By Dr Ananya Mandal, MD
Diagnosis of glaucoma is difficult based on symptoms alone. Usually the condition is diagnosed in early stages by regular screening. This is because glaucoma takes a slow course and if detected late may have already damaged the vision to a large extent. It is therefore important to have regular eye tests and screening so that problems can be detected and treated as early as possible.
Eye screening tests are recommended every two years or more frequently if there is presence of other risk factors. Common risk factors include age, ethnicity, family history of glaucoma, diabetes and other eye diseases.
Eye tests for glaucoma
There are several glaucoma tests that can help detect the intraocular pressure and the extent of damage. The tests should be carried out during the same appointment to ensure results are as accurate as possible. These tests include:-
Tonometry or test for intraocular pressure
This uses an instrument called a tonometer to measure the intraocular pressure. Before the test a small amount of local anesthetic is sprayed or applied over the cornea. Thereafter a dye is placed onto the cornea. A blue light from the head of the tonometer is held against the eye to measure the intraocular pressure. Tonometry can diagnose ocular hypertension as well. Normal readings are between about 10 mmHg and 21 mmHg. There are various tonometers available but the most frequently used in a hospital setting is Goldmann's applanation tonometer.
Measurement of the central corneal thickness
The thickness of the central part of the cornea is measured as this gives an idea of the damage caused by the raised intraocular pressure.
This is an examination of the angle from where the aqueous humor drains. The angle is formed between the cornea and the iris. A gonioscopy can help to determine whether this angle is open or closed (blocked).
Visual field test (perimetry)
This is a computerized test that checks for loss of vision field or peripheral visual fields due to glaucoma. The individual is placed before the instrument and shown a sequence of light spots and asked which ones they can see. These spots are placed randomly all over the field of vision. Those in the periphery may not be visible to individuals who have damage to their visual field due to glaucoma.
Assessment of the retina and optic nerve
The optometrist first uses eye drops to dilate the pupils. Then the back of the yes or the retina is examined using a slit lamp (a microscope with a very bright light). This can show optic nerve damage. The optic disc is visulaized. Optic disc damage is assessed by looking at the vertical ratio of the pale centre (cup) to the overall size of the disc. A small cup and a thick rim (the darker edge surrounding the cup) may give a ratio of 0.3 or less (normal). A small number of people have a cup:disc ratio up to 0.7 but anything over that may indicate a pathology.
Reviewed by April Cashin-Garbutt, BA Hons (Cantab)