How Does Your Optometrist Check Your Eye Pressure?

At a previous eye examination, you may remember having a puff of air blown into each of your eyes before seeing the doctor. Some people do not mind this, while others are quite afraid of this portion of the testing. This article will describe what this test measures, why it is done, and, although it is certainly not fun to have performed on you, why it is quite valuable in maintaining your eye health.

What is Eye Pressure?

The eye is filled with a gelatinous material at the back, called the vitreous humour, and a watery substance at the front, called the aqueous humour. The vitreous grows as the eye grows throughout development and then begins to shrink, condense, and liquefy with age.

The aqueous, however, is constantly being produced and drained. This balance of fluid production and drainage must be maintained to make sure the eye is constantly inflated and its structure is healthy.

There is a normal range of eye pressure, or intraocular pressure (IOP), that most people fall in most of the time. There are regular fluctuations in IOP throughout the day and the pressure can also be affected by exercise, wearing a tight neck tie, etc.

At your eye examination with us, we are primarily looking for significant changes in your IOP or overly low or high IOP. Low eye pressure might be a sign of injury, inflammation, or other conditions.

High IOP can be a sign of injury or inflammation as well, but the most well known disease associated with increased eye pressure is glaucoma. Glaucoma is the gradual death of nerve fibers at the back of the eye that carry visual information from the eyes to the brain, resulting in a loss of peripheral vision and, if severe and left untreated, eventual complete loss of vision.

Applanation Tonometry and Your IOP

IOP is often measured with applanation tonometry, where a small constant area of the clear cornea at the front of the eye is flattened and the force required to do so is converted into a pressure reading.

There are several types of applanating tonometers that range in how much of the cornea they push on and their respective accuracy. 

Some touch the eye but are hardly felt, while the most accurate method noticeably pushes on the eye and thus requires a numbing drop to be put into the eye so that the patient does not blink or feel the probe. The numbness wears off after 10-15 minutes and there is no damage to the eye done during this measurement.

The Air Puff Test

The non-contact computerized tonometer, also known as the “air puff test”, is the applanation tonometer that uses air for IOP measurement and is extremely common for screening patients in comprehensive eye examinations. The test is automated and shoots air as soon as it is properly lined up in front of the eye.

The value that is measured is the amount of time it takes for the cornea to be flattened a specific amount, meaning that the longer it takes for the air puff to flatten the eye, the higher the internal pressure. The thickness and rigidity of the cornea can influence the measurement and this is taken into account by our eye doctor when they are examining the results.