What is it for?

Measuring the pressure inside your eyes.

Optometrists and ophthalmologists measure eye pressure (also known as ‘intraocular pressure’, or IOP) as part of a routine eye examination, to screen for eye diseases such as glaucoma.

One method of tonometry is the ‘air-puff’ method. In this test, you rest your forehead on a padded support and stare straight into the examining instrument. The instrument then blows a small puff of air onto your eye.

The tonometry instrument estimates the pressure inside your eye by measuring the change in the light reflected off the cornea as it is temporarily indented by the air-puff.

The clinician may perform the procedure several times for each eye.

Alternatively (or additionally) your optometrist may perform Goldmann tonometry (or a similar ‘applanation method’). In applanation methods, anaesthetic drops containing a fluorescein dye are placed in the eyes to numb the surface. Your optometrist will ask you to stare straight ahead whilst resting your chin and forehead against special supports on an instrument called a slit lamp.

A slit lamp is a special microscope with a light source, to which a tonometer is attached.

The optometrist will direct a broad beam of blue light from the slit lamp into your eye while they gently move the slit lamp forward until the tonometer probe lightly touches your eye. Your optometrist can directly measure your intraocular pressure by adjusting a tension dial on the tonometer.

How does it feel?

In air-puff tonometry, nothing but air directly touches the eye. You will hear the puffing sound and feel a coolness or mild pressure on your eye. This may make you flinch, but it is not uncomfortable or painful. Goldmann tonometry is not painful.

The optometrist will use anaesthetic eye drops to numb the surface of your eyes so that you will not feel the instrument touching your cornea during the test.

How does it benefit you?

The inside of the eye is nourished by the production of fluid, which drains out of the eye; if there is an imbalance between the rate at which the fluid is produced and the rate at which it is drained away, the intraocular pressure may increase or decrease from its normal level.

Measuring changes to the IOP is important, as these changes can affect other structures in the eye such as the optic nerve. Increased pressure inside the eye is often associated with glaucoma. Glaucoma is a treatable condition but, if left untreated, it can be devastating – which is why early diagnosis is so important.