Hello.
To know what Arachnoid Cyst is, you must first know what is Arachnoid mater. In simple terms, "mater" is a term for membrane. Dura, arachnoid and pia are the three membranes that cover and protect the entire brain and the spinal cord. Arachnoid and dura are the outer one. Pia mater is closely adherent to the brain tissue.
In early stages of development, if the membranes are not formed properly, the cysts may form. A cyst is a localized collection of fluid. (in this case, the cerebrospinal fluid).
Hope this helps you.
Regards
I had an Arachnoid Cyst in the Posterior Fossa, pressing on my cerebellum that was quite large. I had brain surgery in May/06 to have it decompressed, which seemed to be successful. In the spring of /08, I began to again have problems with my balance which was just like my original symptoms. I have seen my Neurologist, who did a CT scan and said the Arachnoid Cyst has refilled, and will talk to the Neurosurgeon who did my surgery. He does not think though, that there is anything further that can be done. Just wondering if anyone has any suggestions of any further treatment that might work ?