Agreement in Measurement of Optic Cup-to-Disc Ratio with Stereo Biomicroscope Funduscopy and Digital Image Analysis: Results from the Nigeria National Blindness and Visual Impairment Survey

ABSTRACT

Purpose: To determine agreement in estimations of vertical cup-to-disc ratio (VCDR) between clinical stereo-biomicroscopic funduscopy and digital fundus image analysis.

Methods: Systematic sampling of 1-in-7 from a sample of 13,591 participants aged ≥40 years gave a subsample who were examined in detail. VCDR was estimated clinically by 60 diopter aspheric lens biomicroscopic funduscopy (c-VCDR) and by digital fundus images (i-VCDR) graded at the Moorfields Eye Hospital Reading Centre. Spearman’s correlation coefficient, paired t-test and the Bland-Altman method to assess limits of agreement (LOA) between the two methods were applied.

Results: Of 1759 participants in the subsample, 848 participants (48%) with normal frequency doubling technology (FDT) visual fields and data for i-VCDR and c-VCDR in both eyes (n = 1696 eyes) were included in the analysis. By absolute difference of VCDR values for each eye between the two methods, 1585 eyes (94%) differed by ≤0.2. Mean i-VCDR was 0.381 (standard deviation, SD 0.156), and mean c-VCDR 0.321 (SD 0.145). i-VCDRs were significantly larger by a mean difference of 0.061 (SD 0.121; 95% confidence interval, CI, 0.055–0.066; p < 0.001). The 95% LOA assessed by the Bland-Altman method were lower limit −0.182 (95% CI −0.192 to −0.172) and upper limit 0.303 (95% CI 0.293–0.313). The 95% LOA intervals narrowed with higher VCDR.

Conclusion: Digital image analysis and clinical assessment are two distinct methods to measure VCDR; with larger i-VCDRs in this survey. Applying i-VCDR cut-off values to c-VCDR measurements in the Nigeria Blindness Survey might have underestimated glaucoma prevalence. It is recommended that all participants in glaucoma surveys have VCDR by digital image measurement.

KEYWORDS:

• Bland-Altman plot
• glaucoma
• limits of agreement
• Nigeria
• vertical cup-to-disc ratio (VCDR)

Acknowledgments

We thank Tunde Peto, Ferenc Sallo, and Nisha Parmar of the MEHRC for grading the optic disc images.

The findings in this paper have been previously presented, in part, at the 23rd ISGEO Congress, September 2014, London, UK.

Declaration of interest

The authors report no conflicts of interest. The authors alone are responsible for the content and writing of this article.

Funding

The Nigeria National Blindness and Visual Impairment Study was supported by Sightsavers International, CBM, and Velux Stiftung.

The data analysis and writing was supported by the Fred Hollows Foundation as part of a research degree study for FK.

The funding organizations had no role in the design and conduct of the research.

Additional information

Funding

The Nigeria National Blindness and Visual Impairment Study was supported by Sightsavers International, CBM, and Velux Stiftung.