Testing For Glaucoma

Tonometry measures the pressure within the eye. One of the main risk factors for glaucoma is high eye pressure and the best known treatment for glaucoma is lowering the eye pressure. Therefore the accurate measurement of the eye pressure (tonometry) is essential. The Goldman Applanation Tonometer is the most accurate way of measuring the eye pressure. It involves numbing the eye with eye drops first and then the instrument gently contacts the front of the eye to take the measurement.

Gonioscopy is the examination of the drainage angle for the  outflow of intraocular fluid. Fluid is constantly being made in the eye and it flows out of the eye at the drainage angle. This test can determine if the high eye pressure is caused by a closed/blocked angle (angle closure glaucoma) or if the angle is open but just not working well enough (open angle glaucoma). This is important because the management of each sub-type is slightly different. The test involves putting a mirrored lens on the surface of the eye after using a numbing drop, almost like wearing a contact lens.

Ophthalmoscopy is the visual examination of the optic nerve. Since glaucoma is a disease of the optic nerve, this is a key test. Dilating drops are usually given to enlarge the pupil, so that the optic nerve can be more clearly seen. The nerve(s) will be looked at for signs of glaucoma-related nerve cell loss. This can be done using a table top slit lamp or hand held ophthalmoscope. The appearance of the optic nerve can be documented with a drawing, a photo or with an imaging device. This is so any worsening of the nerve appearance can be detected in the future.

Glaucoma initially causes peripheral vision loss that the patient does not notice. This vision loss can be detected with a perimetry test (also known as a visual field test). It involves testing each eye separately with an automated machine that flashes a series of small lights in the periphery to which the patient should react by pressing a button. The perimetry test takes around 3-6 minutes per eye and is repeated 1-2 times a year so that every new test can be compared to the previous ones to look for any worsening.

An OCT scan is an important test in diagnosing and monitoring glaucoma. It is used to measure the retinal nerve fibre layers (RNFL) around the optic nerve which is an important marker of early to moderate glaucoma damage. For glaucoma suspects and those at the very early stages of glaucoma, the visual field test may not show any peripheral field damage however, the measurements from the OCT can detect early signs of glaucoma. The OCT measurements can also help monitor any changes and progression with glaucoma. This can help determine the treatment plan.

Pachymetry is a test that measures the thickness of the front window of the eye (cornea). The eye is numbed with drops then a contact probe quickly measures the thickness in a few seconds. A very thick or very thin cornea can affect the pressure/tonometry readings and also a very thin cornea increases the risk of developing glaucoma.