Monoscopic Cameras Have Low Sensitivity in Glaucoma Detection

While monoscopic cameras are increasingly being used to screen for glaucoma, findings published in Eye suggest that stereoscopic imaging provides a better assessment of vertical cup-to-disc ratio (VCDR).

Due to their low cost, easy portability, and non-mydriatic nature, monoscopic cameras are increasingly being used for these screenings, especially in underserved areas.

Researchers endeavored to compare VCDR estimates attained using images taken with a monoscopic and stereoscopic camera. Participants were selected from the Tema Eye Survey. Eligible subjects had images of at least 1 eye taken with 2 cameras. They were classified as meeting the glaucoma threshold if an eye had a VCDR estimate above the 97.5th percentile (>0.725 for this population). The team used 0.725 as the cutoff to group eyes into 2 categories, positive and negative. Investigators calculated sensitivity, specificity, and predictive values of VCDR assessed by expert readers at a reading center for monoscopic photos using stereoscopic photos as the standard of care.

The team included 379 eyes (of 206 participants) in the study, 16 of which met the glaucoma threshold (VCDR > 0.725). Of these, the VCDR estimates of 14 eyes (87.5%) disagreed on the glaucoma threshold from the 2 cameras. The sensitivity to detect glaucoma with the monoscopic camera was 14.3%. This low sensitivity percentage suggests a high rate of false negatives, which would prevent early diagnosis and treatment.

Additionally, VCDR estimates calculated using a monoscopic device were, on average, smaller in comparison to the estimates obtained via a stereoscopic device. Differences in VCDR estimates increased as the estimates reached closer to the glaucoma threshold.

This study had several limitations, including its small number of incident glaucoma cases. Second, given the small sample size of eyes that met the glaucoma threshold and low incidence of glaucoma in the study, researchers did not calculate kappa statistics, which is generally used as a measure of agreement. Third, monoscopic images were taken before dilation and stereoscopic images after dilation, meaning researchers could not determine the effect of dilation on identifying glaucoma suspects in this study. It was also out of the study’s scope to determine whether pupil dilation impacted the quality of photos and assessment of the cup-to-disc ratio. Furthermore, the cutoff used to classify monoscopic VCDR estimates into 2 categories was based on the distribution of the stereoscopic VCDR estimates for the Tema, Ghana population, which may have contributed to the low sensitivity noted in the study. 

Reference

Shrestha R, Budenz DL, Mwanza JC, et al. Comparison of vertical cup-to-disc ratio estimates using stereoscopic and monoscopic cameras. Eye. Published online January 29, 2021. doi:10.1038/s41433-021-01395-3