We are taught embryology of the heart, right from the first year , when we enter the medical school.We learnt about various holes of the heart in a sincere way .After 15 years or so , 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
Meanwhile, how many of us are aware, there is a big disconnect between basic science and the cath lab cardiology. When I ask my fellows, what is the origin ostium secundum ASD ? majority come with wrong answer. Very few of us have time and interest to go deep into the different layers 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/
Misnomers galore
The first one : The most common type of ASD, what we call, it as OS ASD , Is actually a defect in septum primum.
The second is OP ASD is not due to defect in septum primum , but due to defect in AV cushion.
The third : 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 an 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 as a restless Interventional cardiologist. 150 years of congenital heart disease , thousands of literature and advanced imaging ,interventions, can’t be Ignored. Let us be aware of the reality of embryology and how we are still following the old cheat sheets .Fellows please make a note of it.
Dr.S.Venkatesan MD 