Tonometry

Tonometry An early warning for glaucoma, this simple test can detect low eye pressure, or IOP (intraocular pressure), an indicator of other eye disorders such as inflammation, a detached retina and a poor blood supply to the eye. It’s quick, and usually carried out by an ophthalmologist, but it’s also notoriously unreliable.

Tonometry comes in three varieties. The most accurate is the Goldmann applanation method, which measures the amount of force necessary to flatten the cornea; the higher the IOP, the greater the force needed. Another option is Schiotz, or indentation, tonometry, which uses a preset amount of weight to gently press a plunger into the cornea, making a slight indentation. The third method is non-contact, or air-puff, tonometry, in which a puffer blasts air at the cornea, and the rebound air is then measured. Considered the most inaccurate of the three, it is also the least invasive.

Topical anaesthetic eyedrops are used with both applanation and Schiotz tonometry, with applanation also using eyedrops containing a special dye called ‘fluorescein’. As this dye can permanently stain contact lenses, these should be removed before taking the test. The ophthalmologist should also check if you have an irregularly shaped or deformed cornea – in which case, a special tonometer will need to be used. Regardless of method, the test takes just a couple of minutes, although it may take several hours before the anaesthetic wears off.

Is it accurate?
As tonometry is unreliable, researchers usually stress that the test needs to be carried out as part of a general eye examination, which may reveal problems more readily than tonometry can. Ophthalmologists who rely on tonometry alone to determine whether to carry out a more exhaustive examination may be doing the patient a great disservice.

One study tested all three types of tonometry, and found that none was entirely successful in detecting problems with eyes with known conditions. Although the most accurate was the Goldmann method, even this was not sensitive across the entire range of pressures tested. Ophthalmologists would still need to check the optic nerve and visual field to be sure of a diagnosis of glaucoma (Br J Ophthalmol, 1997; 81: 448-51).

A study by the Moorfields Eye Hospital in London arrived at similar conclusions. In this case, they concentrated only on the applanation test (Eye, 2002; 16: 562-6).

Is it safe?
If the test is unreliable, it is at least safe, or so the eye specialists maintain. The worst that can happen is a slight scratching of the cornea, but this will heal within 24 hours or so, they say. The dye used in the Goldmann applanation test is also relatively harmless, though it can permanently discolour contact lenses.

However, there is a risk that Creutzfeldt-Jakob disease can be contracted from the tonometry equipment (see box above).