Diagnosis of congenital long QT syndrome

ECG: electrocardiogram; LQTS: long QT syndrome; QTc: corrected QT interval; TdP: torsades de pointes.

* Congenital LQTS should be suspected in any patient ≤40 years of age with any of the following: 1) QT prolongation, or 2) personal or family history concerning for ventricular arrhythmia (eg, abrupt syncope, sudden cardiac arrest, TdP, generalized seizures, unexplained drowning), or 3) family history of genetically proven LQTS.

¶ Reversible causes include drugs, electrolyte disturbance, acute myocardial infarction, hypothyroidism, hypothermia, and anorexia nervosa. Refer to UpToDate content about acquired LQTS for details.

Δ QT prolongation refers to QTc >99th percentile (ie, QTc ≥460 ms in prepubertal patients, QTc ≥470 ms in postpubertal males, ≥480 ms in postpubertal females), assuming a normal QRS width <120 ms.

◊ Further workup is indicated for many patients with a reversible cause, even if the QTc normalizes, because underlying congenital LQTS may be present. It is important to address the reversible cause before performing an exercise treadmill test because the resting QTc will impact the validity of the test results.

§ The Schwartz score incorporates personal and family history with findings on testing. Genetic testing is indicated for patients with a score >1. Refer to UpToDate content about the diagnosis of congenital LQTS for details.

¥ Patients with a reversible cause of QT prolongation who meet at least 2 criteria are very likely to have a pathogenic disease variant for congenital LQTS.

‡ Refer to UpToDate content about the interpretation of genetic test results.