My son was diagnosed with an arachnoid cyst in his brain. 2 years ago when it was discovered I was told by his neurosurgeon that it is massive and was splitting both his temporal and frontal lobe. Yesterday he told me that there is no brain matter. That this part of the brain never developed. Instead there is a huge sac of CSF.

No brain? Is this true? I can't find a strait answer anywhere and I'm losing my mind.