Intraocular pressure reduction with a fixed treatment protocol in the Early Manifest Glaucoma Trial

Abstract

To evaluate: (i) the relationship between intraocular pressure (IOP) reduction attained with a fixed treatment protocol and the untreated IOP level; (ii) the consistency of IOP reduction over time; and (iii) whether there is a threshold pretreatment IOP level below which IOP reduction might be less effective. Results are based on 128 patients with glaucoma with field defects, who were randomized to the treatment arm of the Early Manifest Glaucoma Trial (EMGT). The EMGT fixed treatment protocol consisted of 360° laser trabeculoplasty and topical betaxolol eye drops B.I.D. Treatment was unchanged as long as progression did not occur. Analyses assessed the initial IOP reduction after 3 months and also the mean reduction based on all follow-up values; IOP changes over time were evaluated with linear regression analysis. Factors influencing initial and mean IOP reduction were also explored using linear models. Mean age at baseline was 68 years, and untreated baseline IOP ranged from 13 to 30.5 mmHg. On average, eyes with higher baseline IOP experienced larger pressure reductions than eyes with lower baseline IOP, whether expressed in mmHg or as percentages. Each mmHg of higher baseline IOP was associated with approximately 0.6 mmHg larger IOP reduction. IOP changed little over time, with 66% of patients changing less than 0.5 mmHg/year, and only 13% (17/128) changing >1.0 mmHg/year. The treatment protocol did not achieve any average IOP reduction in eyes with baseline pressures ≤ 15 mmHg. Factors related to more IOP reduction at 3 months were higher baseline IOP and positive refractive error, while higher baseline IOP and male gender (more reduction) and cardiovascular disease history (less reduction) were associated with mean IOP on treatment. With a fixed treatment protocol, the IOP reduction achieved depended very strongly on baseline untreated IOP levels. There seemed to be a lower threshold around 15 mmHg, where therapy did not result in any reduction of IOP. Our results suggest that when effects of IOP-lowering treatment are reported, whether expressed in mmHg or as a percentage of untreated pressure levels, the baseline IOP levels should be specified as well.