More than 1 million patients are treated for glaucoma in the United States every year. Yet until recently, little research was done on the typical clinical characteristics of these patients or the type of therapy they receive. Now, studies are providing evidence that many of these patients don't have glaucoma at all, which means an unknown number of people are being unnecessarily subjected to drug side effects, medical expenses and the fear of possible blindness.

To prevent this kind of overmedication, doctors need to be more aware of the potential for misdiagnosis. They also need to have access to tests that can more definitively determine whether a patient does have glaucoma. Our experience in the latter area may be helpful. By using a modified version of the Arden contrast sensitivity test (developed by Scott Brodie, M.D.), along with other ancillary testing, we've been able to distinguish between those patients who require treatment and those whose treatment can be safely decreased or eliminated.

In 1992, the American Academy of Ophthalmology (AAO) published recommendations defining glaucoma as the presence of glaucomatous optic nerve damage in the disc or nerve fiber layer, or abnormalities of the visual field. Although glaucoma patients usually have elevated intraocular pressure (IOP), elevated IOP, by itself, is not definitive evidence that a patient has glaucoma.

Why would so many patients who don't have glaucoma end up being treated for it? The answer appears to be an over-reliance on elevated IOP as a symptom of glaucoma.

For example, Rhonda L. Bohn, MPH, ScD, et al, evaluated 544 patients who were diagnosed with primary open angle glaucoma at a group model HMO located in central Massachusetts. (The study was reported in the Journal of Glaucoma, Vol. 9, No. 1, 2000.) They found that 44.7% of the patients had an elevated IOP, but no other clinical symptoms.

The authors also noted that 9% of the same group met none of the criteria necessary for the diagnosis of glaucoma, as defined by the presence of 3 clinical findings:

• IOP greater than or equal to 22 mm Hg
• optic disc changes, including a cup-to-disc ratio equal to or greater than 0.8
• cup-to-disc asymmetry equal to or greater than 0.2, or morphologic disc changes consistent with glaucomatous optic neuropathy, including visual field deficit consistent with glaucoma.

The authors concluded that the most common reason for initiating long-term therapy for primary open angle glaucoma was an elevated IOP, even when other risk factors were absent.

At St. Luke's Cataract & Laser Institute, we've been using GLSCRN, a glaucoma screening software program designed by Scott Brodie, M.D., to assist us in diagnosing patients who may or may not have glaucoma.

GLSCRN is a modified version of the contrast sensitivity test originally developed by Dr. Geoffrey B. Arden. Arden's original test required the use of very costly research-grade display equipment. Dr. Brodie's modified test can be performed using a standard VGA computer monitor, making it easy to use in a clinical setting.

When taking the modified Arden test, the patient sits facing a computer screen (see photo, above, right). He focuses on a Landolt C target, which is shown in various degrees of color and intensity. The target's contrast relative to the background is shifted up and down in increasingly precise increments to determine the threshold at which the patient can no longer visualize the target.

Like Arden's original diagnostic test, this simplified version measures parafoveal tritan contrast sensitivity. This approach to detecting glaucoma is based on four premises:

• The blue color vision system is mediated by a pathway that is relatively rare in the visual system; only about 5% of the total number of ganglion cells detect blue.
• A large number of acquired color vision defects are of the blue-yellow variety.
• There isn't much redundancy in the blue color vision system, so deficits in the performance at different regions are easy to quantify.
• Early glaucomatous visual field defects have been found as central focal abnormalities in glaucoma patients using multifocal electroretinograms.

While the modified version of the test can't achieve the level of accuracy of Arden's original, it achieves a sufficiently high level of accuracy (something approaching 90% of the original test's accuracy) to be a useful glaucoma screening tool -- at a small fraction of the equipment cost.

Tests conducted by Dr. Brodie in the early 1990's demonstrated that the modified test does indeed help to determine the presence of glaucoma. In his tests, the average parafoveal tritan contrast sensitivity of normal patients almost always fell below 15, whereas the vast majority of glaucomatous patients had a threshold greater than 15.

By using this program in conjunction with other diagnostic tests, we've been able to diagnose patients who do not have glaucoma and discontinue their medication. Here are case histories of two patients we've diagnosed:

• Case #1.
A 78-year-old white male presented for evaluation of cataract and glaucoma. He related a history of primary open angle glaucoma, diagnosed 2 years prior, with a peak IOP of 21 mm Hg OD and 23 mm Hg OS. He was being treated with Timolol 0.5% OU qam, and Xalatan OU qhs, with IOP on presentation of 15 mm Hg OD and 12 mm Hg OS.
The patient had a cup-to-disc ratio of 0.2 OU with full neural rims, a full Humphrey visual field 30-2, and modified Arden values averaging 9 OD and 8 OS.
Because of these test results, we discontinued his glaucoma drops, and he underwent uneventful cataract surgery. His IOPs have remained in the low teens, without medication, and he maintains a normal visual field.
• Case #2.
A 79-year-old white female was diagnosed with glaucoma 8 years prior to presentation, at which time topical drops were started. She sought care for glaucoma after moving from another state to Florida. On presentation, she was taking Timoptic 0.25% OU qam, and Alphagan OD bid. Her IOP measured 17 mm Hg OD and 18 OS.
Her cup-to-disc ratios were 0.2 OU with full neural rims. Humphrey visual field 30-2 was full. Modified Arden testing showed an average value of 9.5 OU. The evidence strongly suggested that this patient did not have glaucoma.
Timoptic was discontinued and the patient was rechecked 1 month later with IOPs of 19 and 17 mm Hg. Alphagan was discontinued at that visit. The patient was again checked 1 month later, when IOP was recorded at 20 and 15. The IOP has remained below 21 mm Hg OU on subsequent visits.

Unnecessary glaucoma treatment is a serious matter, and one that puts a financial, emotional and sometimes physical burden on the patient. However, our success using the GLSCRN software at St. Luke's indicates that it is possible
for doctors to minimize the problem -- without having to purchase expensive equipment.

The GLSCRN software may be obtained free of charge by contacting Dr. Scott Brodie at brodis01@doc.mssm.edu. OM