Here’s a question that’s been bothering me for a while:
I know that the eyeball is full of a liquid. One of the checks for glaucoma is to ‘tap’ the eyeball with a burst of air to find the pressure within the eyeball. So, what pressure is normal? What pressure indicates problems? Is there any disease that can cause a lower-than-normal eyeball pressure?
http://www.medicina.univr.it/glaucoma/Clinica_Di_Verona/clinic/ipertens.html
Simple Intraocular hypertension
The normal intraocular pressure is ranging from 10 to 21 mmHg, the average being 16 mmHg.
Higher levels of pressure can injury the optic nerve as is the case with the various forms of glaucoma.
From Prevention or Treatment??:
And try Emanuel M. Josephson’s Nutritional Theory of Myopia.
All I found in a quick search for below normal eye pressure was a site on ANTERIOR UVEITIS that included:
If I did the math right 20 mmHg equals about 270 mmH2O which is about 1.4 PSIG. How the puff of air measures that I have no idea. I would just drill a hole into the eyeball and put a gauge there but then, I’m not an eye doctor.
And you’ll damn sure never be MY eye doctor, no matter what sort of sheepskin you present!!!
I don’t know exactly how that puff of air works either, but I do know that it’s only a rough tool. For more precise pressure readings, my eye doctor has used a direct-contact device that requires that she numbs my eyeballs. (Oooohhh!! I hate that!!!)
My doctor hasn’t explained to me, and I’d really like to know, what causes elevated intraocular pressure?
Here’s my WAG as to how the air puff works:
Imagine a soda can. Full and closed, it is noticeably more pressurized than the outside air. Tap the can just briefly with your finger. It vibrates for a while, and you get a neat little sound. After you open the can, and some carbonation escapes, the can will still vibrate, because the liquid is more dense than the air, but not as much as when the liquid was under pressure. So by tapping the eyeball with a puff of air and measuring the vibrations, you should be able to get a rough estimate of how much pressure the vitreous humor (is that what it’s called?) is under.
And as for the probe, well, try to compress the can before and after you open it.
That’s my WAG. I’m not a physician.
Hmmm… I’m getting .4 not 1.4 psi. Am I mis-remembering one of my conversion factors?
20 mm hg x 13.8 (specific gravity of mercury) = 276 mm water.
276 mm water / 25.4 (mm per inch) = 10.87 inches of water.
10.87 inches of water / 28 (approx height of water column that equals 1 psi) = .39 psi.
All from memory, but I think it’s correct.
Ugly
RJKUgly is right. Normal atmospheric pressure is about 14.7 psi, which is about 760 mm Hg, so 20 mm Hg is 20/760*14.7 psi, which equals 0.39 psi.
>> RJKUgly is right
Yep, obviously I didn’t do the math right.
There are several methods used to measure introocular pressure (IOP). All of them involve measuring the deformation of the eyeball, specifically the cornea, when challenged by a known pressure.
Schiotz tonometry employs a weighted plunger in contact with the cornea. The Schiotz tonometer is a mechanical device that reads the deformation of the cornea directly.
The “applanation” tonometer is usually attached to a “slit lamp” (biomicroscope), and allows pressure to be applied to the cornea while a reading is made of the amount of corneal deformation. The “air puff” tonometers allow the same measurements to be made without solid contact of a mechanical device with the cornea.
Hardly anyone uses the Schiotz method anymore. I have a Schiotz tonometer in my medical bag, and have used it very rarely when I didn’t have access to the applanation tonometers and it was an urgent matter to measure IOP.
As an aside…several years ago I had a contract to do medical exams on FBI and Secret Service agents. For some reason (perhaps fear of infection), the exam protocols prohibited contact tonometry…