Spontaneous Remission of Congenital Non-immune Mediated Heart Block in a Foetus at 21 Weeks Gestational Age: A Case Report

Isolated congenital Atrioventricular Block (AVB) in structurally normal hearts has an estimated prevalence of 1 in 15,000-22,000 live births. In non-immune cases, up to 50% are associated with structural abnormalities like atrial isomerism or congenitally corrected transposition of the great arteries. Rarely, AVB of unknown origin occurs in the absence of maternal antibodies, structural heart defects, trauma, metabolic disorders, or channelopathies. The natural history of idiopathic AVB remains poorly understood. A 29-year-old primigravida with no comorbidities was found to have foetal bradycardia, with atrial contractions at 144 beats per minute and ventricular contractions at 70 beats per minute during a routine anomaly scan at 21 weeks of gestation. Foetal Echocardiography (ECG) using M-mode confirmed a 2:1 AVB in an otherwise structurally normal heart. Maternal serology was negative for anti-SSA/Ro and anti-SSB/La antibodies. Repeat foetal ECG at 23 weeks showed spontaneous resolution to normal sinus rhythm. Serial prenatal assessments remained normal, and the pregnancy progressed without complications. At term (40 weeks), she delivered a healthy female neonate with a normal ECG and sinus rhythm. Postnatal follow-up confirmed normal cardiac function and development. This case underscores the potential for a benign outcome in isolated, non-progressive AVB in structurally normal hearts without maternal autoantibodies. Although the mechanisms of spontaneous resolution remain unclear, this case supports the importance of individualised evaluation and careful monitoring before initiating corticosteroids or invasive treatment. Additional case reports and prospective studies are vital to guide future management and counselling.