Dr. Alessandro Giardini
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Long QT syndrome (often shortened to LQTS) is a condition that affects the electrical system of the heart. In simple terms, it means the heart takes a little longer than normal to reset between beats.
We see this on an ECG (electrocardiogram) as a prolonged “QT interval”. When that interval is too long, the heart becomes more vulnerable to slipping into a dangerous rhythm, called torsades de pointes.
If that happens, a child may suddenly faint. Very rarely, it can lead to cardiac arrest. This is why, although many children are completely well, the condition deserves proper attention.
Long QT syndrome is not particularly rare. It affects around 1 in 2,000 people and is an important cause of unexplained faints or sudden events in otherwise healthy children and teenagers.
In most cases, Long QT syndrome is inherited. It is caused by a change in one of the genes that control how electrical signals move through the heart.
There are a few main types, and each behaves slightly differently. Some children are more likely to have symptoms during exercise, particularly swimming. Others may be triggered by sudden noises, like an alarm going off at night. In a smaller group, events tend to happen at rest or during sleep.
Because it is inherited, there is often a family pattern, although not always an obvious one. A parent may carry the gene without ever having had symptoms.
Less commonly, the QT interval can become prolonged because of external factors. Certain medications, electrolyte imbalances (like low potassium), or acute illness can all temporarily affect the heart’s electrical system. In these cases, the problem is often reversible once the underlying cause is addressed. A very useful resource for checking medications is CredibleMeds.org.
This is usually the first question parents ask, and understandably so. Long QT syndrome affects the heart's electrical system and can, in certain situations, trigger abnormal heart rhythms. These may cause fainting or episodes that resemble seizures. Very rarely, more serious events can occur.
Perspective matters here. Once a doctor recognises the diagnosis and starts appropriate treatment, the risk of serious problems drops significantly. Many children grow up leading full, active lives with some sensible guidance.
Every child's risk profile differs depending on the specific type, symptoms and family history. Certain triggers, including intense exercise, sudden loud noises, emotional stress, fever and particular medications, can increase the likelihood of symptoms. Part of my role involves understanding these individual factors and tailoring advice accordingly.
What matters most is not the diagnosis itself, but how we manage it. With regular follow-up, appropriate treatment and clear guidance, families can move forward with confidence rather than uncertainty.
One of the tricky things about Long QT syndrome is that many children have no symptoms at all. Quite often, the diagnosis comes to light only after another family member is identified.
When symptoms do occur, they are usually quite striking. The most common one is a sudden faint. This can happen during exercise, when a child is upset, or sometimes just after being startled. It often comes without much warning, which is one of the clues that this is not a simple faint.
Some children are initially thought to have epilepsy, because a prolonged faint can be accompanied by brief jerking movements. Others describe episodes of their heart racing or fluttering.
Thankfully, cardiac arrest as a first presentation is rare, but Long QT syndrome is one of the conditions we always think about when a young person collapses unexpectedly.
If there is any doubt, this is exactly the sort of situation where I would strongly recommend a proper assessment by a paediatric cardiologist.
The starting point is always an ECG. This allows us to measure the QT interval and correct it for heart rate (QTc). In children, a QTc of around 460 milliseconds or more would raise concern, and values above 500 milliseconds are associated with a higher risk.
That said, it is not always straightforward. Some children with genuine Long QT syndrome can have a normal ECG at rest. For that reason, we often go a step further. An echocardiogram is usually performed to rule out other causes of sudden faint. A prolonged ECG recording (Holter ECG) is also usually performed to assess the QT interval over longer periods of time and to detect asymptomatic arrhtyhmias.
An exercise test can be very helpful, as it shows how the QT interval behaves when the heart rate increases and then recovers. In some cases, genetic testing is also arranged. This can confirm the diagnosis and, importantly, allows us to screen other family members.
Family screening is absolutely key. In many cases, we identify relatives who were completely unaware they carried the condition.
Hearing that your child's ECG looks "abnormal" can feel unsettling, but context matters. An ECG picks up many changes that turn out to be harmless or simply normal variations for that particular child.
The first step involves looking at the result alongside your child's symptoms, such as fainting or palpitations, and any relevant family history. A repeat ECG often helps, as findings can vary from one recording to the next.
Should the result raise concern about a condition such as Long QT syndrome, a more detailed assessment follows. This may include an echocardiogram, a 24-hour ECG monitor or an exercise test. Genetic testing sometimes forms part of the workup too.
For many children, this process leads to reassurance rather than a diagnosis. Where we confirm a condition, it allows us to put a clear plan in place covering treatment, activity guidance and long-term follow-up.
An abnormal ECG is not a diagnosis. It simply opens the door to understanding your child's heart more clearly, and in most cases the outcome is either full reassurance or a well-managed condition with a very good outlook.
The reassuring part is that, once diagnosed, Long QT syndrome is usually manageable.
Most children are treated with a beta-blocker, such as propranolol or nadolol. These medications reduce the effect of adrenaline on the heart and significantly lower the risk of dangerous rhythms. They are generally very well tolerated and form the backbone of treatment.
Alongside medication, we talk through practical day-to-day considerations. This might include being sensible around certain triggers, checking medications carefully, and making sure schools are aware of the diagnosis. Children can and should remain active, but occasionally we tailor advice depending on the specific type of LQTS.
In a small number of higher-risk cases,, particularly if a child has already had a serious event or continues to have symptoms despite medication—we may recommend an implantable defibrillator (ICD). This is a device that can automatically correct a dangerous rhythm if it occurs.
There is also a surgical option called left cardiac sympathetic denervation, which is used in selected situations.
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This is usually the first question parents ask, and understandably so. The honest answer is that, with the right treatment and a bit of awareness, the vast majority of children with Long QT syndrome grow up to live full and active lives. They go to school, play sports, and do all the normal things you would expect.
The key is recognising the condition early and putting the right measures in place.
Parents often ask whether their child has to stop sport altogether. In most cases, the answer is no. We might adapt things slightly depending on the situation, but we very rarely advise children to avoid activity completely.
Another common question is whether parents or siblings should be tested. The answer here is yes because this is an inherited condition, checking close family members is an essential part of care.
I am also often asked whether a normal ECG rules things out. Unfortunately, it does not entirely. If the story suggests Long QT syndrome, we still need to look a bit deeper.
Some parents wonder whether this is the same as Brugada syndrome. It is not. They are different conditions, although both affect the heart’s electrical system.
If your child loses consciousness suddenly and unexpectedly, you should call 999 immediately.
If they are not breathing normally, start CPR straight away. It is well worth learning basic life support as a family. Organisations such as the British Heart Foundation and St John Ambulance offer excellent courses.
In some families, we also discuss whether having a home defibrillator is appropriate.
If your child has had an unexplained faint, has been diagnosed with epilepsy without a clear cause, or there is a family history of sudden death, it is important to have a proper cardiac assessment. This is very much an area where seeing an experienced paediatric cardiologist makes a real difference. If you are unsure, I am always happy to review symptoms and advise on whether further testing is needed.
Long QT syndrome (LQTS) is a condition affecting the heart's electrical system. After each heartbeat, the heart needs a moment to reset. In children with LQTS, this reset takes longer than it should, which can occasionally cause the heart to beat dangerously fast, leading to fainting or, rarely, a cardiac arrest. Most children and teenagers with LQTS, when properly looked after, grow up to live completely normal lives.
LQTS usually runs in families. If one parent has it, each child has a 50/50 chance of inheriting it. There are different types (LQT1, LQT2, LQT3), each caused by a different gene change. Some children have a brand-new gene change that neither parent carries. A rarer form called Jervell and Lange-Nielsen syndrome is linked with deafness from birth and tends to be more serious. When a child or teenager is diagnosed, testing of parents and siblings is strongly recommended.
It is very common for children and teenagers with LQTS to have no symptoms at all. The condition is often found during a routine heart tracing, family screening, or checks before starting medicines like ADHD medication. Feeling well does not mean there is no risk. Picking it up early means your child's cardiologist can start protective treatment and give clear advice on things to avoid, well before any problems develop.
The triggers depend on which type of LQTS your child has. In LQT1, swimming, sports, and sudden physical effort are the main concerns, which is especially relevant for active children and teenagers. In LQT2, sudden loud noises (like alarm clocks or school bells), being startled, and emotional stress are common triggers. In LQT3, episodes tend to happen during rest or sleep. Across all types, certain medicines can make the condition worse, including some antibiotics and hay fever tablets. Dehydration from illness, hot weather, or not drinking enough during sport is also a risk. Always check any new medicine against the CredibleMeds website (crediblemeds.org) before giving it to your child.
Most children and teenagers are given a medicine called a beta-blocker, which calms the heart and makes dangerous rhythm problems much less likely. This is usually a lifelong daily treatment. It is very important that your child takes it every day, including on weekends and holidays, and never stops it suddenly. As teenagers take on more independence, helping them build a reliable routine with their medication is one of the most important things you can do. The dose is adjusted as your child grows.
Most children and teenagers do not need one. An ICD is usually only considered if your child has survived a cardiac arrest, continues to have episodes despite full medication, or has a particularly high-risk form of LQTS. For the majority who do well on daily medication and sensible lifestyle adjustments, an ICD is not needed.
This is one of the biggest worries for families. The answer depends on the type of LQTS, your child's individual risk, and the activity. For LQT1, competitive swimming is generally not recommended. For other types, specific triggers matter more than exercise itself. Many children and teenagers with well-managed LQTS can enjoy PE, join sports clubs, and take part in recreational activities, as long as they are on the right medication and have been individually assessed. Your cardiologist will give clear guidance so your child can stay as active as possible without unnecessary restrictions.
Yes. Parents and siblings should all have a heart tracing (ECG) and, ideally, a genetic test looking for the same gene change. Family members who test negative can be reassured. Those who carry the gene, even if they feel well and their heart tracing looks normal, should be seen by a cardiologist. This is especially important for siblings who may be going through adolescence and growth spurts themselves.
The school should have a clear written plan covering your child's diagnosis, daily medication, triggers to avoid, what to do if they faint or collapse, and the importance of calling 999 if they are unresponsive. PE teachers should know which activities are suitable and which need adjusting. Staff should ideally know basic CPR and how to use a defibrillator (AED). As your child moves through school, the plan should be updated and shared with new teachers each year. Older teenagers should also be encouraged to understand and communicate their own needs.
The outlook for children and teenagers with LQTS is good. With daily medication, avoiding problem medicines, knowing their personal triggers, and regular check-ups, the vast majority grow up to lead full, active, normal lives. As your child becomes a teenager and then a young adult, they will gradually take ownership of managing their condition, including their medication routine, understanding their triggers, and attending their own appointments. Your cardiologist will support this transition. Women with LQTS should discuss pregnancy planning with their cardiologist when the time comes.
Author: Dr. Alessandro Giardini, MD, PhD
Written 30/03/2026