Glaucoma Screening Tests
Three tests are commonly available to screen for glaucoma: tonometry, ophthalmoscopy, and perimetry.
In tonometry, a device (a tonometer, such as the air puff device) measures intraocular pressure. Unfortunately, no one knows whether elevated IOP is an accurate indicator of either current or future glaucoma. As many as 70% of those with high IOP never have vision problems caused by glaucoma. Also, single measurements of IOP are not particularly good indicators of glaucoma. When IOP is measured on a random basis, only half of all patients who have glaucoma also have IOP at the "ocular hypertension" level (over 21 mm Hg). There is no single cutoff value for IOP that can be used as a guideline when screening for glaucoma.
In ophthalmoscopy, a healthcare provider uses a viewing instrument (an ophthalmoscope) to look directly at the optic disc (the area where the nerves come together and leave the eye). He or she is looking for the changes that often precede glaucoma. As a screening tool, ophthalmoscopy has some limitations. First, because even experts vary greatly in their interpretation of what they see, this method has poor accuracy in pinpointing glaucoma: ophthalmologists using ophthalmoscopy alone may detect fewer than half of all cases of glaucoma. Your primary-care physician probably would be even less accurate. Second, no combination of features of the optic nerve can be used with certainty to distinguish people with glaucoma from normal people.
In perimetry, which is the measurement of visual fields, you respond to objects of varying brightness that are presented at various spots in your field of vision. Although perimetry is the gold standard for diagnosing glaucoma, it, too, has limitations as a screening test. It is expensive, time-consuming, and technically cumbersome. Also, perimetry detects patients late in the course of glaucoma.
