We are taught embryology of the heart, right from the first year , when we enter the medical school.We learn about various holes of the heart in a most sincere way .After 15 years, it is fascinating for a fellow to become full-fledged structural interventional cardiologist , and close an ASD with an Amplatzer device with absolute ease.
Can’t ignore the basics
How many of us are aware, there is a big disconnect between the basic science and the cath lab cardiology. When I ask a fellow what is ostium secundum ASD ? majority come out with wrong answer. Very few of us have time and interest to little deep into the different palen of IAS development. Now, let us be aware there is a host of errors in the way we have understood the embryological basis of ASD.
Image source : https://radiologykey.com/embryology-and-physiology-of-the-fetal-heart/
The first shock : The most common type of ASD, we call it as OS ASD , Is largely a defect in septum primum.
The second shock is the commonest cause of OP ASD is due to defect in AV cushion , not in septum primum
The third shock , the so called sinus venosus type of ASD is not an ASD at all, where IAS is totally intact.it is just an vascular Unroofing, between PV and SVC.
The fourth and ultimate shocker : Embryologically, there may not be anything called true septum secundum, it is just interatrial fold, we named it as septum secundum and include it as apart of part of IAS .
| The traditional Name | What it Sounds Like | What it Actually Means Anatomically |
|---|---|---|
| Ostium Secundum ASD | A defect in the septum secundum. | A hole or deficiency in the septum primum (the fossa ovalis floor). |
| Ostium Primum ASD | A defect in septum primum, that comes lower in IAS than OS ASD | It is an endocardial cushion defect at the AV junction , to precise they are defects of IAS rather Partial AVSD). |
| Sinus Venosus Defect | A true hole within the interatrial septal wall. | An unroofing/missing wall between the pulmonary vein and a vena cava (SVC or IVVC) outside the true septum. |
| Septum Secundum | An ingrowth of septal tissue left of septum primum , that forms the upper part of IAS | It is not true component of IAS. It is a thick in-folding of the outer roof wall (interatrial groove) pushed downwards and incompletely fuse with septum primum, forming the fossa ovalis in the process. |
Final message
A hole is a hole is a hole.If you have a device close it, can’t waste time to bother about the origin of it . May be you are right. 150 years of congenital heart disease , thousands of literature and advanced imaging ,interventions has helped us close these ASDs.Let us also be aware of the reality of the embryology and how we are still following the old cheat sheets .Fellows please make a note of it.
Dr.S.Venkatesan MD 