Long QT syndrome (KCNQ1 and other genes): inherited heart rhythm condition

The QT interval is the stretch of an electrocardiogram that reflects the heart's electrical reset between beats. In long QT syndrome this interval is prolonged, creating a vulnerable period in which a dangerous rhythm can start. The NHS describes it as a rare condition that can cause serious rhythm problems, though there are things that can be done to reduce the risk.

Many people do not know they have it. The NHS says that many people with LQTS have no symptoms and that it is sometimes discovered when an ECG is done for another reason. Where symptoms occur they include palpitations, dizziness, fainting, a seizure and, in the most serious cases, cardiac arrest. The NHS Genomics Education Programme notes that the first presentation can be cardiac arrest or sudden death, that symptom burden rises at puberty, and that risk differs by sex, age and genetic subtype.

A long QT interval is not always inherited. The NHS lists causes that include certain medicines such as some antibiotics, antihistamines, antidepressants and heart medicines, other heart problems, very low potassium, magnesium or calcium, a brain bleed or stroke. Doctors must exclude these before concluding it is inherited.

The majority of people who test positive carry a variant in one of three genes, each defining a type of LQTS. Beyond these, rarer subtypes have distinctive features, such as Andersen-Tawil syndrome (KCNJ2) with periodic paralysis, Timothy syndrome (CACNA1C), and Jervell and Lange-Nielsen syndrome, a recessive form in which QT prolongation is accompanied by sensorineural hearing loss.

Most LQTS is autosomal dominant. A person who has it carries one changed and one working copy of the gene, and each child has a 1 in 2 chance of receiving the changed copy. Sons and daughters are equally likely to inherit it. The NHS Genomics Education Programme says penetrance is high but incomplete, so some carriers never develop the condition, while others in the same family are more strongly affected, probably because of modifier genes and silent carriers.

In the rare recessive form, such as Jervell and Lange-Nielsen syndrome, both parents are carriers. Each pregnancy then carries a 1 in 4 chance of an affected child, a 1 in 2 chance of a healthy carrier and a 1 in 4 chance of a child who is neither.

Taking a three-generation family history is an essential part of genetic counselling for LQTS, according to the programme. Unexplained sudden deaths or faints in the family are part of that history.

A GP who suspects LQTS refers to a cardiologist. The NHS says diagnosis uses an ECG, possibly with a 24-hour heart monitor or an ECG during exercise, and that a genetic test may be offered because the condition is usually inherited. If you are diagnosed, the NHS says your immediate family should also be offered testing. It also advises asking for an urgent GP appointment if your parent, child, brother or sister has been diagnosed with LQTS or died unexpectedly.

The NHS Genomics Education Programme says that depending on the quality of the clinical picture, a genetic result is found in roughly 60% of those tested, so a normal gene test in someone with a clearly long QT does not exclude the condition.

The NHS says beta blockers, which help control heart rhythm, are recommended for most people with LQTS. An implantable cardioverter defibrillator is recommended if the risk of serious problems is high, and surgery that cuts certain nerves affecting the heart is only for rare cases when other treatments are unsuitable. The NHS Genomics Education Programme adds that mexiletine is useful in type 3, and that a defibrillator is advised after a survived cardiac arrest or arrhythmic fainting despite optimal beta blockade.

• Check with a doctor or pharmacist before starting any medicine or supplement, because some can lengthen the QT interval. The NHS Genomics Education Programme refers people to the CredibleMeds list of drugs to avoid.

• Keep well hydrated if you have vomiting or diarrhoea, using oral rehydration solution, and have a low threshold for going to hospital.

• Eat foods containing potassium; a doctor may recommend supplements.

• Avoid triggers specific to your type: for LQTS 1 this includes cold-water swimming and diving.

• Avoid sudden noises such as alarm clocks, energy drinks with a lot of caffeine, and strenuous exercise or sport including swimming (NHS advice).

• Ask for an urgent GP appointment or call NHS 111 if you have fainted and recovered, if palpitations keep returning or last more than a few minutes, or after a seizure.

The NHS signposts the British Heart Foundation and Cardiac Risk in the Young for support. Families often benefit from an inherited cardiac conditions clinic, where cardiologists and genetic counsellors see relatives together.