Aneurysm actually refers to ballooning / dilatation due to weakness in the wall of a vascular structure.
Aneurysm of the Inter atrial septum (IAS ) would result from pressure differences across the septum, due to valvular abnormalities( Like Mitral regurgitation , MVP ).
In the absence of these accompanying disorders, IAS aneurysm could be due to a primary weakness in the septum.;
This aneurysm is not like a vascular aneurysm , which will bleed profusely , if it bursts.
if IAS aneurysm gives away, then it would cause mixing of blood between left and right atria, ( Shunting ). This would not be life threatening, however, would need to be repaired in the cathlab.
Apart from this, you should monitor this lesion by periodic echocardiography to rule out further changes.
Hope you understand now ?
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