Macular GCC thickness in the nine o'clock sector showed the highest correlation coefficient to BCVA. However, this finding is just one among many that correlates with visual impairment in glaucoma.  Photo: Michael Cymbor, OD. Click image to enlarge.

The relationship between visual acuity and ganglion cell complex (GCC) thickness is not linear, and there is a “floor” past which further thinning does not result in additional loss of visual acuity. Researchers in Japan believe that it is necessary to consider a variety of factors when evaluating the visual acuity of patients with loss of GCC thickness. Their study’s results suggested that research on the structure-function relationship in the retina requires not only measurement of retinal thickness but also evaluation of various factors, such as aging, ocular blood flow, oxidative stress and corneal hysteresis.

In 515 eyes of 515 patients with open-angle glaucoma (mean age, 62.6 years; mean deviation, -10.95dB), the researchers used swept-source OCT to measure macular GCC thickness in sectors classified as corresponding to circumpapillary RNFL clock-hour sectors from seven o'clock (inferotemporal) to 11 o'clock (superotemporal). They noted that macular GCC thickness in the nine o'clock sector showed the highest correlation coefficient to BCVA and a cutoff of 76.17µm.

So, the team decided to focus exclusively on cases with GCC thickness loss in the nine o'clock sector and to explore various clinical and ocular parameters that were associated with VA decline. Age, biological antioxidant potential, corneal hysteresis and temporal-tissue optic nerve head blood flow were significantly associated with BCVA, and the diagnostic ability of specific factors to predict VA decline (<20/25) significantly improved) after adjusting for these risk factors.

In the patients below the cutoff value for GCC thickness, the researchers observed no significant correlations among these parameters, even though they were all considered relevant to glaucoma progression. This suggested that various mechanisms are involved in the visual acuity disturbance associated with glaucoma, making it difficult to explain the cause of visual acuity decline in a unified manner.

“This nonlinear relationship between structure and function highlights the need to consider factors other than GCC thickness in assessing glaucomatous damage and visual acuity,” the study authors wrote in their paper.