A 22-year-old primigravida was referred for fetal echocardiography at 24 weeks' gestation given suspicion of fetal tachyarrhythmia on screening. The four-chamber view showed moderate pericardial effusion, hyperechoic endocardium, papillary muscles, and both atrioventricular valve annuli [Figure 1a and b]. The heart was otherwise structurally normal. There was ventricular diastolic dysfunction as evident in the left ventricle inflow-outflow Doppler (presystolic flow in ascending aorta) [Figure 2a and b] with no atrioventricular valve regurgitation.
Left ventricle inflow-outflow Doppler showed periods of fetal sinus bradycardia [Figure 3] alternating with frequent conducted atrial ectopics triggering episodic atrial tachyarrhythmia. Left brachiocephalic vein-aorta Doppler showed the beginning and termination of the tachyarrhythmia. It was a 1:1 long VA tachyarrhythmia (VA interval 600 ms, AV interval 267 ms) triggered by an atrial ectopic, and spontaneous termination of the tachyarrhythmia followed by an atrial ectopic and bradycardia [Figure 4]. Hence, a diagnosis of TBS was made. There was no evidence of atrioventricular block. Maternal serum SSA and SSB antibody levels were elevated 145 U (normal <20) and 181 U (normal <20), respectively. There was no family history of sinus node dysfunction.
Transplacental therapy with oral dexamethasone at a dose of 4 mg/day was given initially for 7 days. Reassessment after 1 week showed absent pericardial effusion and improved ventricular diastolic function [Figure 5]. However, fetal bradycardia, conducted atrial ectopics, and episodic atrial tachycardia persisted. Hence, oral dexamethasone dose was increased to 8 mg/day. Reassessment after a week showed no further improvement.