Reassessment of the Collaborative Normal-Tension Glaucoma Study: Statistical Evidence and Implications for Current...

The Collaborative Normal-Tension Glaucoma Study (CNTGS) is frequently cited as evidence that a 30% reduction in intraocular pressure (IOP) slows progression in normal-tension glaucoma (NTG). This study re-examines the statistical methodology of CNTGS to assess how its conclusions are supported by the data.

This study reviews the CNTGS design with emphasis on survival analysis methodology, including the definition of time zero, censoring rules, and intention-to-treat (ITT) versus per-protocol comparisons. Particular attention is given to the post hoc redefinition of baseline and the handling of cataract-related visual decline, assessing their impact on the reported treatment effect.

CNTGS shifted the analytical baseline for the treatment group to the point of IOP stabilization, thereby excluding early progression events and introducing immortal time bias. Additionally, cataract-related visual decline, more frequent in the treatment group, was censored rather than treated as a competing risk or time-dependent covariate. These methodological choices reduced the number of counted progression events in the treatment arm. Although the adjusted per-protocol analysis yielded a statistically significant treatment effect, this effect disappeared under the original ITT analysis, which included all randomized eyes from time zero and all progression events.

The potential treatment benefit reported in CNTGS depended largely on post hoc analytical modifications, whereas the original ITT analysis did not support a statistically significant effect of IOP reduction. These findings highlight the importance of transparent survival analysis methods and strict adherence to ITT principles in future NTG trials. Well-designed prospective studies that avoid immortal time bias and model treatment-related events appropriately are needed to clarify the true role of IOP reduction on NTG management.