Our eyes are filled with a fluid called aqueous humor. This fluid circulates between the anterior and posterior chambers, through the pupil. This process maintains the pressure in our eyes. This pressure is called eye pressure, or intraocular pressure.
Why is it important to measure eye pressures?
The technical name for measurement of eye pressure is tonometry. As part of your eye test, your eye doctor will measure your eye pressure or intraocular pressure, IOP. Generally, the normal range of eye pressure is considered to be between 11-21mm of Hg.
However, your ophthalmologist will determine the eye pressure range that is healthy for your eyes. An increase in eye pressure is called Ocular Hypertension. When this is accompanied by damage to optic nerve and visual fields, the disease is called glaucoma. Sometimes, some people with “normal” eye pressures may also have glaucoma.
Since glaucoma has no symptoms, and can cause irreversible blindness, your eye doctor measures your eye pressures to assess if you are at risk for glaucoma. In addition to this, lowering eye pressure is important for all patients of glaucoma. Decreasing the IOP can prevent the progression of disease. This is why your eye doctor will measure your eye pressure at each visit.
How Is Eye Pressure Measured?
The two most common tests used for measuring eye pressure are:
1. Non Contact Tonometery
2. Applanation Tonomtery
Both processes are painless, and quick. However, you need to cooperate during the test by keeping your eyes open, and looking straight ahead. Do you remember the episode of FRIENDS in which the eye doctor tried to measure Rachel’s eye pressures? And could not, because she would just wince so much? He wasn’t actually measuring her eye pressure, just doing a slit lamp evaluation. But enjoy the memory anyway.
Non Contact Tonometry
The most commonly done test for eye pressure is the air puff, or non-contact tonometry, NCT. The doctor will ask you to sit comfortably with your chin in the chinrest. She will also ensure that your forehead is resting comfortably against the machine. Once you look at the light inside the apparatus, the doctor will press a button. During the test, you will feel a puff of air in your eye, but no pain. The instrument does not touch your eye at any point. This is why non-contact tonometry is a very popular way of measuring eye pressure. . You might, however, be startled by the accompanying noise during the air puff.
Applanation tonometry
Applanation tonometry is the other way of measuring eye pressure. During this test, your doctor will first put numbing eye drops in your eyes. Thereafter, the doctor will either put yellow colored drops, or touch a paper impregnated with the yellow eye to your eyes. This dye is called fluorescein and stains your tear film yellow. The doctor will then ask you to sit at the slit lamp, and look straight ahead. She will then measure your eye pressures using a blue light. You will see a prism like instrument approaching the eye. Even though the prism with gently touch your eye, the procedure is completely painless.
Other methods
There are other methods of measuring your eye pressures. They are not used so commonly, but are of tremendous use in certain cases.
• Schiotz tonometer
• Tonopen
• iCare Home tonometer
• Rebound tonometer
• Perkins tonometer
• Dynamic Contour Tonometer
What is Target Eye Pressure?
If you have glaucoma, your eye doctor will want to decrease your eye pressure. She will set a target eye pressure for your eyes. It’s an eye pressure that should keep your vision safe, without making your life miserable, since all treatment has side effects.
The World Glaucoma Association (WGA) defines Target IOP is “an estimate of the mean IOP at which the risk of decreased vision related quality of life due to glaucoma exceeds the risk of the treatment.”
How do we determine Target IOP?
The magic number actually is dependent on several factors. These include
- Stage and severity of glaucoma
- Eye pressure at the time of diagnosis
- Age and life expectancy
- Associated diseases like Diabetes and high blood pressure
- Risk factors for glaucoma progression
- Quality of life impact of treatment for individual patient
