my baby has an arachnoid cyst

my 17 month old son has a 4cm cyst in his left temporal region. it was found when he had an mri to see if he had cerebral palsy. fin has development delays and muscle weakness, he has little balance, doesnt talk, is floppy and at times bangs his head on the floor or pulls at his hair. do you think this could be the cyst? if it is what could they do?

also i have read about it being a birth defect, is it like spina bifida?

thanks