AV melformation

i was 6 years old when i had my first AVM which resulted in me having brain surgery. 8 months or so after having this surgery i had another to remove a sist that formed over top of the surgical sit. my question is how does a AVM start? what causes them to be created?  do to i was so young i did not get the chance to ask any questions about what was happening. i hope some one out there can help me out.

thanks

[rumpled]

I found this:

"Unlike brain aneurysms, there are no well established risk factors for AVM formation, growth and rupture. They are regarded as "developmental" or "congenital" vascular anomalies, i.e. you're born with them and they typically increase in size as the brain enlarges. There are rare instances of persons with multiple AVMs (Wyburn-Mason syndrome, which involves multiple central nervous system AVMs, including in the eye's retina) said to be nonhereditary, but probably due to some yet-unknown genetic event)."

I hope this helps.