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Long QT syndrome (LQTS) means that the heart's electrical recovery phase takes too long, which is seen as a prolonged QTc interval on the ECG and can cause sudden fainting or cardiac arrest. The condition can be congenital via genetic changes in the heart's ion channels or acquired as a result of certain medications and electrolyte disturbances. The diagnosis is made by weighing up the ECG, blood tests, symptoms and heredity, and is treated primarily with beta-blockers, correction of underlying causes and adapted lifestyle advice.
Have you ever heard of a young person fainting while swimming, when suddenly alerted by an alarm or in the middle of an intense workout – even though the heart otherwise seems healthy? Behind such events may be long QT syndrome. It is not a blockage in the blood vessels, but rather that the heart's electrical system needs too long to recharge between beats.
What does long QT syndrome mean – when the heart's electrical recovery takes too long?
Long QT syndrome, often abbreviated LQTS, is a disorder of the heart's electrical recovery. On an ECG, it appears as a prolonged QT interval, i.e. the time from when the heart chambers are activated until they are ready for the next beat.
It is easy to think that each heartbeat is just about the heart contracting. But before the next beat, the heart muscle cells must restore the electrical balance. When this process takes too long, the risk of dangerous rhythm disturbances increases, especially a particularly fast and chaotic ventricular rhythm that can lead to fainting, seizures or sudden cardiac arrest.
LQTS can be congenital or acquired. The congenital form is most often due to genetic changes in ion channels, small “gates” in the heart cells that control the flow of sodium, potassium and calcium. The acquired form is more often triggered by medications, electrolyte disorders or other medical conditions.
What symptoms can long QT syndrome cause?
The most common symptom is fainting that comes on suddenly. It can happen during physical exertion, strong emotions or after an unexpected noise, but in some people the symptoms occur at rest or during sleep. Some describe heart palpitations or a fluttering in the chest just before they faint.
Sometimes the episodes are misinterpreted as epilepsy, because lack of oxygen to the brain can cause convulsive movements. This means that the diagnosis is sometimes delayed, especially in children and young adults with otherwise normal heart examinations.
A concrete example is the person who faints in the pool without clear warning. Another is the person who wakes up with palpitations after an alarm clock or mobile notification. In about 1 in 10, sudden cardiac arrest or sudden death can be the first sign of the disease, which explains why even short fainting episodes must be taken seriously.
At the same time, many have no symptoms at all. It is sometimes called silent or “silent” LQTS. Therefore, the disease may only be discovered when someone in the family is diagnosed or after an unexplained sudden death of a close relative.
Why does long QT syndrome occur?
In congenital long QT syndrome, there is usually an inherited change in genes that control the heart's ion channels. The most common genetic forms affect how the heart reacts to stress, rest or sleep. This is of practical importance, as different subtypes may have different typical triggers. In some forms, arrhythmias occur more often during exercise, especially swimming. In others, sudden noises or emotional stress are more typical, and in still others, events are seen more often at rest or during sleep.
Acquired long QT syndrome is more common than many people think. It can be triggered by drugs that affect the heart's electrical repolarization, such as certain antibiotics, antidepressants, antihistamines and other drugs with a known QT risk. This is why a medication review is a central part of the investigation.
The salts in the blood also play a major role. Low potassium, magnesium or calcium can prolong the QT interval and increase the risk of arrhythmia. Such disorders can occur with prolonged diarrhea, vomiting, eating disorders or certain hormonal diseases, including thyroid disorders.
The risk is also affected by gender and life situation. Women with LQTS generally have a higher risk of certain events, and the postpartum period in particular is mentioned as a period where the risk may be elevated.
How is the diagnosis made in cases of suspected long QT syndrome?
The basis is an ECG where the QT interval is measured and corrected for heart rate, so-called QTc. However, a single normal ECG does not rule out the disease, as the QT interval can vary over time. Therefore, some people need to have repeated ECGs, long-term Holter recordings or exercise tests.
The doctor also considers more than just numbers. An unexplained fainting during exertion, a family history of sudden death at a young age and the presence of QT-prolonging drugs are often as important pieces of the puzzle as the ECG curve itself.
Clinical scoring systems are used where ECG findings, symptoms and family history are combined. According to GeneReviews, referring to European guidelines, a score above 3 speaks in the absence of another explanation for the diagnosis of LQTS. At the same time, a higher risk of serious cardiac events is seen when the QTc exceeds 500 milliseconds, and the risk is considered very high at QTc above 600 milliseconds.
Genetic testing is often valuable when a hereditary form is suspected. It can confirm the diagnosis, contribute to risk assessment and make it possible to offer family screening. However, a negative genetic test does not completely rule out congenital LQTS, since not all genetic causes can yet be detected with today's tests.
In practice, blood tests for electrolytes and sometimes thyroid function tests are often also included. This is a clear example of how laboratory medicine and cardiac examination are related: an ECG shows the pattern, but blood tests can explain why it has arisen.
How is long QT syndrome treated and what can you do yourself?
The goal of treatment is not primarily to "normalize an ECG", but to prevent fainting, dangerous rhythm disturbances and sudden death. For many, beta-blockers are the basic treatment, especially for congenital LQTS. Nadolol and propranolol are often mentioned as established alternatives.
In certain subtypes, the drug mexiletine can be used as an adjunct, especially when you want to reduce the QT interval and lower the risk of serious events. Treatment is always individualized according to genotype, symptoms and risk level.
If QT prolongation is acquired, the cause must be corrected. This may mean pausing or replacing a medication, correcting potassium or magnesium, or treating the underlying disease. You should never stop taking prescription medication without a doctor's advice, as the benefits and risks must be weighed against each other.
A smaller group needs more advanced protection. In this case, an ICD, an implanted defibrillator, may be considered. If the risk remains high despite medication, left-sided sympathetic denervation can also be used, an operation that reduces the heart's sensitivity to stress.
Self-care is often about knowing your triggers and reducing unnecessary risk. This could include, for example:
- avoiding medications that can prolong the QT interval
- being extra careful if you have a stomach ache with vomiting or diarrhea
- not swimming alone if you have been diagnosed with LQTS
- informing healthcare professionals about the diagnosis before starting new medications or anesthesia
- informing close relatives about how to act in the event of fainting or cardiac arrest
For those who exercise or compete, it is not possible to give a general yes or no. Decisions about sports need to be adapted to the type of LQTS, treatment, symptoms and previous events. Therefore, physical activity should be discussed with a cardiologist, not decided on your own.
When should you seek care or further testing?
You should seek care urgently if you have fainted without a clear explanation, especially during exertion, in water or after a sudden fright. The same applies if you have had recurrent fainting spells, palpitations with dizziness or if a close relative died suddenly at a young age without a clear cause.
For many, the investigation begins at the right end with a combination of medical history, ECG and blood tests. Blood tests are relevant when you want to discover contributing factors such as low potassium or magnesium, and they can also be important when following up on treatment or other diseases that affect the heart's rhythm.
Long QT syndrome shows how closely linked blood values, medications and the heart's electrical function actually are. A seemingly small deviating test result can, in the right context, become an important clue, especially when the symptoms are vague or occur infrequently.
