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.
Now that I looked a little closely at the pictures, I think I can make out the valve that can be pumped with finger pressure if the line plugs. This could really be ironic because a couple of years ago when he appeared on Leno, he made some jokes about hydrocephalus.