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THE Ocular Hypertension Treatment Study (OHTS) has made the prevention of glaucoma in ocular hypertensives a real possibility, although the identification of at-risk patients will require more effort on the part of the physician. The OHTS study was the first of its kind to scientifically demonstrate that lowering eye pressure can delay and possibly prevent the development of glaucoma. It also provided some strong indications of which patients were most at risk of developing glaucoma if left untreated, said M Roy Wilson MD at the 4th International Glaucoma Symposium. "The study showed that you can identify people who are going to be at particularly high risk and those who will be at low risk. You may want to consider treating those at high risk," Dr Wilson said. The study involved 1,636 ocular hypertensives with an IOP ranging from 24mmHg to 32mmHg. It showed that only 4.4% of patients randomised to receive IOP lowering medications went on to develop glaucoma over the five-year period, compared with 9.5% of those in the observation group. In addition, a multivariate risk factor analysis of the trial results indicated that increased IOP, age, cup-to-disc ratio and thin corneas were all risk factors for the development of primary open-angle glaucoma among ocular hypertensive patients. "Using various combinations of these risk factors one can develop a risk profile. For example, you can place corneal thickness on one axis and baseline IOP on the other and the combination of those two factors can give you either low risk or very high risk. You can do the same thing by using corneal thickness and vertical cup disc ratio," he said. Dr Wilson presented some case studies to illustrate how risk factor profiling might be applied to individual patients. The first was a 70-year-old with an IOP of 27mmHg and a cup-to-disc ratio of 0.7, with a moderate corneal thickness of 520 microns. In that case, the five-year risk of developing glaucoma is going to be quite high at 30% to 40%, he said. The second case study involved a 50-year-old patient with an IOP of 23mmHg and a corneal thickness of 590 microns. In that patient, the five-year risk of developing glaucoma would be fairly low, probably between 1% and 5%. "However there are other factors you may want to consider. They include the general health and life expectancy of the person. You have to remember that these are people with a non- discernible disease," Dr Wilson said. In countries without fully socialised medicine, a patient’s finances may also play a role in the decision, he pointed out. Some patients may have to choose between taking an IOP-lowering medication and another medication which may be more important for their general well being. Dr Wilson noted that the OHTS findings have led many to arrive at conclusions that may be dangerously premature and inaccurate. One is that patients of African descent will not benefit from IOP-reducing treatment and an other is that diabetes reduces the risk of glaucoma. The study showed that the rate of progression among hypertensive African Americans was 6.9% in the treatment group, compared to 12.7% in the observation group at five years follow-up. The difference was not statistically significant. However, the lower rate of glaucoma prevention among African Americans, compared to the overall group, may have been partly attributable to the fact that the study’s investigators extended the recruitment of African Americans for an additional six months in order to have the number necessary for a statistically significant result. The six-year results have shown a greater divergence between treated and untreated African Americans. In fact, the glaucoma reduction in that sub-group now has reached statistical significance. "I think this is important because I’ve heard a few people presenting the results from OHTS in a way that suggests that treatment is not effective among African Americans. I think that is the wrong conclusion," Dr Wilson said. As regards diabetes, one of the study’s surprising outcomes was that diabetics actually had a lower rate of glaucoma progression than the overall group. Previously, most glaucoma specialists had considered diabetes to be a risk factor for the disease. However, Dr Wilson strenuously cautioned against taking the results to mean that diabetes might actually prevent glaucoma. He pointed out that clinical confirmation of diabetes was not part of the study’s protocol and the diagnosis was based only on the patients’ history. Therefore many patients without diabetes may have been misclassified as having the disease and many with the disease may have been misclassified as not having it. Furthermore, the number of diabetics in the study was a small and very select group. In order to be included in the study, patients had to be free of any retinopathy. The diabetics in the study would therefore have been somewhat atypical. "I don’t actually believe and I don’t think that most people believe that diabetes is protective in the development of glaucoma," Dr Wilson said. The risk factors identified by OHTS also pointed to the need for a more extensive examination of ocular hypertensives before making a decision of whether or not to treat them. In particular, optic nerve head assessment and corneal pachymetry are now emerging as essential components for the initial work-up of such patients, he said. Changes in the optic nerve generally antedate changes in visual field and they are often the earliest sign of glaucoma. Moreover, they can occur in the absence of an elevated IOP. Early detection of such changes through optic nerve head assessment can therefore enable early treatment and preserve patients’ vision. Corneal pachymetry, meanwhile, is useful not only in identifying those who have thin corneas - the strongest risk factor in the study - but also in determining the patient’s true IOP. Thinner corneas are less resistant to applanation instruments. In eyes with such corneas, tonometry can fail to detect the imbalance between the inflow and outflow of the aqueous humour which would be measured as an increased IOP in an eye with normal corneal thickness. The reverse is true in eyes with unusually thick corneas, he explained. Therefore, without corneal pachymetry, many patients with thin corneas who need treatment will not receive it, while many with thick corneas will receive treatment unnecessarily, he noted. "One should look at optic nerve assessment and corneal pachymetry very carefully in all ocular hypertensives, particularly in the initial examination," Dr Wilson said. M Roy Wilson MD |
