USUALLY congenital. Subarachnoid cysts are usually congenital ('born-with')malformations that arise from the unexplained splitting or tearing of the arachnoid membrane. Arachnoid cysts can also occur secondary to other disorders such as Marfan syndrome, arachnoiditis, or agenesis of the corpus callosum. Complications can arise: e.g. Head trauma can cause bleeding in or around the cysts possibly causing symptoms.
What do you recommend if I have a 2.82.1cm subarachnoid cyst near my fascial nerves it cause problems but dr said it wouldnt cause it?
Frequent checkups. It sounds like you have an arachnoid cyst of the posterior fossa. These are usually benign lesions in the brain that are under low pressure and usually do not cause any symptoms. Symptoms in your case might include problems with a facial droop or hearing as these nerves are in this area. You may consider having serial MRI scans (with contrast) to make sure there is no tumor and this doesn't grow.