Glaucoma is a major cause of irreversible visual loss worldwide, and primary open-angle glaucoma (POAG) is a common form of progressive optic neuropathy [1–3]. Retinal ganglion cell degeneration produces characteristic optic nerve head changes and corresponding visual field defects. Because the disease may be asymptomatic until substantial damage occurs, objective structural and functional assessments are important in its evaluation [3]. Spectral-domain optical coherence tomography (SD-OCT) permits quantitative assessment of retinal tissue. The macular ganglion cell complex (GCC), comprising the retinal nerve fibre, ganglion cell and inner plexiform layers, provides a structural indicator of glaucomatous neuronal damage. Related macular ganglion cell and inner plexiform layer measurements have demonstrated diagnostic relevance in glaucoma [4,5]. Standard automated perimetry evaluates the functional consequences of retinal damage. Humphrey 24-2 perimetry provides mean deviation (MD), an index of overall field impairment, and sensitivity values at individual locations. Macular visual field sensitivity (VFS), calculated from central test locations, may show a closer anatomical relationship to macular GCC measurements than global indices [6–9]. However, structural and functional measurements do not necessarily change in parallel, partly because of regional variation and differences in measurement scales [10,11]. This study evaluated correlations of macular GCC thickness with macular VFS and MD in eyes with POAG, together with associations involving cup-to-disc ratio, foveal sensitivity, age and sex. AIM AND OBJECTIVES Aim: To evaluate the relationship of macular GCC thickness measured by SD-OCT with macular VFS and MD obtained through Humphrey automated perimetry in POAG. Objectives: To quantify GCC thickness, macular VFS and MD; determine their correlations; and examine relationships of GCC thickness with cup-to-disc ratio, foveal sensitivity and demographic characteristics

Study design, setting and duration This hospital-based observational study was conducted at Ahalia Foundation Eye Hospital, Palakkad, Kerala, India, from December 2018 to November 2020 (24 months). The analytical sample comprised 78 eyes with POAG from patients aged 20–60 years. Sample size The original sample-size planning used an anticipated correlation of r = 0.3049, 80% power and a 5% significance level, giving a reported minimum sample of 77. Seventy-eight eyes were ultimately analysed. Eligibility criteria Inclusion criteria were POAG, age 20–60 years, intraocular pressure within normal limits at examination, spherical equivalent within ±5 dioptres and cylindrical correction within ±3 dioptres. Exclusion criteria were angle closure on gonioscopy; retinal disease including age-related macular degeneration or diabetic retinopathy; neurological disease affecting visual fields, including cerebrovascular accidents or pituitary tumours; previous ocular surgery; manifest strabismus; and aphakia. Clinical examination and imaging Demographic information, systemic illnesses, family history and antiglaucoma medication use were recorded. Ophthalmological examination included Snellen visual acuity, refraction, pupillary responses, slit-lamp examination, gonioscopy with a Volk G-4 lens, Goldmann applanation tonometry and dilated optic disc assessment with a +78 D lens. Cup-to-disc ratio was documented. Structural assessment employed the Carl Zeiss Cirrus HD-OCT with a 200 × 200-pixel optic disc cube and 512 × 128-pixel macular cube. PanoMap analysis provided the reported average GCC thickness. Scans with signal strength below 6 were excluded. Functional assessment used the Carl Zeiss Humphrey Field Analyser 24-2 programme. Visual fields were evaluated using Anderson criteria and accepted reliability indices. Macular VFS was calculated as the average sensitivity at 12 central test locations; MD and foveal sensitivity were also recorded. Structural and functional examinations were performed on the same day. Statistical analysis and ethics Data were entered in Microsoft Excel 2010 and analysed using SPSS version 20. Continuous variables were described by mean, standard deviation, median and range; categorical variables by frequencies and percentages. Pearson and Spearman correlation coefficients were applied as reported, and an independent-samples t-test compared GCC thickness by sex. A two-sided p < 0.05 was considered statistically significant. Ethical and Scientific Committee approval was obtained from the hospital, and participants provided written informed consent.

Results overview Seventy-eight glaucomatous eyes were analysed. Mean age was 52.42 ± 8.89 years. Most observations belonged to the 50–60-year age group (69.2%). Females accounted for 51.3% and males for 48.7%. Average GCC thickness was 71.74 ± 9.11 µm. Mean macular VFS was 26.94 ± 6.32 dB. Mean MD was −5.56 ± 5.37 dB. GCC thickness correlated significantly with macular VFS (r = 0.565; p < 0.001). GCC thickness also correlated with MD (r = 0.449; p < 0.001). Greater cup-to-disc ratio was associated with thinner GCC (r = −0.516; p < 0.001) and lower macular VFS (r = −0.444; p < 0.001). Correlations involving foveal sensitivity were not statistically significant. Neither age nor sex showed a significant association with GCC thickness. Table 1. Demographic characteristics Characteristic n % Age 20–30 years 2 2.6 Age 30–40 years 4 5.1 Age 40–50 years 18 23.1 Age 50–60 years 54 69.2 Female 40 51.3 Male 38 48.7 Table 2. Descriptive ophthalmological measurements Parameter Mean ± SD Range Median GCC thickness (µm) 71.74 ± 9.11 48–91 73 Macular VFS (dB) 26.94 ± 6.32 6–34.50 28.33 Foveal sensitivity (dB) 33.10 ± 4.21 14–40 34 Mean deviation (dB) −5.56 ± 5.37 −24.89–1.58 −3.95 Cup-to-disc ratio 0.70 ± 0.11 0.40–0.90 0.70 Table 3. GCC thickness and visual field parameters Association Correlation p-value GCC vs macular VFS Pearson r = 0.565 <0.001 GCC vs MD Spearman r = 0.449 <0.001 GCC vs foveal sensitivity Pearson r = −0.021 0.854 Table 4. Cup-to-disc ratio and structural/functional measures Association Pearson r p-value CDR vs GCC thickness −0.516 <0.001 CDR vs macular VFS −0.444 <0.001 CDR vs foveal sensitivity 0.025 0.827 Table 5. Demographic associations with GCC thickness Comparison Finding p-value Age vs GCC Spearman r = −0.114 0.322 Female GCC (n = 40) 70.33 ± 7.38 µm — Male GCC (n = 38) 73.18 ± 10.50 µm — Male vs female GCC Independent-samples t-test 0.171 Table 6. Macular and foveal visual sensitivity Association Pearson r p-value Macular VFS vs foveal sensitivity 0.189 0.115 Table 7. Summary of statistically significant structure–function correlations Comparison Coefficient p-value GCC vs macular VFS 0.565 <0.001 GCC vs MD 0.449 <0.001 CDR vs GCC −0.516 <0.001 CDR vs macular VFS −0.444 <0.001 Table Summary Table 1: The mean age was 52.42 ± 8.89 years (range 20–60); 92.3% of participants were aged 40–60 years. Sex distribution was approximately balanced. Table 2: GCC thickness and macular VFS varied substantially across the sample, as did MD, indicating heterogeneous structural and functional involvement. Mean cup-to-disc ratio was 0.70. Table 3: GCC thickness showed moderate positive associations with macular VFS and MD. Its association with foveal sensitivity was negligible and nonsignificant. Table 4: Larger cup-to-disc ratio was associated with lower GCC thickness and macular VFS; no significant association with foveal sensitivity was observed. Table 5: Age showed a weak, nonsignificant negative association with GCC thickness. The numerical difference in mean GCC thickness between sexes was also nonsignificant. Table 6: Macular VFS and foveal sensitivity had a weak positive correlation that did not reach statistical significance. Table 7: The strongest reported association was between GCC thickness and macular VFS (r = 0.565), followed by GCC thickness and cup-to-disc ratio (absolute r = 0.516). All four listed correlations were significant

The present study demonstrated moderate structure–function associations in 78 eyes with POAG. Macular GCC thickness correlated positively with macular VFS (r = 0.565; p < 0.001) and MD (r = 0.449; p < 0.001). These findings are consistent with earlier investigations demonstrating relationships between OCT-derived macular ganglion cell measurements and perimetric sensitivity [6–9]. Macular GCC thickness and visual field sensitivity The correlation with macular VFS was stronger than that with global MD. This difference is plausible because the central field locations used for macular VFS are anatomically closer to the macular retinal area measured by OCT. Prior investigations have shown that regional correspondence and retinal ganglion cell displacement influence measured structure–function relationships [8,12,13]. The moderate correlation, however, also indicates that GCC thickness and visual sensitivity are not interchangeable. Structural and functional measurements differ in their dynamic ranges, variability and response to disease severity [10,11]. Mean deviation and optic disc cupping The positive GCC–MD association suggests that more negative MD values generally accompany thinner GCC measurements. Previous work has likewise demonstrated relationships between macular structural indices and global visual field measures, although the relationship may be nonlinear across disease stages [7,14]. Cup-to-disc ratio correlated inversely with both GCC thickness (r = −0.516) and macular VFS (r = −0.444), linking optic nerve head cupping with macular structural loss and functional impairment. The combined assessment of optic nerve, OCT and visual field measures may provide a fuller clinical picture than any individual parameter [5,15,16]. Foveal sensitivity and demographic variables Foveal sensitivity was not significantly correlated with GCC thickness (r = −0.021; p = 0.854), cup-to-disc ratio (r = 0.025; p = 0.827) or macular VFS (r = 0.189; p = 0.115). This may reflect the limited anatomical region sampled by the foveal measurement and variable distribution of glaucomatous loss. It should not be interpreted as proof that foveal function remains unaffected during disease progression. Age (p = 0.322) and sex (p = 0.171) were not significantly associated with GCC thickness in this sample; these null results may reflect the restricted age range and modest sample size. Clinical implications and limitations OCT and automated perimetry provide complementary evidence of glaucomatous damage. Combined structural and functional assessment is supported by prior research on retinal ganglion cell estimates, glaucoma staging and longitudinal progression [15–18]. The present single-centre observational design, absence of a non-glaucomatous control group and lack of longitudinal measurements limit causal or prognostic conclusions. Sector-wise correspondence was not evaluated, and the sample was restricted to ages 20–60 years. In addition, the analysis involved 78 eyes; any dependence between eyes from the same patient could not be evaluated from the reported aggregate data.

Macular GCC thickness measured by SD-OCT correlated significantly with macular VFS and MD in POAG. Cup-to-disc ratio was inversely associated with GCC thickness and macular VFS, whereas foveal sensitivity showed no significant correlation with the examined parameters. These results support combined OCT and Humphrey perimetry assessment, while recognising that structural and functional measurements are complementary rather than interchangeable

1. Quigley HA, Broman AT. The number of people with glaucoma worldwide in 2010 and 2020. Br J Ophthalmol. 2006;90(3):262–267. doi:10.1136/bjo.2005.081224. PMID:16488940. 2. Tham YC, Li X, Wong TY, Quigley HA, Aung T, Cheng CY. Global prevalence of glaucoma and projections of glaucoma burden through 2040: a systematic review and meta-analysis. Ophthalmology. 2014;121(11):2081–2090. doi:10.1016/j.ophtha.2014.05.013. PMID:24974815. 3. Weinreb RN, Aung T, Medeiros FA. The pathophysiology and treatment of glaucoma: a review. JAMA. 2014;311(18):1901–1911. doi:10.1001/jama.2014.3192. PMID:24825645. 4. Mwanza JC, Durbin MK, Budenz DL, et al. Glaucoma diagnostic accuracy of ganglion cell-inner plexiform layer thickness: comparison with nerve fiber layer and optic nerve head. Ophthalmology. 2012;119(6):1151–1158. doi:10.1016/j.ophtha.2011.12.014. PMID:22365056. 5. Mwanza JC, Warren JL, Budenz DL. Combining spectral domain optical coherence tomography structural parameters for the diagnosis of glaucoma with early visual field loss. Invest Ophthalmol Vis Sci. 2013;54(13):8393–8400. doi:10.1167/iovs.13-12749. PMID:24282232. 6. Cho JW, Sung KR, Lee S, et al. Relationship between visual field sensitivity and macular ganglion cell complex thickness as measured by spectral-domain optical coherence tomography. Invest Ophthalmol Vis Sci. 2010;51(12):6401–6407. doi:10.1167/iovs.09-5035. PMID:20631238. 7. Kim NR, Lee ES, Seong GJ, et al. Structure-function relationship and diagnostic value of macular ganglion cell complex measurement using Fourier-domain OCT in glaucoma. Invest Ophthalmol Vis Sci. 2010;51(9):4646–4651. doi:10.1167/iovs.09-5053. PMID:20435603. 8. Na JH, Kook MS, Lee Y, Baek S. Structure-function relationship of the macular visual field sensitivity and the ganglion cell complex thickness in glaucoma. Invest Ophthalmol Vis Sci. 2012;53(8):5044–5051. doi:10.1167/iovs.11-9401. PMID:22700706. 9. Boling W, WuDunn D, Cantor LB, Hoop J, James M, Nukala V. Correlation between macular thickness and glaucomatous visual fields. J Glaucoma. 2012;21(8):505–509. doi:10.1097/IJG.0b013e318239c32b. PMID:22104726. 10. Hood DC, Kardon RH. A framework for comparing structural and functional measures of glaucomatous damage. Prog Retin Eye Res. 2007;26(6):688–710. doi:10.1016/j.preteyeres.2007.08.001. PMID:17889587. 11. Hood DC, Anderson SC, Wall M, Kardon RH. Structure versus function in glaucoma: an application of a linear model. Invest Ophthalmol Vis Sci. 2007;48(8):3662–3668. doi:10.1167/iovs.06-1401. PMID:17652736. 12. Raza AS, Cho J, de Moraes CGV, et al. Retinal ganglion cell layer thickness and local visual field sensitivity in glaucoma. Arch Ophthalmol. 2011;129(12):1529–1536. doi:10.1001/archophthalmol.2011.352. PMID:22159673. 13. Garway-Heath DF, Poinoosawmy D, Fitzke FW, Hitchings RA. Mapping the visual field to the optic disc in normal tension glaucoma eyes. Ophthalmology. 2000;107(10):1809–1815. doi:10.1016/S0161-6420(00)00284-0. PMID:11013178. 14. The relationship between macular cell layer thickness and visual function in different stages of glaucoma. Eye (Lond). 2011. doi:10.1038/eye.2011.17. PMID:21350568. 15. Medeiros FA, Lisboa R, Weinreb RN, Liebmann JM, Girkin C, Zangwill LM. Retinal ganglion cell count estimates associated with early development of visual field defects in glaucoma. Ophthalmology. 2013;120(4):736–744. doi:10.1016/j.ophtha.2012.09.039. PMID:23246120. 16. Medeiros FA, Lisboa R, Weinreb RN, Girkin CA, Liebmann JM, Zangwill LM. A combined index of structure and function for staging glaucomatous damage. Arch Ophthalmol. 2012;130(9):1107–1116. doi:10.1001/archophthalmol.2012.827. PMID:23130365. 17. Medeiros FA, Zangwill LM, Girkin CA, Liebmann JM, Weinreb RN. Combining structural and functional measurements to improve estimates of rates of glaucomatous progression. Am J Ophthalmol. 2012;153(6):1197–1205.e1. doi:10.1016/j.ajo.2011.11.015. PMID:22317914. 18. Medeiros FA, Zangwill LM, Anderson DR, et al. Estimating the rate of retinal ganglion cell loss in glaucoma. Am J Ophthalmol. 2012;154(5):814–824.e1. doi:10.1016/j.ajo.2012.04.022.