Background— Fetal and neonatal atrioventricular (AV) reentrant tachycardias can be life-threatening but resolve in most cases during the first year of life. The transient presence of accessory AV myocardial connections during annulus fibrosus development may explain this phenomenon.

Methods and Results— A total of 45 human embryonic, fetal, and neonatal sectioned hearts (4 to 36 weeks of development) were studied immunohistochemically. Accessory myocardial AV connections were quantified and categorized according to their specific location, and 3D reconstructions were made. Between 4 and 6 weeks of development, the atrial and ventricular myocardium was continuous at the primitive AV canal. At 6 to 10 weeks, numerous accessory myocardial AV connections were identified in the left (45%), right (35%), and septal (20%) regions of the AV junction. Most right-sided accessory connections comprised distinct myocardial strands, whereas left-sided connections consisted of larger myocardial continuities. At 10 to 20 weeks, all accessory AV connections comprised discrete myocardial strands and gradually decreased in number. The majority of accessory connections were located in the right AV junction (67%), predominantly in the lateral aspect (45%). Seventeen percent of the accessory connections were observed in the left AV junction, and 16% were observed in the septal region. 3D reconstructions of the developing AV nodal area at these stages demonstrated multiple AV node–related accessory connections. From 20 weeks until birth, and in neonatal hearts, no further accessory myocardial AV connections were observed.

Conclusions— Isolation of the AV junction is a gradual and ongoing process, and right lateral accessory myocardial AV connections in particular are commonly found at later stages of normal human cardiac development. These transitory accessory connections may act as substrate for AV reentrant tachycardias in fetuses or neonates.