Hi, Thank you for your question. Arachnoid cyst is a brain cyst that may remain asymptomatic for long years. But if it is large it can cause headache, paralysis, seizures and gait disorders. Therefore, surgical intervention is essential to remove the large cyst and if permanent damage needs to be avoided. Craniotomy, needle aspiration and cerebral shunts are common surgical treatment that may help. Please consult a neurologist right away in this regard. Hope this helps.
I recommend you join the Arachnoid Cyst Awareness Network on the Web. There are many of us who have Arachnoid Cysts there and we discuss many aspects of living with them. Facebook also has a couple of AC groups--Arachnoid Cyst Family and Arachnoid Cyst Awareness.