First of all, Whoo hoo! I am SO excited! Season Eight of Buffy has arrived!
Georges Jeanty did (as expected) a great job and there are some laugh out loud moments in the story line: a very Buffy episode. I can't wait for issue 2!
Second, RE Bus company stuff, I'll keep you posted as to what else we find out. Grrrr.....
Third, I've had so many eye questions I may just have a "Ask the Eye Doc" section in my journals. Or a Big Eyeball Friday a la the Colonel. Here's one from AmandaJ3162 about a recent trip to an optometrist. She asks about the function of some tests she had. I'll talk about the first question here. (I'll get to the others!) Specifically she describes:
...the air machine. I hate that. *wait....wait....wait....WOOSH! air hits my eye*
First, I know this was an optometrist, and not an ophthalmologist, because the "puff of air" machine is an estimator of eye pressure. It isn't as accurate as the applanation tonometer or the tonopen (see below). However, to use those devices, you have to physically touch the eye, and in many states optometrists (non MDs) are not allowed to do that.
All of these devices check the pressure in the eye, which has been proven as a good way to check for a potentially blinding and common condition called glaucoma. In glaucoma, the pressure inside the eye is too high for the optic nerve and the nerve slowly dies off... it is much like having your arm fall asleep if you have it hanging over a railing... the pressure on the nerve deadens it. With glaucoma the pressure slowly kills the nerve. Patients frequently don't realize they are losing vision until a scary amount of their peripheral vision is gone. As I've mentioned in other journals, your brain fills in the missing spots, so folks can walk around not realizing that they truly can't see to the sides.
Okay, so about the tests. The "air puff" test basically shoots a puff of air at your eye, thus deforming the surface of the cornea. The higher the pressure in your eye, the more difficult it will be to deform it. If you have a nice low pressure, your eye will be (relatively) squishy, and the puff will deform the cornea slightly more. There is a little monitoring device in the machine that electronically measures the deformation (I can't remember offhand if it looks at reflections of light or sound) and then calculates from that what the pressure in the eye would be.
The gold standard is something called Goldmann applanation tonometry. If you will remember that yellow stuff your eye doc might put on your eye? It stings for a moment and then gets numb? That is an organic dye called fluorescein (a brick of which, incidentally, is used to turn the Chicago River green for Saint Patrick's day). It lights up neon green in blue light and really helps eye docs see scratches on the cornea. For instance, here is a patient with dry eyes. you can see how there are little dry lit up spots in the center of his cornea. Fluorescein also lights up the tear film a bright yellowy-green (see the pic again).
So here is what the apparatus looks like in the eye doc's office. The end of the tonometer is a round, flat tip about 3mm across. If you put that flat tip on the (numb) cornea, you see a round circle of that yellowy-green tear film. However, the round flat tonometer tip has a little prism in it which takes advantage of the fact that when you apply just the right amount of pressure, the prism will split the circle of light that you see into two circle halves:
The device has a little dial on the side which moves the tip forward and back by fractions of a millimeter so you can line up the inner edges of the split circle, which works out to be related to the force required to flatten that small area of the cornea, which in turn is related to the pressure in the eye. The handy dial on the side also tells you the eye pressure. No electronics needed, just good old physics. For those interested in the calculations, you can go here: www.opt.indiana.edu/riley/HomePage/new_Goldmann_tonometry/2Goldmann_Tonometry.html
A good ophthalmologist can get a routine cornea exam and pressure measurement done in a matter of about 5-10 seconds while you look at the blue light.. And you won't feel it at all; most folks don't even realize I touched their eye, because the drop I put in the eye numbs them up so well.
There is a new device, out in the last ten years, called a tonopen which is portable, and when calibrated, is pretty accurate. The problem with that is it tends to walk around in people's coat pockets and gets jostled a lot, so it has to be recalibrated constantly. But it is great for checking eye pressure on patients who can't get up to the slit lamp (like tiny babies, or people in bed in the hospital, or dogs or sheep).
For some photos of these and the really old fashioned (but highly accurate) Shiotz tonometer, I found this site here sponsored by the U of Toronto.: eyelearn.med.utoronto.ca/ClinicalSkills/Measure.htm
Okey dokey. Please see me after class if you have any further questions. Now go read that comic book!
Today's Vocabulary Words
Air-Puff Non-Contact Tonometry
Goldmann Applanation Tonometry