I saw a documentary once about a child in UK, who was born with, or developed fluid on the brain. Imaging showed almost all fluid in his skull, with a sort of shell of brain tissue surrounding it. He was a twin. Despite the small amount of brain tissue, he was developing at a normal rate. Here’s a link to a similar story:
http://www.mirror.co.uk/tv/tv-news/boy-no-brain-stuns-doctors-9778554
I recall reading about this or similar as well. The brain is probably the biggest terra incognito still remaining in medicine.