Abstract 11929: Rare Presentation of Fetal Long QT Syndrome With 2:1 Atrioventricular Conduction

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Case Presentation: A 34 year-old G2P1 mother with Long QT syndrome Type 2 (LQTS2), history of sympathetic denervation, implantable defibrillator, and strong family history of LQTS2 including sudden death, presented to fetal cardiology at 29+3 weeks gestation with fetal 2:1 atrioventricular (AV) conduction and a ventricular rate of ≈60 bpm on fetal echocardiogram without hydrops. Findings persisted at 30+5 weeks by echo. The patient’s KCNH2 p.Met645Ile variant was previously classified as a Variant of Uncertain Significance, but segregation analysis of the family by a genetic counselor upgraded the variant to Likely Pathogenic. Prior to pregnancy, the patient was taking nadolol, then started on 2000 IU of vitamin D at 30+6 weeks due to borderline vitamin D level. Within 24 hours of initiating vitamin D, fetal magnetocardiography (fMCG) demonstrated predominant sinus rhythm with 1:1 conduction at ≈120 bpm and occasional 2:1 AV conduction. The QTc measured ≈550 msec during 1:1 conduction, and ≈720 msec during 2:1 conduction. Following the multidisciplinary diagnosis of fetal LQTS, several decisions were made. The patient declined amniocentesis, as the risk outweighed the benefit of additional genetic information. Daily home fetal heart monitoring, with goal heart rate 70-170 bpm, allowed for fewer fetal echos. Maternal vitamin D intake was liberalized to 4000 IU/day. Follow-up fMCG was recommended to exclude late-onset Torsade de Pointes, which would require transplacental treatment and complex perinatal management. Discussion with electrophysiology outlined longitudinal outcomes for the family, including potential need for neonatal pacing wires. Discussion: LQTS affects ~1/2,500 live births, with even fewer presenting with fetal AV conduction abnormalities. Optimized maternal vitamin D level may be associated with improved conduction. A multidisciplinary approach is crucial in diagnosis and management of these high-risk fetuses.