First time checking this board.. Not my particular area of cardiology, but i feel bad this question went unanswered so long.
No doubt you probably have already received the info you need and wont read this, but for other users ill do my best to reply.
The risk of an asd is dependent on the size and 18mm is pretty darn big!
The big concerns we have with asd's are complications later in life, particuarly with lifestyle factors such as smoking increasing the risk for pulmonary hypertension and shunt reversal.. And stroke from clots that form in the venous system making there way over to the left side.
At 18mm a significant amount of shunting could result in chronic hypoxemia, casing the same symptoms youd expect to find in someone with emphysema or some other serious lung disease.
Thankfully, it is absolutely treatable through surgery. Shunts (asd's or vsd's) can be corrected by ct surgery or through transcutaneous septal closure devices (a significantly less risky procedure).
However as i stated this really isn't my area of expertise, i am more heart rhythm oriented, so at 18mm i could not tell you whether it meets the criteria for ct surgery or whether they even make transcutaneous septal occluders that large. That information is something that would fall under the purvue of a pediatric cardiologist, an interventional cardiologist, or ideally a ct surgeon.
Good news is cases like these are rarely approached alone. If the size of the asd is larger then what is typically seen then the case is typically brought up in conference.
Conference is the term we use to describe weekly meetings in which basically the entire cardiology department gets together and discusses the more complex patients.
In attendence is at least 1 ct surgeon, 1 radiologist, 1 or more interventional cardiologists, an electrophysiologist, and the entire staff of fellows and physician residence, along with some nurses and techs. All in all its not uncommon for the case to be discussed among 20 or more doctors before the decision is made as to how to move forward. So rest assured whatever answer you get will likely be the best course of action possible.