Early in development, the atria are one single chamber. The sinus venosus is the posterior entryway for blood returning to this primitive atrium.
Eventually, the sinus venosus closes and moves rightward due to hemodynamic shifts during development.
In adults, the sinus venous becomes the smooth posterior wall of the adult right atrium called the sinus venarum and is separated from the anterior wall of the RA by the cristae terminalis.
If a persistent channel through the sinus venosus remains into adulthood, it can result in an intra-cardiac shunt. This is termed a sinus venosus defect and accounts for 10-15% of all inter-atrial shunts.
As above, sinus venosus defects are associated with PAPVR (with RUPV → SVC).
📝 a sinus venosus defect is NOT a defect in the atrial septum and so is not an “ASD”. Rather it is a defect connecting either the SVC-RA junction (more common) or the IVC-RA junction to the LA. The former is associated with a RUPV PAPVR and the latter is associated with a RLPV PAPVR.
Management
Surgery
ACC/AHA guidelines recommend closure of a sinus venosus defect if:
PA systolic pressure is < 50% systemic pressures AND
PVR is <1/3 of SVR.
⚠️ Class III recommendation (potentially harmful) to close a defect if PA systolic pressure is >2/3 of systemic systolic pressure and/or PVR >2/3 SVR.
Surgical correction is:
Class I (B-NR) if the systolic PA pressure is less than 50% of the systemic pressure and the PVR is less than one third of the SVR.
Class III, or potentially harmful, (C-LD) if the PA systolic pressure is greater than 2/3 of the systemic systolic pressure or if the PVR is greater than 2/3 of the SVR and/or if there exists a right to left shunt.
Those with PA systolic pressures between 50% and 2/3 systemic pressures and PVRs between 1/3 and 2/3 should be considered for repair on a case-by-case basis.