I need medical evidence that it is harmful, or potentially harmful,
to stifle sneezes by holding your nose closed during the sneeze. Note
that I'm not talking about keeping the sneeze from happening, but
holding the nose closed once the sneeze begins to prevent the blast of
air from exiting the nose.

First, I would like to note that this information is for general
educational purposes. Keep in mind that Google Answers is not an
authoritative source for medical advice, and that you should consult
your physician or other medical professional for your health needs.

Now on to the explosive question... 

I found literally hundreds of non-medical Web sites which contained
this interesting factoid, almost always worded in precisely the same
way:

"If you try to suppress a sneeze, you can rupture blood vessel in your
head or neck and die. If you keep your eyes open by force [while you
sneeze], they will pop out."

While this may be both oversimplifying and exaggerating the hazard,
there is some medical truth underlying our mothers' admonition that
stifling sneezes is dangerous.

The "Dear Doctors" section of the University of Alabama at Birmingham
Health System Web site discusses this issue. Here are some excerpts
from the UABHS physician's answer:

"Suppressing a sneeze can be harmful, particularly to your ears...
Studies have found that the air expelled during a sneeze travels at an
amazing 100 miles per hour, and spews out up to 5,000 droplets, which
may be propelled up to 12 feet in a single sneeze.... By clamping your
mouth closed an pinching your nose shut, you force the "sneeze" into
the eustachian tube (which connects the back of the throat to the
middle ear) and then to your eardrum. Sneeze strong enough, and you
could rupture an eardrum, causing acute pain, infection, bleeding, and
even hearing loss."

The full text of this article may be found here:

University of Alabama at Birmingham Health System
http://www.health.uab.edu/show.asp?durki=8418

Cecil Adams, who writes an informative and entertaining syndicated
column, is billed as "The World's Smartest Human Being." While Cecil
is not (as far as anyone knows) a doctor, he is very good at
explaining medical concepts to the layman. Here is a portion of his
answer to the question posed by a reader, "Will suppressing a sneeze
give you a brain aneurysm?"

"The chances of your getting a brain aneurysm from a sneeze, stifled
or otherwise, are pretty slim. But it could happen. ... Is suppressing
a sneeze bad? Could be, due to something called the Valsalva maneuver,
better known as a way of relieving pain in the ears caused by a rapid
change in elevation. While pinching your nostrils shut, you blow into
your nose hard. This opens the Eustachian tubes connecting your inner
ear with your throat and equalizes the pressure on either side of your
eardrums. But because the Valsalva maneuver increases pressure in the
chest, it also briefly blocks the blood flow entering the heart,
causing a sharp fluctuation in blood pressure. Conceivably this could
cause an aneurysm to rupture."

Cecil also points out that a full-blown, unstifled sneeze has perils
of its own. You can read his entire answer to this question in the
archives of The Straight Dope online:

The Straight Dope: Archives
http://www.straightdope.com/columns/010427.html

A column about sneezes written by the Mayo Clinic's Health Information
Services has this to say:

"Once a sneeze begins, don't try to stop it. Suppressing a sneeze
forces the air into your eustachian tubes, which lead from the back of
your mouth to your middle ears. You could damage your eardrums or
transport infectious organisms into your ears."

Blue Shield of California: My Life Path
http://makeashorterlink.com/?T23112681

In a column of advice for allergy sufferers, there is this, from Dr.
Donald Donovan, Associate Professor of Otorhinolaryngology at Baylor
College of Medicine:

"Stifling a sneeze out of politeness may do more harm than good. In
rare cases, increased pressure from holding your nose and closing your
mouth can blow out the eardrums. 'When you stifle a sneeze, you can
prevent the clearance of the germs or irritants from your body and
increase your needs to keep sneezing or develop an infection,' said
Dr. Donald Donovan, an associate professor of otorhinolaryngology at
Baylor. 'The best thing is to sneeze with your nose and mouth open
into a tissue away from other people.' That same pressure activates a
reflex in most people to close their eyes while they sneeze.
Scientists speculate that the reflex evolved to help protect the eyes
from the particles a sneeze expels, but not everyone has it. 'The old
wives tale that if you sneeze with your eyes open, you will blow them
out is absolutely untrue.'"

Baylor College of Medicine
http://public.bcm.tmc.edu/pa/alergysuffers.htm

Hampton Sides, a columnist for Outside Magazine, has this to say about
suppressed sneezing:

"When you sneeze, you're expelling aerosol particles at a speed of
more than 100 feet per second, a force akin to a fire hose that clears
out any pollen or dirt lodged in your nasal tract...Hold in a sneeze,
though, and you run the risk of fractures in the nasal cartilage,
heavy nosebleeds, burst eardrums, detached retinas, temporary swelling
called facial emphysema, even fatal strokes. And, if you've got an
early winter cold, the act of sneezus interruptus can drive millions
of tiny pathogenic particles deep into the sinus tissues, causing
serious infection. Gruesome outcomes all, and compelling proof that
the only safe sneeze is an explosive one."

Outside Magazine Online
http://www.outsidemag.com/magazine/1198/9811wild.html

This gives an overview of the matter of suppressed sneezes. The
medical consensus is that it can be dangerous to stifle a sneeze, but
fortunately it is NOT true that sneezing with your eyes open will blow
your eyes out.

Last, and probably least, I cannot resist including this little gem
that I found while researching your question:

There was a young man from Belize
Who tried hard to hold in a sneeze.
But it came out at last
As a powerful blast
That knocked down a number of trees.

Gesundheit!
pinkfreud

Google search strategy:

"suppress" + "sneeze" + "danger"
://www.google.com/search?hl=en&ie=UTF-8&oe=UTF-8&q=suppress+sneeze+danger

"stifle" + "sneeze" + "infection"
://www.google.com/search?hl=en&ie=UTF-8&oe=UTF-8&q=stifle+sneeze+infection

"hold in a sneeze"
://www.google.com/search?hl=en&ie=UTF-8&oe=UTF-8&q=%22hold+in+a+sneeze

Terrific answer. Just what I was looking for. This was my first try
using Google Answers and it was well worth it...I'll definitely use
the service again. Thanks PinkFreid-GA for your comprehensive and
well-documented response!