Anatomy of the inferior interatrial route in humans

To explore the morphology of the proximal coronary sinus (CS) and the surrounding tissues in order to identify possible routes for interatrial conduction.

Specimens containing interatrial septum and proximal CS were taken from 21 necropsied hearts and sliced into 10-microm thick parallel histological sections in 1-mm steps starting from the valve plane, up to the atrial roof (40-80 sections per heart). The sections were stained with van Gieson's stain.

Media in the proximal CS consists of smooth muscle cells that do not form a continuous layer. CS was not surrounded by striated atrial myocardium in 10 specimens in which posterior CS wall was covered by epicardial fat only. In seven specimens, striated muscle bundles of up to 2-mm width connected the myocardium surrounding the CS with the left atrium. Regardless of their presence, variable posterior and/or anterior interatrial muscular connections were identified in all specimens.

Variability of the striated atrial myocardium surrounding proximal CS may affect interatrial conduction. Striated muscular fascicles connecting the proximal CS with the left atrium are not obligatory cardiac structures and may be considered as supplementary to the larger interatrial connections outside the CS.