Short QT syndrome: symptoms, diagnosis and treatment

Each heartbeat is triggered by an electrical signal that spreads through the heart muscle, and the muscle then recharges before the next beat. On an electrocardiogram (ECG) the time this takes is called the QT interval. MedlinePlus Genetics explains that in short QT syndrome the heart muscle takes less time than usual to recharge between beats, so the QT interval is abnormally short, and this can disturb the heart's normal rhythm.

myheart describes the condition as similar to but distinct from long QT syndrome. In short QT syndrome the affected potassium channel genes let too much potassium through too quickly, whereas in long QT syndrome too little gets through. The shortened reset time makes the heart prone to dangerous rhythm problems in the lower chambers (ventricular arrhythmias), and myheart notes there is also an increased risk of atrial fibrillation, an irregular and rapid rhythm from the upper chambers.

It is a type of channelopathy, meaning a condition caused by faulty ion channels in heart cells. The British Heart Foundation groups channelopathies, such as long QT syndrome and Brugada syndrome, among inherited heart conditions that can cause abnormal heart rhythms.

Short QT syndrome appears to be rare. MedlinePlus Genetics says at least 70 cases had been identified worldwide in the years after the condition was first recognised in 2000, and that it may be underdiagnosed because some affected people never have symptoms.

The Genomics Education Programme describes the cause as pathogenic gene variants that alter ion channels responsible for the electrical currents of the heart. It lists KCNH2, KCNQ1, KCNJ2, CACNA1C and CACNB2B among the genes implicated and says these are covered by a gene panel test in the National Genomic Test Directory. It states that the condition usually follows an autosomal dominant pattern, meaning there is a 50% chance of an affected parent passing it on to each child, although new (de novo) cases have been reported. In some people no gene change is found.

According to the Genomics Education Programme, people with short QT syndrome may present with palpitations, syncope (fainting) or cardiac arrest, and that the condition appears to carry high rates of lethality in all age groups. MedlinePlus Genetics says symptoms can occur from early infancy to old age and that the condition may explain some cases of sudden infant death syndrome, while some people have no problems at all. There are usually no physical signs unless atrial fibrillation is present, in which case a doctor may feel an irregular, fast pulse, according to myheart.

The NHS advice on heart rhythm problems applies here. It says to call 999 or go to A&E if you have heart palpitations together with chest pain, shortness of breath, feeling dizzy or lightheaded, or fainting or feeling like you might faint. If these symptoms have stopped, it advises asking the GP surgery for an urgent appointment or calling NHS 111. It also says not to drive yourself to A&E. The NHS heart palpitations page says to see a GP if palpitations keep coming back or if you have a history of heart problems in your family.

The first step is usually an ECG, which records the heart's electrical activity through pads placed on the skin. myheart says the abnormality may be detected on a standard ECG or a 24-hour Holter monitor, and an electrophysiological study may help the doctor reach a diagnosis. Genetic testing may find a potassium channel variant in an affected family that can then be looked for in relatives.

The ECG is interpreted together with symptoms, family history and sometimes genetic results, which is specialist work. The NHS says a GP may refer someone with palpitations to a heart specialist (cardiologist) for tests and treatment, and a family history of heart problems is one of the reasons it gives for seeing a GP about palpitations.

Family screening is a key part of the process. The Genomics Education Programme says taking a three-generation family history is important, and that first-degree relatives should be offered clinical screening, with predictive testing where the pathogenic variant is known. The British Heart Foundation likewise explains that some people with an inherited heart condition have no symptoms, which is why relatives are checked.

The Genomics Education Programme sets out the approach for clinicians. People with the condition should avoid medicines that shorten the QT interval, with nicorandil given as an example. An implantable cardioverter-defibrillator (ICD) is recommended after surviving cardiac arrest or documented sustained ventricular arrhythmia, and should be considered for arrhythmic syncope. Close monitoring with an implantable loop recorder should be considered in young children.

Medicines have a smaller role. The Genomics Education Programme says quinidine may be considered if an ICD cannot or will not be used, as an adjunct to prevent ICD shocks, to treat atrial fibrillation, as an alternative in young children, and in young people without symptoms who have a family history of sudden cardiac death. myheart says it is unclear whether it is safe to rely on medication alone, and that tablets may also be used for atrial fibrillation.

A diagnosis can be worrying, particularly when it is picked up on a family screening ECG in someone who feels well. A specialist in inherited cardiac conditions will explain the level of risk in an individual, whether a device is appropriate, and which activities need discussion. Because risk depends on the person's history and results, advice about sport, driving and work is individual and should come from the cardiac team.

Practical steps include telling every doctor, dentist and pharmacist about the diagnosis so that new medicines can be checked, keeping any ICD or loop recorder review appointments, and encouraging relatives to take up screening.

In the NHS, someone with suspected short QT syndrome is usually seen by cardiology, often a specialist inherited cardiac conditions service linked to clinical genetics, and genetic testing is provided through the National Genomic Test Directory when criteria are met. Family screening is organised through the same service.

Private cardiologists can arrange ECGs, rhythm monitoring and consultations, and some people use them for a second opinion. Implanted devices and genetic testing for inherited rhythm conditions are largely provided through specialist centres, so it helps to confirm which team will manage long-term follow-up and family screening.