Long QT syndrome (LQTS) is a congenital or acquired heart condition in which the QT interval (i.e., ventricular depolarization and repolarization) is prolonged. Most patients with LQTS are asymptomatic, but some present with seizures, syncope, or even life-threatening arrhythmias and sudden death. Treatment depends on the underlying cause: Beta blockers and implantable cardioverter defibrillator (ICD) insertion are commonly used for congenital LQTS, whereas treatment of the underlying cause (drug, electrolyte abnormality, etc.) is the first-line therapy for acquired LQTS.
A prolonged QT interval may be congenital or acquired.
- Congenital LQTS arises from mutations in genes that code for ion channels within myocytes (e.g., KCNE1 gene, located on chromosome 11p that encodes potassium channels in the heart and inner ear)
- These mutations all cause ventricular action potentials to be prolonged, resulting in a lengthened QT interval on ECG. 
- LQTS type 1 
- Most common type of congenital LQTS
- Loss of function mutation on the KCNQ1 gene located on chromosome 11p → defective slow delayed rectifier voltage-gated potassium channel → affected repolarization phase 
Acquired LQTS 
Drug-induced LQTS: Usually substances that block potassium outflow during the rapid repolarization phase 
- Antiarrhythmics ; 
- Antibiotics (e.g., macrolides, fluoroquinolones)
- Antihistamines (e.g., diphenhydramine)
- Antidepressants (most tricyclic antidepressants and tetracyclic antidepressants, some SSRIs, lithium)
- Antipsychotics (e.g., haloperidol, ziprasidone)
- Anticonvulsants (e.g., fosphenytoin, felbamate)
- Antiemetics (ondansetron)
- Antifungals (e.g., azoles)
- Antiparkinson medications
- Protease inhibitors
- Electrolyte imbalances: hypokalemia, hypomagnesemia, hypocalcemia
- Acute CNS insult: Ischemic stroke or intracranial hemorrhage 
- Endocrine disorders: hypothyroidism
- Nutritional deficiencies: severe vitamin D deficiency (uncommon) 
- Diagnosis of LQTS can be difficult because a slightly prolonged QT interval can be a normal variant (i.e., not congenital LQTS), and some patients with LQTS do not have a prolonged QT interval.
The primary finding in LQTS is a long QT interval corrected for heart rate (QTc) interval.
- Males: > 440 ms
- Females: > 460 ms
- Other diagnostic criteria take patient history and ECG findings into account.
- Genetic testing confirms the diagnosis of LQTS.
Both congenital and acquired LQTS
- Avoid activities that stress the heart
- Avoid cold temperatures (e.g., swimming, diving, skiing)
- Congenital LQTS
- Acquired LQTS: treat cause; (remove offending drug, fix electrolyte imbalances, etc.)
All treatment modalities aim to reduce the risk and severity of cardiac events.