1) essentially yes - a cyst mean a collection of fluid material, and the word before denotes what type of cells the lining of the cyst comes from - the arachniod mater covering over the brain (part of the meninges), sub just means under this lining, that is betweem the arachnoid layer and the actual brain surface. Arachnoid could mean within the layer itself also
2) I would have to see the scan to tell you if the cyst was large enough to be causing compression of adjacent structures. The MRI will be more revealing. Usually these cysts (which are quite common in the general population and are often an incidental finding on brain scan done for eg headache) do not cause symptoms unless quite large. In the back of the brain (depending on where) they could affect vision in the occipital lobe, or lower down, the cerebellum (off balance, difficult with walking, dexterity) or top of the brainstem (double vision, facial weakness)
3) not by itself, but you should wait until the full results of the MRI are available and your neurologist clears her for this
4) see 2
5) this is possible - the MRI shows the brain in greater detail and some tumors have a cystic component - but usually there is a noncystic component that can be seen also - subarachnoid cysts are more common, and the likliehood is that this is what it is, the MRI is needed to make sure though
Thank you so much for the prompt and detail response! I really appreciate this site so much! She has her MRI on June 2nd, so I may try to post again!