From the last three months of research I have found from what I have read they are very rare for the most part. It seems to be something like 1 in every 10,000 or so. There are various web sites that are out there that are available, and it is best that I leave it at that, go to google and let your mind wander...
I have been diaganosed with ACM1 Arnold Charia Malformation and the odds are 1 in 1,000. Which makes it even more interesting it is 3X more likely that it will happen in women. It is a form of an aneurism in a way, there are different types of aneurisms as well. IT is thought that perhaps it is genetic. As far as ACM1 it begins in the womb while in the embyro stage, such might be the case with all aneurisms, don't take my word for it.
It seems to be a common thing that I have found that happens in both situations. Some studies feel that perhaps it is something that the mother lacks while the baby is at that stage of development.
My mother comes from a family of 13 and many of her sisters and brothers have large families and they have families and so on. I am trying to contact them and find out if this has happened to them. So far I am the only one. Lucky me.. ( LOL ). Another thing about aneurisms they can show up at the teen years, 20'3,30'3, 40's and 50's and so on. I had no idea I had this unitll this spring. I am now 47 years old. There are no real answers that I have found. I spend alot of time on the net and google and find out information on this web site as well as many others that are out there.
I have yet to scratch the surface and if by it is what you think it may be. Please go forward and research, I have found this to be very very interesting reading and research so to speak...
We all deal with things in different ways, this is the way that I deal with my situation.
Best of luck,