Re: subarachnoid cyst
How was it diagnosed? Were you exhibiting symptoms before you had the MRI or CT scan. I also have a arachnoid cyst/Chiari Malformation that extends from my cerebellum into my cisterna magna.
Unless you are exhibiting the symptoms you should not worry too much about it from what I have been told, it is a chance discovery.
I will tell you you that neurologists more often than not refuse to believe an AC can be anything BUT Asymptomatic(No symptoms) and they will use the standard "it is all in your head" if you do complain of symptoms.