What's The Purpose Of The Air Puff Eye Test?
During an eye exam, you're just minding your own business when all of a sudden you get a blast of air directly into your eye. Sound familiar? This is the dreaded air eye puff test. You're not alone in hating it. I don't think I've met anyone who doesn't dislike the test. But the test does serve a very useful purpose.
The purpose of the eye air puff test
The air puff test checks the eye pressure (also known as the intraocular pressure or IOP for short). It does it without actually touching the eye - it is a non contact test. Tests that check the intraocular pressure within the eye are called tonometry tests. So putting it all together, the air puff test is a non-contact tonometry test.
Flow of aqueous within the eye; http://www.nei.nih.gov/photo/eyedis/index.asp, Public domain, via Wikimedia Commons
The pressure of the eye is largely determined by fluid within the very front part of the eye. This fluid, called aqueous humor is produced in a part of the eye called the ciliary body behind the iris and flows into the anterior chamber of the eye before finally flowing out at the angle of the cornea. If there is too much of this fluid, the pressure of the eye increases. This is what we want to check.
For this test, you sit at a machine and the machine blows a column of air at your cornea. The force of this column of air is increased until the force equals the pressure within the cornea. By laws of physics, if the pressure of the eye is greater than the pressure of the air, nothing will happen. The eye will "push" back harder against the air. Once the pressure of the air equals (or exceeds) the pressure of the eye, the air will actually push the eye around and actually cause the cornea to flatten a little (by flattening the cornea and decreasing the space in the anterior chamber, the pressure inside the eye increases to equalize the air burst pressure). Once the machine detects that the cornea has flattening in response to the column of air, it stops and records the air pressure as the pressure of the eye. Pretty straightforward. Physics can be fun!
Reasons why the test is done
- One of the most common reasons eye pressure in a healthy eye is checked is to screen for glaucoma. Glaucoma is a complex condition which causes permanent vision loss. Frequently glaucoma is associated with a high pressure within the eye. And many types of glaucoma have no symptoms other than a higher than normal eye pressure (which you won't be able to detect on your own). By lowering the pressure within the eye through medication or procedures, one can slow or halt the progression of glaucoma and prevent vision loss. But this can only happen if glaucoma is detected. As glaucoma commonly has high pressures within the eye, screening for glaucoma often involves checking the pressure within the eye to see if it is high or not.
- But outside of glaucoma, there are also good reasons to check the pressure of the eye. Certain eye diseases can cause a high pressure within the eye. If the eye is having a problem, checking the pressure can help the doctor manage the issue. One example is that inflammation inside the eye (also known as iritis) can cause the pressure to go up. Whether this inflammation was caused by being hit in the eye or by some unknown cause, it's important to know if the pressure is high. High eye pressures can cause vision loss (same way it causes vision loss in glaucoma); but if the high eye pressure is detected, eye drops can be started to lower the pressure and prevent any issues.
Advantages to the air puff eye test
Let's face it, there are advantages to a non-contact test. Otherwise it wouldn't be used. You don't have to worry about anything touching the eye. This is great for a few reasons.
- With nothing touching the eye, you don't have to worry about anything being properly cleaned. There is no risk of transmitting any infection like pink eye. Preventing acquired infections is a great thing.
- In addition, you eye doesn't have to be numb to have the eye puff test. While it is uncomfortable to have a puff of air in the eye, it doesn't cause any pain. You don't need any anesthetic drops to numb the eye prior to the test. Anesthetic eye drops aren't bad for you, but they will burn upon putting the drop in. And afterwards, you don't blink as much; because you don't blink as much, the tears on the surface of the eye will evaporate away and you will develop a little more dryness after having numbing eye drops. This dryness can cause some blurriness to your vision or some extra irritation after the numbing eye drops wear off.
- The accuracy of the test isn't dependent on the operator. Because a machine does all the work, you don't have to worry whether someone is doing the test incorrectly. The learning curve is simply learning how to work the machine. Aside: but other tonometry tests, described below, don't really have a steep learning curve either.
Disadvantages of the air puff eye test
Not everything is peachy for the air puff test. Despite the conveniences, the air puff test has limitations. It won't be the most accurate test for everyone. When the pressure is high inside the eye, the eye puff test can underestimate the pressure of the eye. It will give a reading lower than the actual pressure. Conversely, when the intraocular pressure is low, the eye puff test can overestimate the pressure and give a reading which is higher than the actual pressure.
Because of these disadvantages, the air puff eye test is only best used for screening purposes. If you actually have glaucoma, there are better more accurate ways to check the pressure.
Alternatives to the eye air puff test
Goldmann tonometry; Jason7825 at English Wikipedia, CC BY-SA 3.0, via Wikimedia Commons
The current gold standard way of checking pressure in the eye is via something called Goldmann applanation tonometry. This test is performed at the exam room microscope or slit lamp. The technician or doctor numbs the eye and puts an orange dye called fluorescein in the eye. A cylinder device is then pressed up against the cornea until the cornea flattens and the pressure measured.
While the pressure readings from Goldmann tonometry can still be affected by different variables, such as the thickness of the cornea, it is still one of the most accurate tests used to check pressure. This test is used very commonly to follow intraocular pressure in patients with glaucoma. The downside of this test is that it takes a trained technician or doctor to perform correctly. It isn't the easiest or quickest test to perform. It's not the best test if your goal is simply to screen for high intraocular pressures.
There are two other commonly used tonometry tests that also easy to use: the tono-pen and rebound tonometry. Both of these are handheld devices and quick for a technician to learn how to use. The tono-pen is tapped against a numb cornea and the force of resistance to the tap is measured. The rebound tonometer bounces a tiny plastic tip against the cornea. A neat feature of rebound tonometry is that anesthetic drops are also unnecessary. Both of these devices correlate well with the gold standard Goldmann applanation tonometry and you may find these in use instead of the air puff eye test.
Next time you are at your eye doctor getting your eye puff air test, rest assured that this inconvenience is all for the benefit of your eyes. By screening for high intraocular pressures within the eye, you can make sure your eyes stay healthy.
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