To try to hide this would make sense because often children with these problems end up with some neurologic damage, although many also turn out just fine.
However, even the question about this possibility could have been a political disaster for anyone seeking to be POTUS.
It could explain the secrecy around the birth and medical records.
Again, this is speculation based on the assumption that the shadows on the photo really represent something. If they are real, then I think this is the most likely explanation.
Just to clarify my speculation, if he had hydrocephalus as an infant, often no burr hole needs to be drilled because the fontanelles are not yet closed. Usually, burr holes only need to be drilled after that happens which would make it a later occurrence. Again, just my speculation.
Bingo!