In this particular episode of Ask an Eye Doc, you will learn:
- What the puff of air test is really called
- What it measures
- What is aqueous humor
- How eye pressure is related to glaucoma
This episode will make more sense if you have already listened to the following episodes:
For you readers out there, here’s the answer in written form:
It’s everyone’s favorite test: the puff of air! If you’ve never had the puff of air test at the eye doctor, you’re missing out. It’s great fun! You put your forehead on a bar, your chin on a chinrest and stare at a blinking green light while a quick puff of air shoots out of the instrument at your eye. Some patients love this test so much that they get to do it two or three or four times!
Setting the sarcasm aside, what exactly does this test do besides torture my patients?
First of all, I should say that for most people this test actually isn’t really that bad. Mostly it’s the anticipation of when the little puff of air is coming that throws people off. But why are we poofing air at your eyes anyways? The answer is to measure the pressure of the fluid inside your eyes.
The “puff of air test” is called a non-contact tonometer (NCT). “Non-contact” because the instrument doesn’t touch your eye (the air does), “tono-” meaning “pressure” and “-meter” meaning “measure”.
Measuring eye pressure is one of the standard eye health measurements that is assessed with each comprehensive eye exam and sometimes for medical eye visits as well. For example, glaucoma, an eye condition that leads to blindness, is associated with high eye pressure. Also, certain medication eye drops can elevate your eye pressure.
So how does puffing the eye actually measure your eye pressure?
A little aqueous humor
Remember in episode 19 and episode 20 we learned about the iris and the cornea? The space between the cornea and the iris is filled with a clear fluid called aqueous humor. This is the fluid I was referring to earlier.
When the air shoots out of the NCT, it temporarily flattens the center of your cornea. The time it takes for the cornea to bounce back to its shape (in milliseconds) is then converted to a pressure value. If the pressure behind the cornea is low, it will take a longer time for the cornea to return to it’s shape. If the pressure is high, it will bounce back more quickly.
Eye pressure is measured in units of millimeters of mercury (mmHg). Normal eye pressures range between 8-22 mmHg. As long as your are within this range and your two eyes are no different than 4 mmHg, you have normal eye pressures.
Not the glaucoma test
One last thing: you shouldn’t call this the “glaucoma test” like a lot of people do because you cannot diagnose glaucoma by only measuring eye pressure. Some glaucoma patients actually have normal eye pressures.
So now you know. If you want to impress the eye doctor at your next exam, tell him or her that you’d like to know your non-contact tonometry results in millimeters of mercury.