Glaucoma is a common disease of the eye that causes damage to the optic nerve. It is the No. 1 cause of irreversible blindness in the world. In almost all cases the damage to the optic nerve is caused by the build up of pressure in the fluid of the eye. If caught early and treated, the damage can normally be controlled, so a glaucoma test is performed routinely during eye exams. The three main types of glaucoma test are tonometry, ophthalmoscopy and perimetry, and all are quick and painless.
Tonometry measures the internal pressure in the eye by recording the resistance of the cornea to pressure. There are several tonometry methods used by doctors, including the “air puff” method that many people are familiar with. For this test, the patient is told to look straight ahead into a special machine that blows a measured puff of air onto the open eye. The machine measures how much the air puff indents the eye surface and uses this information to calculate the pressure inside the eye, which is known as intraocular pressure. While the air-puff method is not the most accurate testing method available, it is simple to administer, totally painless, does not require the use of numbing eye drops, and is easy to use on children.
Other tonometry methods do require numbing eye drops, and then the gentle application of a small probe to the cornea to record how much pressure is required to lightly flatten or indent the cornea. This type of tonometry is very accurate and is usually done as a follow-up after an air-puff glaucoma test has shown increased or higher-than-normal intraocular pressure. It is painless, and the numbing drops wear off approximately 30 minutes after application.
Ophthalmoscopy is a type of glaucoma test that looks for damage to the optic nerve by using a special instrument called an ophthalmoscope. Drops are used to dilate the pupils of the eye, and then the ophthalmoscope shines a small, bright light into the eye. If glaucoma is present, the pressure increase in the eye can cause visible changes to the optic nerve that can be seen through the ophthalmoscope; these changes include cupping in the center of the nerve and changes to its normal color.
Perimetry is a method of testing for vision loss using computerized light sequences. The patient wears a patch over one eye and uses the uncovered eye to look at the middle of a large, white disc suspended on the wall. A computer flashes tiny lights in a seemingly random pattern in the disc, and the patient signals each time he sees a light flash. The computer records the responses and then prints a very detailed map of the field of vision in each eye. Glaucoma causes damage to the vision field in a very specific pattern that the computer can detect.
Glaucoma is the leading cause of blindness, so screening is done routinely by optometrists and ophthalmologists. Although the tonometry air-puff glaucoma test is not the most accurate test available, it is a very valuable first test to help single out people whose pressure is borderline or above and who need to be watched or checked further. The irreversible blindness caused by glaucoma is almost always preventable if the disease is caught early and treated.