Natural Course and Risk Factors of Glaucoma Development in the Untreated Fellow Eye in Unilateral Normal-Tension Glaucoma

PURPOSE. To test whether the level of intraocular pressure (IOP) significantly affects the development of glaucoma in the untreated fellow eye of patients with unilateral normal-tension glaucoma (NTG). METHODS. Long-term observational study on consecutive patients newly diagnosed with unilateral NTG and followed up for >5 years with more than five IOP measurements. We investigated the factors associated with NTG development in the untreated fellow eye, including the longitudinal IOP profile. RESULTS. A total of 194 patients with a baseline age of 50.5 ± 12.1 years were followed up for 8.4 ± 1.6 years. NTG development in the untreated fellow eye occurred in 48 patients (24.7%), with a mean interval of 5.2 ± 2.1 years. Patients with fellow eye NTG development had significantly higher baseline untreated IOP in both eyes and higher follow-up IOPs in the fellow eye compared to those who did not (all P < 0.05). Kaplan–Meier survival analysis and multivariate Cox regression analysis revealed that an average follow-up IOP ≥14 mm Hg in the fellow eye was associated with a significantly increased risk of glaucoma development compared to those with IOP <14 mm Hg (hazard ratio, 3.626; 95% confidence interval, 1.519–8.654; P = 0.004), particularly for the eyes with baseline IOP ≥14 mm Hg. CONCLUSIONS. In this 8-year cohort, we observed NTG development in the untreated fellow eyes in 24.7% of the patients with unilateral NTG. The untreated average follow-up IOP was significantly associated with NTG development, with an average IOP ≥14 mm Hg exhibiting a threefold increased risk.