The European Glaucoma Prevention Study and the Ocular Hypertension Treatment Study: why do two studies have different...

To understand the results of the Ocular Hypertension Treatment Study and the European Glaucoma Prevention Study and to explain differences in their findings and recommendations.

The value of lowering intraocular pressure with medical therapy to delay or prevent the progression from ocular hypertension to primary open-angle glaucoma has been the subject of two recently completed large clinical trials, the Ocular Hypertension Treatment Study and the European Glaucoma Prevention Study. In the Ocular Hypertension Treatment Study, patients were randomized to either medical treatment to achieve a 20% reduction in intraocular pressure or to careful observation. European Glaucoma Prevention Study patients were randomized to receive either topical dorzolamide or the vehicle for the commercially available solution without a specific intraocular pressure goal. The apparent difference in benefit of reducing the conversion rate by lowering intraocular pressure in these two studies may be explained by understanding the methodological differences in study design.

The benefit of lowering intraocular pressure to reduce the rate of progression from ocular hypertension to primary open-angle glaucoma has been established in the Ocular Hypertension Treatment Study. Failure to demonstrate a comparable benefit in the European Glaucoma Prevention Study relates to the selective drop-out of treated and untreated patients with higher intraocular pressure levels and to the failure to achieve sufficient lowering of intraocular pressure. The evidence that the placebo effect was responsible for progressive intraocular pressure-lowering over the 5-year study is not convincing and is likely a result of the regression to the mean phenomenon.