Morphometry of the Optic Nerve Head as a Diagnostic Tool for Glaucoma

Abstract: Glaucoma is a chronic optic nerve head (ONH) disease. Gradual retinal ganglion cell and nerve fiber loss lead to morphological ONH change and visual field defects. Initial loss is often focal. Rate of progression and life expectancy guide treatment. Currently, confocal scanning laser tomoghraphy (HRT) and optic coherence tomography (OCT) are available for ONH imaging. However, there is no consensus for which morphometric measurement of ONH nerve fiber content to use for glaucoma follow-up.Purpose: To measure ONH nerve fiber content as neuroretinal rim area (NRA) with HRT, estimate NRA measurement variation and its impact on designing a follow-up strategy. To develop a custom algorithm, Pigment epithelium central limit-Inner limit of the retina Minimal Distance (PIMD), for measuring ONH nerve fiber content in OCT data cubes. To measure PIMD in glaucomatous eyes, estimate the variance sources for PIMD and their impact on designing strategies for glaucoma follow-up.Methods: NRA was measured with HRT in non-glaucomatous and glaucomatous eyes. Sources of variance for NRA were estimated. An OCT data cube of a non-glaucomatous eye was used in developing the PIMD algorithm. PIMD was measured in 500 radii along the ONH circumference. PIMD averaged over the circumference is PIMD-2π. Sources of variance for PIMD-2π were estimated for glaucomatous eyes. Strategies for following PIMD-2π and segments of PIMD-2π within subject over time were proposed.Results: Variation among subjects was substantial for NRA and PIMD-2π. Contrarily, within subject variation was small for NRA and PIMD-2π. When within subject variation, a previously reported loss rate for progressing glaucoma, and measuring NRA 3 times every 4 months were applied, a significant loss was detected after 54 months. When within subject variation and a PIMD-2π loss rate resulting in blindness after 20 years were applied, a significant PIMD-2π loss was detected in 16 months with visits every 4 months. Within subject segmental PIMD-2π loss can be detected from the 3rd visit. Loss rate of each PIMD can be estimated with linear regression from the 4th visit. Change in segmental PIMD-2π loss rate can be detected at a later visit.Conclusions: Small within subject variation allows for within subject NRA and PIMD follow-up over time. Segmental PIMD-2π has potential to detect focal glaucomatous defects and worsening of existing defects. There is potential to detect a change in segmental PIMD-2π loss rate. Segmental PIMD-2π has potential as a tool for within subject follow-up of glaucoma.