The influence of corneal factors on glaucoma has been a subject of much interest in recent years, with proponents arguing that below-average corneal hysteresis (CH) values reveal optic nerve vulnerability that makes it more susceptible to damage, while elevated corneal resistance factor (CRF) scores may reflect a status that confers some protective benefit. Correcting tonometry measurements for the effects of corneal biomechanics is also believed to provide a more true assessment of intraocular pressure.
Researchers recently investigated change over time in CRF and CH as risk factors for visual field progression. In this prospective observational cohort study, 72 eyes of 48 glaucoma patients or suspects (mean age 68.1) were followed for about 4.5 years. Thirty-one eyes were classified as glaucoma suspects while 41 were deemed glaucomatous (29 mild, six moderate and six severe). All participants were enrolled in the Diagnostic Innovations in Glaucoma Study, designed to evaluate optic nerve structure and visual function in glaucoma.
Many predictors were investigated in addition to CRF change and baseline CH, including age, race and gender.
Although CRF and CH remained stable in the whole spectrum of glaucomatous eyes, a decrease in CRF over time as well as a lower baseline CH was associated with faster visual field progression. This was true even after adjusting for age, race and intraocular pressure.
Mean baselines were 9.0mm Hg for CH and 9.3mm Hg for CRF. Each 1mm Hg decrease in CH and CRF was associated, respectively, with a 0.12 dB/yr and 0.14 dB/yr faster rate of visual field mean deviation (MD) loss.
The authors concluded that their finding of lower baseline CH associated with worsening visual fields is in line with previous studies, and noted that the relationship of hysteresis with glaucoma is more illustrative of visual field status than central corneal thickness (CCT) alone. “Lower baseline CH in particular has been shown to have a stronger association than thinner CCT for visual field progression over time, although thinner CCT is associated more strongly with a worse baseline visual field. Our study reflected this association in that glaucomatous eyes had a thinner CCT and worse baseline MD.”
The lower baseline CH had a greater impact on MD slope, they added. “Although we found no correlation of CH change with visual field progression, the association of decreasing CRF with worsening visual field MD suggests the importance of outer tissue rigidity especially when considered in comparison with viscous dampening.”
The study also notes both CRF and CH have been shown to decline with age in normal eyes, as the physical characteristics of the cornea changes with age.
Based on these results, the researchers argue that follow-up examinations should assess corneal resistance and elasticity at baseline to identify eyes at the highest risk of losing vision.
Chan E, Yeh K, Moghimi S, et al. Changes in corneal biomechanics and glaucomatous visual field loss. J Glaucoma. Epub ahead of print 2021Feb 15. PMID: 33596020.