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The Dreaded Air Puff – Why Do We Do It?

-The EYE Digest-

The Dreaded Air Puff - Why Do We Do It?

Eye exam. A visit to the optometrist. Updated glasses. New contact lenses. Run through these (quite similar) events in your head. Now name the first thing that comes to your head when you think about these events. For many of you, the phrase "which is better 1 or 2?" might pop up. For others the eye chart rings a bell. Maybe that big "E" on the wall. And for many others, something far more daunting may release memories of anticipation and paranoia.c2b18b0e463f893c32a7c6589e99a6ac eye center optometry office

The dreaded air puff.

So why does the air puff happen? Why does a harmonious flow of letter reading, light flashing and number choosing have to be interrupted by a blast of wind, right into the cornea? Are eye doctors just doing it for the fun of seeing you jump back and shriek in panic?

Well to be frank, the air puff presents an important test that gives your eye doctor useful information on the health of your eyes.

The Non Contact Tonometer (NCT), casually referred to as the air puff machine, is a device that measures intraocular pressure.   In simpler terms, the air puff measures the pressure of the fluid inside your eye.

So how does the NCT measure eye pressure?

trampoline ball bounce

A ball bouncing off of a trampoline surface provides an analogy for the underlying concept behind the NCT

When the machine spits out air at your eye, it hits the cornea (front of your eye). Since the eye is a pressurized organ, that air bounces off the eye and back into the machine. To pictures this, imagine throwing a baseball at the center of a trampoline. The baseball would make an indentation in the trampoline and then rebound back to you.

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An image of a commonly used NCT

Now think about the tension on the trampoline surface.   If there is less tension in the trampoline, the baseball would penetrate further into the trampoline before rebounding back to you. As a result, the baseball would take longer to return to you. Now imagine the trampoline is extremely taught with massive amounts of tension. The baseball would not penetrate nearly as deep into the trampoline and would be returned to you much quicker. This is similar to how eye pressure works.

If the eye pressure is high, the air will return much quicker to the machine. Subsequently, if the eye pressure is low, the air will return much slower to the machine. Thus by reading the speed of the rebounding air, the NCT can measure eye pressure.

So why is measuring eye pressure important?

Eye pressure has substantial indications for the overall health of the eye. If eye pressure is higher than normal, it can cause damage to structures inside the eye that are vital for its function. Specifically, a higher eye pressure can damage the optic nerve.   The optic nerve, which attaches to the back of the eye and carries vision from the eye to the brain, tends to be most susceptible to eye pressure due to the anatomy of its insertion.

glaucoma pressure 330×220@2x (1)The process of the optic nerve being damaged by pressure is known as glaucoma. While many, many variants of glaucoma exist, the underlying idea behind most types of glaucoma is that increased eye pressure damages the optic nerve.

Glaucoma represents a progressive disease. The effect is a slow degradation of peripheral vision. High eye pressure (except in certain extreme cases) does not compromise the nerve overnight, but rather takes years of cumulative effect. However, once damage is done, we currently have no way to reverse it.   This is why it is important to check eye pressure at each eye exam.

If we can catch high eye pressure early, we can treat it in a way that minimizes glaucomatous damage and preserves vision to the highest extent.

So next time you come face to face with the air puff, remember: it's not a torture device, rather it's a very important tool for your eye health and longevity.

-Dr. Aaron Neufeld

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Dr. Aaron Neufeld is the Chief Optometrist at Los Altos Optometric Group and primary author and editor of The EYE Digest.

Contact him with questions or ideas for future articles at (650) 948-3700 or


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