Long QT syndrome (LQTS) is a condition affecting repolarization (relaxing) of the heart after a heartbeat, giving rise to an abnormally lengthy QT interval. It results in an increased risk of an irregular heartbeat which can result in fainting, drowning, seizures, or sudden death. These episodes can be triggered by exercise or stress. Some rare forms of LQTS are associated with other symptoms and signs including deafness and periods of muscle weakness.Long QT syndrome may be present at birth or develop later in life. The inherited form may occur by itself or as part of larger genetic disorder. Onset later in life may result from certain medications, low blood potassium, low blood calcium, or heart failure. Medications that are implicated include certain antiarrhythmics, antibiotics, and antipsychotics. LQTS can be diagnosed using an electrocardiogram (EKG) if a corrected QT interval of greater than 480–500 milliseconds is found, but clinical findings, other EKG features, and genetic testing may confirm the diagnosis with shorter QT intervals.Management may include avoiding strenuous exercise, getting sufficient potassium in the diet, the use of beta blockers, or an implantable cardiac defibrillator. For people with LQTS who survive cardiac arrest and remain untreated, the risk of death within 15 years is greater than 50%. With proper treatment this decreases to less than 1% over 20 years.Long QT syndrome is estimated to affect 1 in 7,000 people. Females are affected more often than males. Most people with the condition develop symptoms before they are 40 years old.
This also applies to some genetic variants which by themselves only minimally prolong the QT interval but can make people more susceptible to significant drug-induced QT prolongation. Mechanisms
The various forms of long QT syndrome, both congenital and acquired, produce abnormal heart rhythms (arrhythmias) by influencing the electrical signals used to coordinate individual heart cells. The common theme is a prolongation of the cardiac action potential – the characteristic pattern of voltage changes across the cell membrane that occur with each heart beat. Heart cells when relaxed normally have fewer positively charged ions on the inner side of their cell membrane than on the outer side, referred to as the membrane being polarised. When heart cells contract, positively charged ions such as sodium and calcium enter the cell, equalising or reversing this polarity, or depolarising the cell. After a contraction has taken place, the cell restores its polarity (or repolarises) by allowing positively charged ions such as potassium to leave the cell, restoring the membrane to its relaxed, polarised state. In long QT syndrome it takes longer for this repolarisation to occur, shown in individual cells as a longer action potential while being marked on the surface ECG as a long QT interval.The prolonged action potentials can lead to arrhythmias through several mechanisms. The arrhythmia characteristic of long QT syndrome, Torsades de Pointes, starts when an initial action potential triggers further abnormal action potentials in the form of afterdepolarisations.