What OHTS Means to Clinical Glaucoma Decisions
BY LEO SEMES, OD, FAAO
The prospective results of the Ocular Hypertension Treatment Study (OHTS) looked at patients who were at risk for developing glaucomatous damage solely because they had elevated intraocular pressure. That is to say, they did not have optic-nerve damage nor visual field loss. One of the bottom-line outcomes was that twice as many subjects in the observation group as in the treatment group developed an "endpoint." During the follow-up period, nearly 10 percent of patients untreated progressed to develop either glaucomatous change as indicated by optic nerve appearance consistent with glaucoma or visual field loss indicative of glaucomatous damage. Big deal, you say, elevated IOP is the greatest risk factor for glaucoma, so why not just recommend treatment to everyone with elevated pressure?
Change Is In the Air
The OHTS results reveal interesting clinical findings that will change the way we manage glaucoma. In a companion paper to the OHTS results, the authors demonstrate the powerful indicator that central corneal thickness makes when determining true IOP. As we have been aware for some time, thinner corneas reflect a lower IOP reading and therefore may underestimate the true IOP. It is also well known that African-Americans have, on average, thinner corneas. Combined, these two factors may signify a newly discovered "surrogate" risk representation for African-Americans. Formerly "normal" IOP readings among patients with thin corneas may actually conceal high-pressure glaucoma, not the entity that we have been referring to as normal tension glaucoma (NTG).
TABLE 1 Five-Year Risk for Developing Glaucoma
More Study Results
Other significant findings from the OHTS include age (risk increases 20 to 40 percent per decade). Other major risks include IOP (> 26mm Hg) and C/D (> 0.5). Finally, and most significantly, central corneal thickness (CCT) < 555m (microns) represented a "powerful risk factor" according to author analysis. For example, extrapolating the five-year risk of developing glaucoma, we can look at some case scenarios (Table 1). Note that for three patients of identical age, the risk fluctuates widely with variations in CCT, IOP and C/D ratio. None of these patients has a "disc at risk" judging by C/D ratio, but an arbiter of risk is CCT. Note too that the relatively elevated IOP (27mm Hg) has lower risk due to thicker CCT.
Management treatment decisions now take on a whole new specter. Decisions will be based on these factors as well as patient age and life expectancy, ease of compliance with treatment regimens, not to mention target pressures. Will we routinely measure CCT like we measure IOP?
We can revisit previous issues about how low the pressure needs to be for protection/prevention of glaucoma progression. Armed with the OHTS data, we now know that even in the treatment group, 4.4 percent of patients developed a progression endpoint. Patients with IOP up to 32mm Hg were admitted. In the treatment group, 20 percent reduction or IOP < 24 mm Hg was the control.
What emerged from the OHTS are more questions than answers. What we can take from the results will enlarge our perspective on the management of glaucoma.
Dr. Semes is an associate professor at the University of Alabama at Birmingham School of Optometry.