For decades, ophthalmologists have relied on a simple test called tonometry to check for high pressure inside the eye, often an early warning of glaucoma. Many patients know it as the "air puff" test, although it has been replaced in many offices by a puffless version. But now, ophthalmologists such as Lorin Press in Edison, N.J., are saying that without a second test, called pachymetry, to measure corneal thickness, they can miss someone at risk for glaucoma. Trouble is, most insurers won't cover pachymetry unless the patient is already known to have -- or suspected of having -- the disease. So Press doesn't test most of his patients.
All this may change in a few years as more studies establish the importance of pachymetry, which uses ultrasonic waves to measure the thickness of the transparent tissue covering the iris and pupil. Until then, pachymetry will remain another case where patients may have to take matters into their own hands, asking for the two-minute, painless test by name and agreeing to pay the $50-or-so cost if their insurer won't.
Patients with high intraocular pressure (IOP) are more likely to develop glaucoma, which afflicts about 2.5 million Americans, damages the optic nerve, and blinds tens of thousands. Ophthalmologists have long known that an unusually thin or thick cornea can skew results from the pressure test. Just as it takes less pressure to indent a balloon than an automobile tire -- regardless of the air pressure inside -- a thinner cornea is more easily pushed in, tricking the tonometer into registering a lower pressure. A patient with a thin cornea may seem to have normal eye pressure even if it's high. Conversely, one with a thick cornea may have a high reading, leading an ophthalmologist to treat a problem that may not exist.
What many ophthalmologists didn't know until recent years is just how much corneal thicknesses vary. That means the risk of a false IOP reading is higher than previously thought. In addition, new surgical procedures are creating patients at greater risk of a false tonometry reading. Those who undergo laser surgery to correct their vision typically wind up with thinner corneas and are therefore more likely to test low, meaning problem pressure might be missed, says Dr. James Brandt, director of the glaucoma service at the University of California at Davis and one of the researchers in a landmark study on treating patients with high IOP.
That multicenter study, funded by the National Institutes of Health and reported in the Archives of Ophthalmology in 2002, shows that corneal thickness itself can be a powerful predictor of glaucoma. The study found that in patients already considered at risk because of high IOP, those with the thinnest corneas were three times as likely to develop glaucoma as those with thick corneas.
Because the study didn't include subjects drawn from the general population, the American Academy of Ophthalmology hasn't called for broad pachymetry testing, says Dr. Joel Schuman, chairman of the ophthalmology department at the University of Pittsburgh School of Medicine and a spokesman for the AAO. "The Academy is conservative in the recommendations that it makes," he says. "Without having the hard evidence, it's harder to recommend the test that broadly." In his practice, Dr. Schuman says he tests everyone because he believes it's important to do so. Glaucoma can be treated with medication, laser treatments, or surgery, but vision lost before the disease is discovered cannot be restored.
That's why spotting a problem before the damage is done is crucial. So next time you go for an eye exam, take the extra $50 you had planned to spend on designer frames and pay for a pachymetry test instead. You might not look as stylish, but you could wind up seeing better.
By Carol Marie Cropper