Ophthalmologists use three types of tests to screen for glaucoma in people at risk, make the diagnosis, and follow people during treatment. The three tests are tonometry to measure intraocular pressure (IOP), ophthalmoscopy to inspect the optic nerve, and perimetry to test the visual fields. A high IOP warrants further testing, and the final diagnosis is made by also finding evidence of optic nerve damage or by identifying defects in the visual fields characteristic of glaucoma.
Tonometry. Tonometry measures IOP by assessing the amount of force necessary to make a slight indentation in a small area of the cornea. The most effective way to do this is with applanation tonometry. Anesthetic eyedrops are administered to numb the eye. Then, slight pressure is applied to the cornea with a small instrument while the doctor looks through a table-mounted microscope (called a slit lamp). A handheld tonometry device (Tono-Pen) also is sometimes used and is relatively accurate. Tonometry is painless and poses virtually no risk to the cornea.
Ophthalmoscopy. Examination of the optic nerve is required for the diagnosis of glaucoma, and periodic examinations of the nerve are valuable to follow the progress of the disorder during treatment. To perform ophthalmoscopy, the doctor dilates the patient's pupils with eyedrops and then uses an ophthalmoscope--a special instrument with a small light on the end--to magnify and examine the optic nerve. Another type of ophthalmoscopy uses a specialized microscope that allows three-dimensional visualization of the optic nerve. Signs of a damaged optic nerve include "cupping" in its center and a loss of its normal pink color.
Perimetry. The patient wears a patch over one eye and looks straight ahead at a bowl-shaped white area, while a computer presents lights in fixed locations around the bowl. The patient indicates each time he or she sees a light. Perimetry provides a "map" of the visual fields. The type of vision loss associated with glaucoma is relatively specific, and perimetry can detect the typical visual-field defects of glaucoma.