0471: Left juxtaposition of the atrial appendages: where are the pectinate muscles?

Abstract

Left juxtaposition of the atrial appendages (LJAA) is a rare anomaly in which the two atrial appendages (AA) are located to the left of the great arteries (GA). Although several anatomic studies have been published, the internal architecture of the right atrium (RA) in LJAA was not well described. We reviewed 21 postnatal and 5 fetal human heart specimens with L-JAA. All had normal atrial situs, concordant atrioventricular and discordant ventriculoarterial connections: 20 D-transposition, 6 L-transposition. None had heterotaxy. There were 11 transpositions of the GA, 8 double outlet right ventricle, 3 double-inlet left ventricle, 4 tricuspid atresia. The conus was always abnormal, bilateral in 22, subaortic in 4. Pectinate muscles (PM) were confined inside the right AA (RAA) in 20/26 hearts (77%, group 1) and spilled out it without extending to the crux in 6 (group 2). In 9 of group 1 and 4 of group 2, there was a small accessory RAA in normal position with PM inside it (p=ns). The only significant difference between the 2 groups was the incidence of hypoplastic right ventricle (65% in group 1 vs 0 in group 2, p<0.03). In 77% of LJAA, PM do not extend to the crux and can be completely absent of the RA wall. This suggests that PM are constitutive of the morphologically RA only if the RAA is in normal position and raises several questions concerning the development of the atria. In other terms, the extent of the PM (to the crux of the heart or not) might be determined by the situation of the RAA relative to the great vessels, and thus would not be an intrinsic property of the morphologically RA. Normal atrial situs and absence of heterotaxy could indicate that outpouching of the AA from atrial walls might occur after the establishment of the left-right asymmetry, as assessed in one of the rare experimental models of LJAA. Further experimental studies are warranted to elucidate this anatomic and embryologic enigma.