June 30, 2021

Improving the Power of Glaucoma Neuroprotection Trials Using Existing Visual Field Data

Author(s): Giovanni Montesano (1), Harry A Quigley (2), David P Crabb (3)

1 City, University of London Optometry and Visual Sciences (G.M., D.P.C.), London, United Kingdom; NIHR Biomedical Research Centre, Moorfields Eye Hospital NHS Foundation Trust and UCL Institute of Ophthalmology (G.M.), London, United Kingdom.
2 Wilmer Institute, Johns Hopkins School of Medicine (H.A.Q.), Baltimore, MD, USA.
3 City, University of London Optometry and Visual Sciences (G.M., D.P.C.), London, United Kingdom. Electronic address: david.crabb.1@city.ac.uk.

PURPOSE: Selecting reliable visual field (VF) test takers could improve the power of randomized clinical trials in glaucoma. We test this hypothesis via simulations using a large real world data set.

DESIGN: Methodology analysis: assessment of how improving reliability affects sample size estimates.

METHODS: A variability index (VI) estimating intertest variability was calculated for each subject using the residuals of the regression of the mean deviation over time for the first 6 tests in a series of at least 10 examinations for 2,804 patients. Using data from the rest of the series, we simulate VFs at regular intervals for 2 years. To simulate the neuroprotective effect (NE), we reduced the observed progression rate by 20%, 30%, or 50%. The main outcome measure was the sample size to detect a significant difference (P < .05) at 80% power.

RESULTS: In the first experiment, we simulated a trial including one eye per subject, either selecting randomly from the database or prioritizing patients with low VI. We could not reach 80% power for the low NE with the available patients, but the sample size was reduced by 38% and 49% for the 30% and 50% NE, respectively. In the second experiment, we simulated 2 eyes per subject, one of which was the control eye. The sample size (smaller overall) was reduced by 26% and 38% for the 30% and 50% NE by prioritizing patients with low VI.

CONCLUSIONS: Selecting patients with low intertest variability can significantly improve the power and reduce the sample size needed in a trial.

Am J Ophthalmol. 2021 Apr 24;229:127-136. doi: 10.1016/j.ajo.2021.04.008.

PMID: 33905747

Keywords: Clinical trial; Glaucoma; Neuroprotection; Perimetry; Visual field

NGP Papers manager: Carlo Cutolo