PO-629-04 PHENOTYPES OF OVERDIAGNOSED LONG QT SYNDROME

Abstract

Long QT syndrome (LQTS) is a potentially lethal, yet highly treatable, genetic heart disease that predisposes individuals to arrhythmic syncope/seizure, sudden cardiac arrest, or sudden cardiac death (SCD). Although well intended, increased physician and public awareness of LQTS-associated warning signs and an increase in ECG screening programs may contribute to an overdiagnosis of this condition. To identify the various avenues or phenotypes that can lead to an overdiagnosis of LQTS. Electronic medical records were reviewed for all patients evaluated in Mayo Clinic’s Windland Smith Rice Genetic Heart Rhythm Clinic between July 2000 and March 2021 who arrived with an outside diagnosis of LQTS but were dismissed subsequently as normal. Data was abstracted for patient demographics, clinical characteristics, and cardiac and genetic test results. Overall, 291/1909 (15%) originally diagnosed LQTS patients [174 (60%) female, mean age at first Mayo evaluation 22 ± 14 years, mean QTc of 426 ± 25 ms] were dismissed as either normal (276, 95%) or having a different diagnosis altogether (15, 5%). The main cause of LQTS misdiagnosis was misinterpretation of the QTc in 93 (32%) patients, including a borderline QT, inclusion of the U-wave in QTc calculation, or prolonged QTc associated with exercise training. This was closely followed by a prolonged QTc recorded in the emergency department following vasovagal syncope (n=89 [31%]). Furthermore, 47 patients (16%) were diagnosed because of positive family history of LQTS or SCD but dismissed as normal after the SCD was found to be unrelated to LQTS or the patient was negative for family’s LQTS-associated variant. Forty-seven (16%) patients had a variant of uncertain significance (VUS) in one of the main LQTS genes (KCNQ1, KCNH2, SCN5A), however after evaluation, the variant was demoted to likely benign and/or there was no LQTS phenotype in family members. Knowing that the four main determinants of discordance between a previously rendered diagnosis of LQTS and full diagnostic reversal/removal were misinterpretation of the QTc, vasovagal syncope, family history of LQTS, and a VUS in LQTS-causative genes, awareness and screening strategies can be fine-tuned to reduce this ongoing burden of overdiagnosed LQTS.