Testing for Glaucoma
Photo by Rita Stern
Intraocular Pressure Check
All glaucoma patients are familiar with the “pressure check” in which a drop of anesthetic is placed on the eyeball and a device to measure the pressure is placed directly on the eye. The gold standard for IOP measurement is the Goldmann tonometer, which measures the force required to flatten a small area of the central cornea. The accuracy of the Goldmann tonometer is well established to within 1 to 2 mm Hg.
Gonioscopy is a test that allows the doctor to look at the angle of the eye. The angle is located at the very edge of the cornea, where the white of the eye joins the iris (colored part) and the cornea. The angle cannot be seen by looking directly at the eye. A special lens with a mirror, the goniolens, is required to actually see the angle. The goniolens has to be placed directly on the surface of the eye. Although an anesthetic drop is used, the test can be a little unpleasant, but it is generally a pretty safe procedure.
The cornea is the clear covering of the eye, like the crystal on a watch. The cornea covers the iris, the colored part of the eye. It focuses much of the light onto the retina. The thickness of the cornea is measured with a pachymeter. Some pachymeters use light; the newer ones use ultrasound. The average cornea is about 545 microns thick, a little more than half a millimeter. The central corneal thickness apparently introduces an artifact in the IOP measurement, so that in patients with thick corneas, the real IOP is actually lower than the measured IOP. In thin corneas, the opposite is true. The result is that people with thin corneas actually have a higher real IOP than what is measured, which puts them at a greater risk.
Visual Field Testing (Perimetry)
Visual field tests are designed to map a person’s visual field, to document the level of peripheral vision. As most glaucoma patients know, the eye not being tested is patched, your chin is placed on a chin rest and with your eye being tested you are asked to focus on a small yellow spot in front of you. The machine itself looks like a large concave bowl. The test consists basically of responding every time a flash of light is perceived, all the while looking straight ahead. The ideal visual field test would be easy to take, easy to administer, and 100% reliable. We have no such test, but fortunately for everyone involved, recent years have seen substantial improvements in all of these areas. Especially welcome to the glaucoma patient are tests that are faster and less tedious.
SWAP (Short Wavelength Automated Perimetry)
SWAP stands for Short Wavelength Automated Perimetry. The visual field is tested on a standard perimeter, but instead of using a white spot on a white background, a blue spot is used on a yellow background. There is some evidence that this type of visual field testing can detect abnormalities at an earlier stage. The test, however, is much longer than the standard test. SWAP is most useful in very early cases with normal or questionable standard visual fields.
HRT – Heidelberg Retina Tomograph
The HRT (Heidelberg Retina Tomograph) uses a special laser to take 3-dimensional photographs of the optic nerve and surrounding retina. This laser, which is not powerful enough to harm the eye, is first focused on the surface of the optic nerve and captures that image. Then it is focused on the layer just below the surface and captures that image. The HRT continues to take images of deeper and deeper layers until the desired depth has been reached. Finally, the instrument takes all these pictures of the layers and puts them together to form a 3-dimentional image of the entire optic nerve.
GDx Nerve Fiber Analyzer
The GDx is a tool that uses laser to determine the thickness of the nerve fiber layer. The nerve fiber layer is important because it is the tissue of which the optic nerve is made. Damage to the nerve layer could be a sign of glaucoma. The test cannot distinguish glaucoma from other causes of nerve fiber damage, such as a vein occlusion. Care must be taken when interpreting the results.