Glaucoma detection based on the waist of nerve fiber layer at the optic nerve head, average thickness or cross-sectional area?

Abstract

Purpose : The waist of the nerve fiber layer at the optic nerve head (ONH) may be expressed as cross-sectional thickness or surface area averaged over the circumference of the ONH. The purpose of the current work was to establish if there is a difference in efficiency between the two quantities for detection of glaucoma.

Methods : A 3D-volume of the optic nerve head was captured with OCT-2000 or OCT-Triton (Topcon, Japan) in one eye without glaucoma in each of 2 groups of 20 subjects aged [20;69]. Each group had a similar age distribution and a gender ratio of 1/1. The waist of the nerve fiber layer at the optic nerve head (ONH) was segmented fully automatically in the OCT volume.
In one group, minimal cross-sectional area was estimated over the circumference of the ONH (PIMA-2π) and in the other group minimal cross-sectional thickness was estimated over the circumference of the ONH (PIMD-2π). On the assumption that both quantities are normal distributed for subjects, a threshold for glaucoma was defined as the probability, α, to classify an eye as glaucoma although it is normal. 1-α then expresses the specificity for detection of glaucoma. The threshold for glaucoma, defined as a fraction below the expected mean of not glaucomatous eyes, is then directly proportional to the variation coefficient with the inverse of the cumulated normal distribution at 1-α as a proportionality constant. A 95% confidence interval for the difference between thresholds for glaucoma estimated as PIMA-2π and PIMD-2π was estimated by iteration based on the estimated mean and standard deviation for PIMA-2π and PIMD-2π, respectively using the t-distribution with d.f. =19.

Results : The estimated mean and standard devation, respectively, for PIMA-2π was 2.01 mm² and 0.54 mm² (c.v. = 0.26) and for PIMD-2π 350 and 62 μm (c.v. = 0.18). A 95 % confidence interval for the difference between the fraction of PIMA-2π below the expected mean and the fraction of PIMD-2π below the expected mean at specificity 0.95 was estimated to [-4;31] x10^-2 rel. (D.f 38).

Conclusions : If the threshold for detection is based on variability among subjects and specificity is set to 0.95, there is no difference in efficiency to detect glaucoma between PIMA-2π and PIMD-2π. The lack of difference is probably due to substantial variability among subjects.

This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.