Tonometry is a simple test that chiefly measures the intraocular pressure (i.e., the pressure inside the eye cavity). The preliminary purpose of this test is to help determine whether a person is at risk of glaucoma or how well the treatment given for glaucoma is working.

Also Read: Visual Acuity Test: What Is It And What To Expect?
Doctor testing for tonometry

What is Glaucoma?

Glaucoma can be defined as a group of eye condition that mainly damages the optic nerve which is quintessential for vision.  This damage mainly occurs due to pressure build up owing to the increase in the optic fluid that normally bathes and nourishes the eye. It is one of the leading causes of blindness in people over the age of 60. Although it can occur at any age, it is normally witnessed in older individuals. Changes that occur in the eye due to glaucoma are usually painless and slowly progresses over the years without anyone noticing.

Also Read: Glaucoma: Causes, Symptoms And Treatment

Eye And It’s Functions

Our eyes are extremely crucial for one’s quality of life. It mainly collects light from the visible world around us and converts the light into nerve impulses. The optic nerve then transmits these signals or nerve impulses, to the brain which forms an image, thereby providing sight. It is extremely crucial for one's quality of life as it helps us read, admire art, cherish the nature and connect with our near and dear ones.

What Is Tonometry?

A tonometry test is extremely critical for detecting the changes in the eye at an early stage long before we are conscious about them. In case of positive result, the doctor might conduct additional testing to confirm the disease.

Types of Tonometry

Goldman Applanation Tonometry Test:

It is the most common type of tonometry test that has been considered as the international gold standard for measuring the pressure within the eye for decades. It mainly involves the use of a flat-tipped probe that is gently pressed against the surface of the eye.

Non-Contact Tonometry

In this case, air pressure is applied to the eye using an instrument that slightly flattens the cornea. The instrument then slowly blows a brief puff of air at the cornea, measuring the pressure in the eye. This method is generally considered less accurate than Goldmann applanation tonometry.

Electronic Tonometry

This method mainly uses a handheld, mobile device, i.e., a Tono pen that the eye doctor or practitioner can carry from room to room to check eye pressure.

Schiotz Tonometry

Schiotz tonometry is a form of indentation tonometry. An indentation tonometer measures eye pressure by measuring the depth of deformity caused by a small metal plunger, i.e., the iCare device as it rests on the cornea.

Who Requires A Tonometry Test?

Our eyes are moistened with different fluids to keep it healthy. It works perfectly when new fluid is made, and the old fluid is drained out. But in some cases, the drainage system of the eye malfunctions causing fluid build-up and leading to pressure inside the eye.

The pressure within the cavities may also increase due to an eye injury or traumatic incident. With passing time, if the eye pressure increases, it can damage the optic nerve and ultimately lead to glaucoma.

Tonometry test, specifically Goldman applanation tonometry test is conducted by the doctor if he or she suspects that you may be at risk of glaucoma.

In some cases, Tonometry is also suggested by the doctor if the person:

  • Is over 40
  • Have a family history of glaucoma
  • Is African, Hispanic or Asian
  • Have diabetes
  • Have circulation (blood flow) problems
  • Have high blood pressure
  • Have had an eye injury in the past
  • Get migraines
  • Is far-sighted or near-sighted
  • Have other chronic eye problems
  • Have used corticosteroid medications for a prolonged time period in the past

The doctor may also suggest a Tonometry test if you are experiencing any of the following symptoms:

  • Gradual loss of peripheral vision
  • Tunnel vision
  • Halos around lights
  • Reddening of your eye
  • Severe eye pain
  • Blurred vision

How To Prepare For Tonometry?

It doesn’t require any special preparation for the Tonometry test. But in case you are wearing contact lenses, your doctor might ask you to remove it as the dye used during the test can permanently stain the lens. One should also reveal the doctor about any history of corneal ulcers or eye infections or a family history of glaucoma or intake of any prescription medications before the test.

What Is The Procedure Of Tonometry?

There are 3 methods to find the pressure of the eye but the most used one in Goldmann tonometry test. In this test, the ophthalmologist first puts numbing eye drops in the eye so that you don’t feel anything touching the cornea during the process. Once the numbing effect occurs, the doctor may touch a small strip of paper that contains orange dye to the surface of the eye to stain it. This mainly helps to increase the accuracy of the test. Next, the doctor will put a machine called a “slit-lamp” in front of the patient and ask him or her to rest the chin and forehead on the supports provided. Blue light is used so that the orange dye will glow green. The lamp is then moved towards the eye until the tip of the tonometer probe touches the cornea. The doctor looks through the eyepiece on the slit-lamp and adjusts a dial on the machine to give the pressure reading.

A second method uses a Tono pen (a handheld device shaped like a pen). After numbing the eye with eyedrops, the device is used to touch the surface of the cornea and it instantly records eye pressure.

The last method is the noncontact method (air puff). In this method, the patient’s chin rests on a device similar to the slit lamp. The patient should stare straight into the examining device. When the patient is at the correct distance from the device, a tiny beam of light reflects off of the cornea onto a detector.

When the test is performed, an instrument blows a puff of air into the eye while the patient is looking at the light. This will slightly flatten the cornea; how much it flattens depends on the eye pressure.

What Are The Risks Associated With Tonometry?

In general, there aren’t any risks associated with tonometry test if done carefully. But in very rare cases if the applanation method is used, there is a small chance that the cornea may be scratched due to corneal abrasion with the device. Nevertheless, the scratch will normally heal within a few days.

How To Interpret The Results?

Eye pressure usually differs from person to person.

If the pressure ranges between 12-22 mmHg (“millimeters of Mercury”), it is considered Normal. A normal test result usually signifies that the pressure inside the eye is within the normal range and the patient most probably doesn’t have glaucoma or other pressure-related eye problems.

If the pressure reading is higher than 20 mm Hg, then the patient might have glaucoma or pre-glaucoma. High eye pressure is just one symptom of glaucoma and the doctor would then carry out additional testing to confirm the diagnosis.

Sometimes abnormal results might occur due to glaucoma, hyphema (blood in the front chamber of the eye), inflammation of the eye or injury to the head.