Tonometry is a test that is commonly performed during a routine eye check-up. A tonometry test may be done by an optometrist or by an ophthalmologist. This test measures intraocular pressure (the pressure inside the eye).Intraocular pressure that is higher than normal is an important indicator of certain diseases, such as glaucoma.
Who Needs a Tonometry Test?
Pediatric patients should receive a thorough eye exam before the age of 2, at about age 3, when in the first grade, and every two years to five years thereafter. More frequent exams or re-evaluation could be necessary for children who are at risk for any hereditary eye conditions. The American Optometric Association recommends that healthy adults between the ages of 18 and 50 receive an eye exam at least every two years. For those over the age of 50, an annual eye exam is recommended. A tonometry test is recommended for all adults during routine eye exams, unless there is a contraindication. Tonometry may be postponed if there is an eye injury or if there is an ulcer on the cornea. Children are not typically given a tonometry test, unless they have had surgery to repair a cataract.
Why Tonometry Is Done
Tonometry is a relatively simple test that can screen for diseases such as glaucoma, for which high intraocular pressure is a risk factor. Glaucoma, if left untreated, can lead to blindness. Normal intraocular pressure is between 9 and 20 mg Hg, and pressure higher than 20 mg Hg may indicate the presence of glaucoma.
Preparing for a Tonometry Test
In general, there is no special preparation needed for a tonometry test. The patient must take out their contact lenses before intraocular pressure can be measured, and therefore eyeglasses should be available for use until contacts can safely be put back into the eyes. Because this is a test of eye pressure, patients should try to relax and avoid restrictive neck wear that could increase intra-ocular pressure. If the test requires anesthetic eye drops, the eyes will be numb for anywhere from 10 to 30 minutes. The patient should tell their healthcare professional if they have an allergy to eye drops or if there is a family history of glaucoma.
Types of Tonometry Tests
There are several ways of measuring intraocular pressure. The type of tonometry test performed will depend on several factors, including the patient’s age, risk factors for high intraocular pressure, and available equipment.
Non-contact tonometry is the air puff test done at older offices.
Applanation, indentation, and electronic tonometry may be performed if a more accurate reading is necessary. Most eye doctors who treat glaucoma and other eye diseases only rely on applanation (also known as Goldman Tonometry) because it is the most accurate method:
- Applanation tonometry: In this type of tonometry, pressure is measured by flattening the cornea. The patient is given anesthetic eye drops to lessen any discomfort from this test. The eye drops also contain a small amount of fluorescein, a type of stain. The patient is asked to sit with their head supported by a chin-and-forehead rest, to keep their eyes wide open, and to be very still. A small probe called a prism is then gently pressed against the cornea in order to flatten it. Each eye is tested separately. The amount of force that is needed to flatten the cornea is used to calculate the patient’s intraocular pressure.
- Noncontact tonometry: This type of tonometry (sometimes also called air-puff tonometry) is fast, simple, and does not require eye drops or contact with the eye, which makes it easier to use on children. The patient sits with their head supported in chin and forehead rests and looks into the tonometry machine. A puff of air is directed at the open eye. The air puff tonometer releases air over a range of pressures at your eye. At the same time, an infrared beam of light is shone off your cornea. When the cornea flattens out under the pressure of the air, the beam of light reflects at a different angle. When this occurs, the device records the pressure of the air that was required, measured in millimetres (mm) of mercury (Hg). In the past, this test was not as accurate as other types of tonometry, but newer technology has improved its accuracy.
- TonoPen tonometry: This test is done with a tool that looks much like a writing pen. It is a common choice for those who need a more portable tonometer. The TonoPen is typically less accurate than other tonometry tests, but its ease of use and portability make it useful for some practitioners. Anesthetic eye drops are administered, and the practitioner holds the eyelids open and presses the tip of the instrument against the cornea. Pressure is shown on an electronic readout on the device. Multiple readings may be taken in order to improve accuracy.
- The Icare® ic100 is based on a rebound measuring principle that requires no drops, air or specialized skills for its use. This device uses patented rebound technology to measure intraocular pressure. Its light-weight probe makes momentary contact with the cornea. Icare’s proprietary algorithm coupled with state-of-the-art software allows the device to evaluate deceleration, contact time and other motion parameters of the probe when it touches the cornea. The premium design and user interface brings IOP measuring to a higher level.
What Tonometry Results Mean
If pressure is measured higher than 20 mg Hg, further testing may be needed. Intraocular pressure is individualized — what is a normal pressure for one person may be considered high for another. This is why tonometry is typically done at every eye exam: a history of testing can help determine whether there is a change in what is considered a normal pressure measurement for that person. High intraocular pressure does not mean that a patient has glaucoma. Conversely, low or normal intraocular pressure does not mean that a patient is not at risk for glaucoma. But patients with higher than their normal lintraocular pressure are considered at high risk for glaucoma and should undergo further evaluation.
Risks of Tonometry
Tonometry is a very safe test, but there are some minimal risks. With tonometry tests that involve touching the eye, there is a risk of scratching the cornea. A scratched cornea can be uncomfortable, even painful, but such injuries tend to heal very quickly within a few days. For those who are allergic to the eye drops or stain used during the test, there is a risk of an allergic reaction. There is also a slight risk of infection from the instruments used to touch the cornea.