The eye clinic tests
A number of tests are performed to examine the eyes. These tests include measurement of the eye pressure (tonometry) and examination of the drainage angle (gonioscopy), the field of vision (visual field test or perimetry) and the optic nerve (with the microscope, photography or imaging).
The standard form of tonometry is called ‘Goldmann tonometry’. An anaesthetic drop containing a yellow dye is given to numb the front of the eye. The doctor or nurse will shine a blue light into your eye. The surface of the eye will be touched lightly with an instrument that measures the pressure. This is more comfortable than the air puff test, which is generally used by optometrists (opticians) to test pressure when screening for glaucoma.
Some hospital clinics use a new form of tonometer, called the Pascal Dynamic Contour Tonometer. This instrument is less affected by differences between individuals in corneal thickness and may be a little more accurate than Goldmann tonometry. This test allows the doctor to monitor the effect of treatment.
This is the term given to the measurement of the thickness of the cornea. An anaesthetic drop is given to numb the eye and then an instrument that looks like a small blunt pencil is brought up to gently touch the surface of the eye. It often makes a ‘peep’ as the measurements are taken. The corneal thickness helps with the accuracy of the pressure measurements.
An anaesthetic drop is given to numb the front of the eye. The doctor holds a special lens against the surface of the eye and examines the drainage angle. The examination is not uncomfortable, although you may feel the lens brush your eyelashes. This test helps the doctor decide the cause for the high pressure in your eye.
Visual field test – Perimetry
This test examines the quality of sight surrounding the central vision. You will be asked to sit in front of a machine with an illuminated bowl and a chin rest. During the test you are asked to keep your eye fixed on a centre point of the bowl while spots of light flash in different parts of the bowl, away from the central part of your vision. You have to press a button each time you see a light flash. You are not expected to see all the flashes because some are very dim. The test lasts less than 10 minutes for each eye. This test tells the doctor how much of the vision has been damaged and is important for monitoring the effectiveness of treatment.
Examination of the optic nerve
Drops are put in the eye to make the pupil bigger (dilating drops). These take about 20 minutes to work and last a few hours. While you sit at the microscope (slit lamp), the doctor holds a lens in front of the eye and looks at the nerve at the back of the eye with a white and green light. The light can seem quite bright, but is harmless. The dilating drops may temporarily blur your vision, preventing you from being able to drive or read. You should not, therefore, drive to your appointment unless someone else can drive you back.
The doctor may order photographs of the optic nerve. These are taken with a special camera and a bright flash of light. Some clinics have more modern imaging devices, such as the scanning laser ophthalmoscope and optical coherence tomography. These devices take pictures with a light that is less bright than a camera. These tests are helpful in the early stages of glaucoma, or before glaucoma damage has occurred, to detect signs of developing optic nerve damage.