As Amy asks, did you have an echocardiogram?
I am astounded two doctors haven't heard of an aneurysm which leads me to conjecture that the ASA dx is Arginosuccine Acidemia (lab test?) also referred to ASA. ASA is one of the urea cycle disorders and is caused by deficiency of an enzyme called argininosuccinic acid lyase in the body. This prevents the conversion of argininosuccinic acid (ASA) into arginine. The build-up in ASA if too high ultimately causes a build-up in ammonia. At the same time, arginine may be deficient.
I just wanted to ditto what Amy said. I also have an ASA, with a PFO, they are both things that I was born with. (I have a bicuspid aortic valve that I was born with as well, but it's completly unrelated to the ASA and PFO.) My doctor also recommends that I take a daily low dose aspirin. I find it kind of strange that your doctors have never heard of this before, since it's actually pretty common.
Linda Hi and welcome. How where you diagnosed with an atrial septal aneurysm? Did you have an echocardiogram to find this out? Do you know the measurements of the septum itself? I believe the size should be between 0.7-1.2 cm. Did they say whether there was a defect or not? This from what I've been told is something you are born with.
I also have an ASA and was told I would have to take a low dose aspirin daily (due to a higher risk of stroke), keep my blood pressure down within normal limits (mine was high and climbs at times; but, not due to the aneurysm), keep hydrated and watch my cholesterol. I hope this helped you some. Maybe others can help you more but I just thought I would share my two cents with you. Take Care; Amy