
Diagnosis involves having an ECG. Sometimes it is possible to tell which ion channel has been affected just by looking at the ECG recording. Unfortunately, in a proportion of people who might be carriers of LQTS, the ECG may not show any sign of the condition. Repeated ECGs, exercise tests and 24-48 hour tape monitoring may be needed before the diagnosis is established.
More recently some doctors have used slow injections of the hormone adrenaline (epinephrine) via a drip to try and improve the diagnosis of some potassium channel long QT syndrome. There is limited evidence however that this is any better than an exercise test.
Genetic testing can identify carriers of the LQTS gene. Unfortunately, this form of testing is limited at the moment, as 3 in every 10 people who are known to have LQTS do not have mutations of the genes known to be associated with LQTS. An additional problem is that many families who do have a mutation appear to have a specific change to the DNA code which is not found in other families (known as a ‘private’ mutation). This sometimes makes it difficult to decide whether a mutation is causing the disease or not. Things are further complicated because people with the same mutation can have no signs of it or be severely affected.
If you have LQTS, your doctor will advise you to avoid excessive exercise or strenuous athletic activities. He or she will also advise you to avoid certain drugs that can make the condition worse and which could increase the risk of blackouts and cardiac arrest.
It is also important to avoid low blood potassium levels, known as hypokalaemia.
Other treatment options will vary depending on the severity of the condition. Those with one or more of the following features will likely need more intervention than those without:
- a previous cardiac arrest
- a very long QT interval on the ECG
- specific genetic forms of the condition
- young, adult women (especially in the first year after pregnancy)
- boys before puberty
The most commonly used drugs for LQTS are beta-blockers. These block the effects of adrenaline and associated natural chemicals in the body that make the heart pump harder and faster. They therefore also block the effects of exercise on the heart. They are effective in the most common forms of LQTS as they reduce symptoms and the risk of cardiac arrest. However, they are less effective in people with the sodium channel form of LQTS. There are other more recent trends in drug treatment that look promising, but their long-term benefits are unknown. These involve using anti-arrhythmic drugs. These drugs block disturbances in the heart rhythm.
Pacemaker or implantable cardiac defibrillator (ICD)
If you are at high risk (for example if you have already had a cardiac arrest), or if drugs have failed to control your symptoms, your doctor may advise you to have a pacemaker or an implantable cardiac defibrillator (ICD) fitted, as well as taking your medication. A pacemaker and an ICD both consist of an electronic box that is inserted under the skin and attached to the heart by special electrical ‘leads’. A pacemaker controls the heart rate and stops any excessive slowing of the heart that could trigger an arrhythmia. An ICD acts in the same way as a pacemaker but it can also identify any dangerous arrhythmias and deliver an electrical shock to reset the heart.
Another option for high risk patients is to perform surgery to disrupt the nerves that release adrenaline and related chemicals at the heart, known as a sympathectomy. This is performed in only a very small number of people and involves operating on the left side of the chest.