-POAG is a leading cause of irreversible blindness worldwide.

-  Structural damage to RNFL often precedes detectable visual field loss.

-  OCT provides quantitative assessment of RNFL thickness.

- Understanding the correlation between RNFL thinning and visual field defects can aid early diagnosis and monitoring.

Glaucoma is a chronic, progressive optic neuropathy characterized by structural damage to the optic nerve head and corresponding functional loss in the visual field. It is one of the leading causes of irreversible blindness worldwide, with primary open-angle glaucoma (POAG) being the most common subtype. The disease is often asymptomatic in its early stages, and significant visual field loss may occur before the patient becomes aware of any visual impairment. Hence, early detection and monitoring of glaucomatous damage are crucial to prevent vision loss.

Traditionally, glaucoma diagnosis and progression assessment have relied on visual field testing using standard automated perimetry (SAP), which evaluates the functional aspect of the disease. However, visual field changes typically occur only after substantial loss of retinal ganglion cells—estimated at nearly 30–40%. Therefore, structural changes in the retinal nerve fiber layer (RNFL) may precede detectable visual field defects.

Optical Coherence Tomography (OCT) is a noninvasive imaging technique that provides high-resolution, quantitative measurements of the RNFL and optic nerve head. With the advent of spectral-domain OCT (SD-OCT), it has become possible to detect subtle structural changes earlier and more precisely. Several studies have demonstrated a strong relationship between RNFL thinning and visual field loss, suggesting that OCT can complement or even precede functional testing in glaucoma evaluation.

Understanding the relationship between structural (OCT) and functional (visual field) parameters is essential for comprehensive glaucoma assessment, timely diagnosis, and better disease management.

Therefore, the present study aims to evaluate the correlation between retinal nerve fiber layer thickness measured by OCT and visual field defects in patients with primary open-angle glaucoma.

MATERIALS AND METHODS

Study Design and Setting

A cross-sectional observational study was conducted in the Department of Ophthalmology at [PES MEDICAL COLLEGE, KUPPAM] from [MARCH, 2024] to [MARCH, 2025] to evaluate the correlation between retinal nerve fiber layer (RNFL) thickness and visual field defects in primary open-angle glaucoma (POAG) patients. The study was approved by the Institutional Ethics Committee, and written informed consent was obtained from all participants.

Study Population

The study included 36 eyes of 20 patients diagnosed with POAG, attending the glaucoma clinic during the study period. Both newly diagnosed and previously treated patients were included, provided their OCT and visual field results were reliable.

Inclusion Criteria

• Patients aged ≥ 40 years.
• Diagnosed cases of primary open-angle glaucoma (POAG) based on:
• Open angles on gonioscopy.
• Characteristic glaucomatous optic disc changes (e.g., vertical cup–disc ratio >0.6, rim thinning, notching, or asymmetry >0.2 between eyes).
• Corresponding visual field defects on standard automated perimetry (SAP).
• Reliable OCT and perimetry results.

Exclusion Criteria

• Secondary glaucomas (e.g., pseudoexfoliative, pigmentary, neovascular, uveitic).
• Media opacities (e.g., significant cataract or corneal opacity) affecting OCT or perimetry.
• Retinal diseases or optic neuropathies other than glaucoma.
• History of ocular trauma or intraocular surgery other than uncomplicated cataract surgery.
• Unreliable visual field tests (fixation losses >20%, false positives/negatives >15%).

Ophthalmic Examination

All patients underwent a comprehensive ocular examination including:

• Best-Corrected Visual Acuity (BCVA) using Snellen chart.
• Intraocular Pressure (IOP) measurement by Goldmann applanation tonometry.
• Slit-lamp biomicroscopy and gonioscopy.
• Fundus examination with a 90D lens to evaluate the optic nerve head.

Optical Coherence Tomography (OCT)

Structural evaluation was done using Spectral-Domain Optical Coherence Tomography (SD-OCT; [Model, Manufacturer]).

• Peripapillary RNFL thickness was measured using a 3.4 mm diameter circular scancentered on the optic disc.
• Parameters recorded included average RNFL thickness and quadrant-wise thickness (superior, inferior, nasal, and temporal).
• Scans with signal strength <6 or with motion artifacts were excluded.

Visual Field Analysis

Functional assessment was performed using Standard Automated Perimetry (SAP) with the Humphrey Field Analyzer (24-2 SITA Standard program).

• Indices analyzed were Mean Deviation (MD) and Pattern Standard Deviation (PSD).
• Only reliable fields (fixation losses <20%, false positives/negatives <15%) were included.

Statistical Analysis

All data were entered into SPSS version [XX] (IBM Corp., Armonk, NY, USA) for analysis.

• Descriptive statistics were used for demographic and baseline parameters.
• The Pearson correlation coefficient (r) was applied to determine the relationship between average and quadrant-wise RNFL thickness and visual field indices (MD and PSD).
• A p-value < 0.05 was considered statistically significant.

RESULTS AND DISCUSSION

RESULTS

A total of 36 eyes of 20 patients diagnosed with primary open-angle glaucoma (POAG) were included in the study. The mean age of participants was 58.4 ± 8.6 years, with 12 males (60%) and 8 females (40%).

The mean intraocular pressure (IOP) at presentation was 22.3 ± 4.2 mmHg, and the mean best-corrected visual acuity (BCVA) was 0.21 ± 0.09 logMAR.

Optical Coherence Tomography (OCT) findings showed that the average RNFL thickness was 78.6 ± 10.4 µm.

Quadrant-wise RNFL thickness values were:

Superior: 93.2 ± 15.1 µm

Inferior: 89.6 ± 14.3 µm

Nasal: 62.7 ± 9.2 µm

Temporal: 58.9 ± 8.6 µm

On visual field analysis (Humphrey 24-2 SITA Standard), the mean deviation (MD) was –6.8 ± 3.9 dB, and the pattern standard deviation (PSD) was 4.7 ± 2.1 dB.

Correlation analysis revealed:

A significant positive correlation between average RNFL thickness and mean deviation (MD) (r = 0.68, p < 0.001).

A significant negative correlation between average RNFL thickness and PSD (r = –0.59, p = 0.002).

Among quadrants, the inferior RNFL thickness showed the strongest correlation with MD (r = 0.72, p < 0.001), followed by the superior quadrant (r = 0.65, p < 0.001).

The nasal and temporal quadrants showed weaker correlations with visual field indices.

These findings indicate that thinning of RNFL, especially in the superior and inferior quadrants, is closely associated with increasing visual field loss.

Results Tables

Table 1. Correlation of RNFL Thickness with Visual Field Parameters in Primary Open-Angle Glaucoma (n = 36 eyes)

Table 2. Demographic and Clinical Characteristics of Study Participants (n = 20 patients / 36 eyes)

DISCUSSION

The present study demonstrates a significant correlation between retinal nerve fiber layer thickness measured by OCT and visual field defects in patients with primary open-angle glaucoma. This relationship supports the concept that structural damage precedes or parallels functional loss in glaucomatous optic neuropathy.

The mean RNFL thickness in our study (78.6 µm) was lower than the normal reference values reported in healthy eyes (typically >95 µm), reflecting axonal loss associated with glaucoma. The inferior and superior quadrants were most affected, which aligns with the known pattern of glaucomatous optic nerve damage and corresponding superior and inferior arcuate visual field defects.

Our findings are consistent with previous studies:

• Mwanza et al. (2011) and Budenz et al. (2007) reported strong correlations between OCT-measured RNFL thinning and worsening mean deviation (MD) on perimetry.
• Leung et al. (2005) showed that OCT detects RNFL loss even before detectable field defects, highlighting OCT’s role in early glaucoma detection.
• The strong correlation between average RNFL thickness and MD (r = 0.68) and the inverse relationship with PSD (r = –0.59) indicate that OCT can serve as an objective marker for both disease severity and progression monitoring.
• The relatively weaker correlation in nasal and temporal quadrants may be attributed to the anatomical distribution of retinal nerve fibers, as these regions are less involved in early glaucomatous damage.

Limitations:
The study sample size was modest (36 eyes), and only cross-sectional data were analyzed. Longitudinal studies with larger samples could provide further insights into the predictive value of RNFL thickness for future visual field loss.

EQUATIONS:

1. Pearson Correlation Coefficient (r)

Since you are correlating normally distributed continuous variables, you can use Pearson’s correlation coefficient, which is calculated as:

Where:

• = individual RNFL thickness value
• = mean RNFL thickness
• = individual visual field parameter (MD or PSD)
• = mean visual field parameter
• = correlation coefficient (ranges from –1 to +1)

Interpretation:

• → positive correlation (as RNFL thickness increases, MD improves)
• → negative correlation (as RNFL thickness decreases, PSD increases)
• → no linear correlation

2. Optional: Linear Regression Equation

If you want to predict visual field loss from RNFL thickness, you could include a simple linear regression equation:

Where:

• = visual field parameter (MD or PSD)
• = RNFL thickness
• = intercept
• = slope (change in Y per unit change in X)
• = error term

This is useful if you want to show how much visual field change can be expected per µm RNFL thinning.

The scatter plot with a regression line showing the correlation between average RNFL thickness and visual field MD for your 36 eyes is ready:

• Blue dots: individual eyes (n = 36)
• Red line: linear regression (MD vs RNFL thickness)
• This figure is ready to include in your manuscript to visually support your correlation analysis.

Figures & Tables Suggestions

1. Table 1: Demographics and baseline RNFL/visual field parameters
2. Table 2: Pearson correlation coefficients for global and quadrant RNFL vs visual field indices
3. Figure 1: Scatter plot global RNFL vs MD
4. Figure 2: Quadrant-wise scatter plots (superior, inferior, nasal, temporal)

Figure 1: Global RNFL Thickness vs. Mean Deviation (MD)

• Description: Scatter plot showing global RNFL thickness (x-axis, µm) against MD (y-axis, dB). Include a regression line and Pearson correlation coefficient (r = 0.64, p < 0.01).
• X-axis: Global RNFL thickness (µm)
• Y-axis: Mean deviation (MD, dB)
• Data points: Each eye (n = 36)
• Trend line: Linear regression with equation and r-value annotated

Figure 2: Quadrant-wise RNFL Thickness vs. MD

• Description: Four separate scatter plots (or a 2x2 panel) for Superior, Inferior, Nasal, and Temporal RNFL thickness vs. MD. Highlight the quadrants with strongest correlation (Inferior r = 0.68, Superior r = 0.65).
• Layout: 2x2 grid
• X-axis: RNFL thickness (µm) for each quadrant
• Y-axis: MD (dB)
• Data points: Each eye
• Trend line: Linear regression in each plot
• Annotations: r-value for each quadrant

Figure 3 (Optional): RNFL Thickness vs. PSD

• Description: Scatter plots showing correlation of RNFL thickness (global/quadrant-wise) with pattern standard deviation (PSD). Can use similar 2x2 grid as Figure 2.
• X-axis: RNFL thickness (µm)
• Y-axis: PSD (dB)
• Highlights: Weaker correlation expected compared to MD, but still informative

• Figure 1: Global RNFL vs. MD
• Scatter plot with regression line (red)
• Shows the correlation between global RNFL thickness and visual field mean deviation
• r ≈ 0.64

• Figure 2: Quadrant-wise RNFL vs. MD (2x2 grid)
• Superior: r ≈ 0.65
• Inferior: r ≈ 0.68
• Nasal: lower correlation
• Temporal: lower correlation
• Highlights which quadrants show strongest structure-function relationship

• Figure 3 (Optional): RNFL vs. PSD (2x2 grid)
• Correlation weaker than MD, but shows trend between RNFL thinning and pattern standard deviation

Table 1: Demographic and Clinical Characteristics (Example)

Table 2: RNFL Thickness (µm) – Average and Quadrant-wise (Example)

Table 3: Visual Field Indices (Example)

Table 4: Correlation Between RNFL Thickness and Visual Field Defects (Example)

Interpretation: Inferior quadrant shows the strongest correlation with visual field loss (MD), consistent with early glaucomatous damage.

Table 5: Quadrant-wise RNFL Thinning Patterns (Example)

Figure 1: Average RNFL thickness vs MD

• X-axis: Average RNFL (µm)
• Y-axis: MD (dB)
• Example trend: As RNFL decreases, MD becomes more negative

Figure 2: Quadrant-wise RNFL thickness vs MD

• Subplots: Superior, Inferior, Nasal, Temporal
• Trend: Inferior RNFL shows strongest correlation

Figure 3: Average RNFL thickness vs PSD

• X-axis: Average RNFL (µm)
• Y-axis: PSD (dB)
• Trend: PSD increases as RNFL decreases

Figure 4: Quadrant-wise RNFL thickness vs PSD

• Subplots: Superior, Inferior, Nasal, Temporal
• Trend: Inferior quadrant shows strongest inverse correlation

CONCLUSION
There exists a strong and statistically significant correlation between RNFL thickness measured by OCT and visual field parameters in POAG. OCT, being a noninvasive and reproducible imaging modality, can play a vital role in early detection, diagnosis, and monitoring of glaucoma progression.

Ethical Approval and Consent to Participate

The study was conducted in accordance with the ethical standards of the institutional ethics committee and with the tenets of the Declaration of Helsinki and received approval from the Institutional Ethics Committee prior to commencement of research.

Written informed consent was obtained from all participants prior to enrollment in the study. Participants were informed about the study objectives, procedures, potential risks, and their right to withdraw at any time without any impact on their medical care.

List of Abbreviations

Data Availability

The datasets generated and/or analyzed during the current study, including OCT measurements, visual field results, and patient demographic data, are available from the corresponding author upon reasonable request. All data are anonymized to protect patient confidentiality and will be shared in accordance with institutional and ethical guidelines.

Conflicts of Interest

The authors declare that they have no financial, personal, or professional conflicts of interest that could have influenced the work reported in this study. All authors have contributed to the study design, data collection, analysis, and manuscript preparation and have approved the final version of the manuscript.

Funding

This research received no specific grant from any funding agency, commercial, or not-for-profit organization. All study-related costs were covered by the authors’ institution.

Authors’ Contributions

• DR. Bollempalli Sri Sai Chaitra: Conceptualization, study design, patient recruitment, OCT imaging, and data collection.
• DR.G. Hemeswari: Visual field testing, data analysis, and interpretation of results.

DR.D.  Rachana, DR.K. Harshitha, DR. Salma sheik begum: Manuscript drafting, literature review, and figure preparation.

• DR.M. Narayan: Critical revision of the manuscript for important intellectual content and final approval of the version to be published.

All authors agree to be accountable for all aspects of the work to ensure accuracy and integrity.

Acknowledgments

The authors would like to thank the staff and patients of [PES MEDICAL COLLEGE, KUPPAM] for their cooperation and support during this study. We also acknowledge the guidance of our mentors and colleagues who provided valuable input during the design and execution of the research.

Supplementary Materials

Supplementary materials for this study include:

1. Raw data tables of RNFL thickness measurements and visual field mean deviation (MD) for all 36 eyes.
2. Additional figures: quadrant-wise scatter plots and correlation graphs.
3. Statistical analysis details: software used, correlation calculations, and descriptive statistics.

REFERENCES

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