In patients with unilateral normal tension glaucoma, a contralateral eye with low central corneal thickness (CCT), high Maximum width of β-zone parapapillary atrophy-disc diameter (MWβPPA-DD), and high intraocular pressure (IOP) are more likely to develop glaucoma, according to research results published in the British Journal of Ophthalmology

Currently, no consensus exists on which risk factors may influence glaucoma conversion of the contralateral eye, nor does a consensus exist on whether and when to treat it. Researchers therefore conducted a study to investigate the rate of glaucoma conversion in contralateral eyes in patients with unilateral normal-tension glaucoma who have been follow-up for at least 5 years. Investigators also sought to determine the potential risk factors associated with glaucoma conversion. 

Participants were followed-up at 4-month intervals. Only eyes with glaucoma received prescriptions for topical hypotensive medications. At each follow-up visit, participants underwent clinical examination, IOP measurement, slit lamp biomicroscopy, and optic disc examination. Once a year, participants also underwent optic disc stereophotography, red-free fundus photography, and SAP. Two independent graders evaluated these results and made clinical decisions about glaucoma conversion in contralateral eyes. 


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The total study cohort included 76 patients with unilateral normal tension glaucoma who were referred for additional analyses. Within this group, 77.6% of patients had myopia of >0.5 diopters (D), and 21.1% had myopia of >6 D. 

Spanning a mean follow-up period of 7.3±2.4 years (median, 6.8 years), 27.6% of patients demonstrated structural glaucoma conversion in the contralateral eye. Mean time to structural glaucoma conversion was 3.6±2.1 years (median, 3.12 years). Investigators saw functional glaucoma conversion in 9 of 21 participants, all of whom had “been observed simultaneously with, or after, structural glaucoma conversion.” Mean time between structural and functional glaucoma conversion was 2.1±2.0 years (median, 1.64 years). 

Between groups, researchers noted no significant demographic difference, and mean deviation and pattern SD values of glaucomatous eyes were not significantly different. Visual field defect stage was classified as either mild (MD>-3 dB), mild (-3 dB≥MD>-6 dB), moderate (-6≥MD>-12 dB), or severe (MD≤-12 dB); this was not significantly different between the groups. 

MWβPPA-DD ratios of nonglaucomatous contralateral eyes were significantly higher in the conversion group compared with the nonconversion group, and DH presence rate and total number in contralateral eyes was also significantly higher. CCT in the contralateral eyes were thinner in the conversion group, with a marginal significance. Researchers noted no significant difference in clinical parameters of glaucomatous eyes between the 2 groups. 

Each group of patients underwent intereye comparisons between the glaucomatous and nonglaucomatous contralateral eyes. In the nonconversion group, baseline IOP and IOP fluctuation were significantly higher in the glaucomatous eye vs the contralateral eye; there were no significant between-eye differences in the conversion group. 

CCT, MWβPPA-DD ratio, and DH presence were also significantly different between eyes in the nonconversion group, and vertical cup-to-disc ratio at baseline was significantly smaller in both groups’ nonglaucomatous eyes. Arteriosclerosis grades showed no intereye differences in either group. 

Results of a regression analysis of the effect of the presence of disc hemorrhage on glaucoma conversion in the contralateral eye was significantly different between eyes. 

After 5 years, the rate of glaucoma conversion in nonglaucomatous contralateral eyes was 19.7% (95% CI, 15%-24.3%). The group with IOP >17 mm Hg demonstrated faster glaucoma conversion compared with the low IOP group, and the low CCT and high MWβPPA groups (<491 µm and >0.32 DD, respectively) also demonstrated significantly faster structural glaucoma conversion in contralateral eyes. 

Using a Cox proportional hazard model, researchers evaluated possible risk factors for glaucoma conversion in nonglaucomatous contralateral eyes. Results of a univariate analysis found that baseline IOP, CCT, and MWβPPA-DD ratio in contralateral eyes were all “possibly associated” with glaucoma conversion. Results of multivariate analyses showed that high IOP, low CCT, and high MWβPPA-DD ratio were all independent risk factors for glaucoma conversion (hazard ratios, 5.05, 4.25, and 6.25, respectively). 

Study limitations include the participation of patients with a wide range of refractive errors, which may have different clinical characteristics than normal tension glaucoma without myopia, a lack of optical coherence tomography findings, and the involvement of systemic factors in glaucoma pathogenesis, which were not considered in the present study. 

“Glaucoma conversion rates for previously nonglaucomatous contralateral eyes of [patients with unilateral normal tension glaucoma] were relatively high,” the researchers explain. “High IOP, low CCT, and high MWβPPA-DD ratio were identified as risk factors…Detailed examinations would be needed for [patients with unilateral normal tension glaucoma] manifesting such risk factors.” 

Reference

Kim J-S, Choi HJ, Park KH. Glaucoma conversion of the contralateral eye in unilateral normal-tension glaucoma patients: A 5-year follow-up study. Br J Ophthalmol. Published online September 5, 2021. doi:10.1136/bjophthalmol-2020-316371