My granddaughter got diagnosed with an arachnoid cyst. what happens to such a baby?

More details please: The location and size of the cyst impact the treatment decisions. Is this a primary or secondary cyst> how old is your granddaughter and is she having symptoms? Was this cyst present at birth? Please resubmit your question with more details and also talk with the physician who made the diagnosis. Sometimes it helps to write down your three most important concerns and ask the doctor.

Maybe no change: If the cyst is not growing or causing pressure on the brain or obstruction of spinal fluid flow there is no need to intervene. However if there is increased size, or obstruction, a neurosurgeon can intervene.