I've heard of cases in which people go to the doctor with chronic
headaches, and when an XRAY or MRI are done, it's discovered that the
sufferer has far less brain mass than most people, and a skull full of
CSF.
 Sometimes these people are fully-functional (aside from headaches),
some of them have some problems.
  A case like this was described in Stephen King's book "The Dead
Zone" and I thought for sure that it was fiction until I read about
these cases in real life.
 Any references/cites would be appreciated.

Hi,

It would take a bit more time than I can devote to this right now to
gather citations for you, but the condition you refer to is acephaly.
It isn't common and I'm not certain that anyone born with this birth
defect survives, but it is a documented birth defect. A quick search
brought up one reference.

http://www.users.zetnet.co.uk/jil/ums/bcps/tbcps_s4.pdf
Case of Acephalous F?tus.

Hope this helps someone track down the info you need.

Regards,

bcguide-ga

What you are describing can be caused by many conditions.  acephaly
refers to no brain a condition not compatible with life.  There are
many conditions that would make a person appear to have less brain
mass on imaging because of an accumulation of fluid (likely by
blockage of CSF pathways)  In these cases, the brain is pushed aside
if you will to make room for the fluid.  tumors, cysts and in-born
defects can cause this.  Sometimes no symptoms are present.  an
example can be seen at
http://www.cami.jccbi.gov/AAM-400A/FASMB/FAS200204/hydro.htm
There are other cases where certain portions of the brain never
developed and inorder to make up for the missing "matter" CSF
accumulates.  It is not uncommon for people to be missing their corpus
callosum, the part of the brain that connects the 2 halves and have no
idea.  http://www.icomm.ca/geneinfo/agencc.htm
If the missing part of the brain is not critical for life, the brain
can often re-map connections necesary for normal functions if the
person is born with the condition.  SO yesy people can be missing PART
of their brain and not know it until it is picked up on a test.  And
YES people can have heads full of CSF without knowing it but they
CANNOT only have their brain stem and CSF :)