Arachnoid cyst within posterior fossa

What does it mean when MRI results show Prominent cisterna magna versus arachnoid cyst within the posterior fossa in an 8 year old child? Sounds like everything else is okay with the exception of an effusion within the left mastoid and left middle ear. Is this something that is serious or will need surgery? We have appointment with Neurologist but want to know more now.

[Dr Vinod]

In brain you have little space and also there is cerebrospinal(CSF) fluid flowing. If there is blockage of CSF then you have cysts and if the size of cyst is small it does not cause symptoms. If it is big it occupies space and cause seizures and other symptoms which i do not want to elaborate or comment without seeing the MRI.
Surgery is an option and after the neuro physican consultation you can go for a neurosurgeon consultation.
Take care!

[sb2009]

Thank you for the information. Neuro said that his cerebellum did not form all the way which created a space for the fluid to build up and caused the cyst.  They said that there is nothing that can be done to fix it, surgery will not help. If they drained it then there would still be a gap where it did not develop and then would just fill up again. Is there some kind of medical term for this? Can this be the reason he has learning difficulties and speech problems? The neuro did not seem to be concerned about the cyst at all. I am so confused as to how to help him.