Long QT syndrome (LQTS) is caused by either inherited gene mutations or outside factors like medications and electrolyte imbalances that disrupt the heart’s electrical recovery between beats. About 1 in every 2,000 to 2,500 live births are affected by the congenital form, and many more people develop the acquired form at some point in life. Both types share the same core problem: the heart’s electrical system takes too long to reset after each beat, creating a window where dangerous, irregular rhythms can develop.
How the Heart’s Electrical Reset Gets Delayed
Every heartbeat involves a wave of electrical activity that triggers heart muscle cells to contract, followed by a recovery phase called repolarization. During repolarization, charged particles (mainly potassium, sodium, and calcium) flow through tiny channels in the cell membrane to restore the cell’s resting electrical state. The QT interval on an EKG measures the total time from the start of contraction to the end of this recovery.
In LQTS, something slows that recovery down. Either potassium channels don’t let enough potassium out of the cell, or sodium channels let too much sodium in during the recovery phase. Both situations keep the cell electrically active longer than it should be. When repolarization drags on, heart cells become vulnerable to firing again before they’re fully reset, which can trigger a rapid, chaotic rhythm called torsades de pointes. That rhythm can cause fainting, seizures, or sudden cardiac arrest.
Genetic Mutations Behind Congenital LQTS
The inherited form of LQTS traces back to mutations in genes that build the heart’s ion channels. Three genes account for the vast majority of cases, and each produces a distinct subtype with its own pattern of risk.
- Type 1 (LQT1) results from mutations in the KCNQ1 gene, which builds potassium channels responsible for a slow outward current during repolarization. When these channels malfunction, potassium leaves the cell too slowly. Physical exercise and emotional stress are the primary triggers for dangerous rhythms in this type, and swimming is a particularly well-known trigger.
- Type 2 (LQT2) stems from mutations in the KCNH2 gene, which builds potassium channels that carry a faster repolarizing current. Sudden loud noises, like alarm clocks or car horns, and emotional surprise are classic triggers for cardiac events in people with LQT2.
- Type 3 (LQT3) involves gain-of-function mutations in SCN5A, a gene that builds sodium channels. Instead of shutting off quickly after the heartbeat begins, these mutant channels allow a sustained trickle of sodium into the cell, prolonging the electrical activity. People with LQT3 most often develop arrhythmias during sleep or rest, when the heart rate is slow.
Types 1 and 2 involve loss-of-function mutations, meaning the potassium channels don’t work well enough. Type 3 is the opposite: a gain-of-function mutation that makes sodium channels overly active. The end result is the same, a delayed electrical reset, but the different mechanisms explain why each type has different triggers and responds differently to treatment.
Two Inheritance Patterns
Most congenital LQTS follows an autosomal dominant pattern, meaning a child needs only one copy of the mutated gene (from one parent) to be affected. This is called Romano-Ward syndrome, and people with it have normal hearing.
A rarer and more severe form, Jervell and Lange-Nielsen syndrome, follows an autosomal recessive pattern, requiring mutated copies from both parents. Children with this form have congenital deafness along with significant QT prolongation. First described in 1957, it carries a higher risk of sudden cardiac death in childhood and tends to respond less well to standard treatments like beta-blockers. Because it requires two copies of the mutation, it is far less common than Romano-Ward syndrome.
Medications That Prolong the QT Interval
Drug-induced QT prolongation is the most common acquired cause of LQTS. Dozens of medications can interfere with the heart’s potassium channels in the same way genetic mutations do, and the risk is highest when multiple QT-prolonging drugs are combined or when other risk factors like electrolyte imbalances are present.
High-risk medications, those associated with QT prolongation of more than 60 milliseconds above baseline, span several drug classes. Heart rhythm drugs like sotalol, dofetilide, quinidine, and procainamide are among the most potent offenders. Certain psychiatric medications also carry significant risk, including haloperidol (especially given intravenously), ziprasidone, quetiapine, and chlorpromazine. The opioid methadone is another well-established cause. Some antibiotics and antifungals fall into a moderate-risk category, including azithromycin, clarithromycin, erythromycin, and fluconazole, typically prolonging the QT interval by 20 to 59 milliseconds.
The antidepressants citalopram and escitalopram carry moderate QT risk, as do certain cancer drugs like dasatinib and crizotinib. Even some older antihistamines, like terfenadine and astemizole, were pulled from most markets specifically because of QT prolongation and sudden death risk.
If you carry a genetic predisposition to LQTS (even a silent one), a QT-prolonging medication can be the factor that pushes the interval into a dangerous range. This is one reason why some people experience a cardiac event on a medication that millions of others take without problems.
Electrolyte Imbalances
The heart’s ion channels depend on the right concentrations of minerals in the blood. Three electrolyte deficiencies can directly cause or worsen QT prolongation:
- Low potassium (hypokalemia) reduces the driving force for potassium to leave heart cells during repolarization, mimicking the same problem caused by genetic potassium channel mutations.
- Low magnesium (hypomagnesemia) impairs the function of several ion channels and often accompanies low potassium, compounding the effect.
- Low calcium (hypocalcemia) prolongs the plateau phase of the heart’s electrical cycle, delaying the overall recovery.
These imbalances can come from many sources. Severe vomiting or diarrhea depletes potassium and magnesium quickly. Eating disorders like anorexia nervosa cause chronic mineral deficiencies that put the heart at ongoing risk. Certain diuretics (water pills), laxative overuse, and heavy sweating without adequate replenishment can also shift electrolyte levels enough to prolong the QT interval. In these cases, correcting the underlying imbalance usually resolves the prolongation.
Other Medical Conditions
Several health conditions create the right environment for QT prolongation even without genetic mutations or offending medications. Hypothyroidism (an underactive thyroid) slows metabolism broadly and can slow cardiac repolarization as well. A very slow heart rate, called bradycardia, stretches out the interval between beats and can unmask or worsen QT prolongation.
Head injuries, stroke, and other conditions affecting the nervous system can also alter the heart’s electrical patterns through surges of stress hormones. These causes are less common but important to recognize because treating the underlying condition can reverse the QT prolongation.
Why Some People Are More Vulnerable
QT prolongation often results from a combination of factors rather than a single cause. Someone with a borderline genetic mutation they’ve never been diagnosed with might live their entire life without symptoms, until they take a QT-prolonging antibiotic during a bout of diarrhea that has already lowered their potassium. That combination of genetic susceptibility, medication, and electrolyte imbalance can push the QT interval past a critical threshold.
Women naturally have slightly longer QT intervals than men, which is reflected in diagnostic thresholds. Using the most common correction formula, a QT interval above 450 milliseconds in men or above 460 milliseconds in women is considered significantly prolonged. Sex hormones appear to account for this difference, which is why the gap emerges after puberty.
Age matters too. In congenital LQTS, the risk profile shifts over a lifetime. Children with LQT1 face the highest risk during physical activity, while adults with LQT2 or LQT3 may not have their first event until they encounter a medication trigger or develop an electrolyte imbalance decades later. The Jervell and Lange-Nielsen form tends to present earliest, often before age five, with the most severe QT prolongation and the highest rate of cardiac events in childhood.