Comparing SDOCT measurements of optic nerve head (ONH) minimum rim width (MRW), minimum rim area (MRA) and peripapillary retinal nerve fiber layer thickness (RNFLT) to complete orbital optic nerve axon counts in experimental glaucoma (EG)

Abstract

 

Purpose

 

To compare SDOCT measurements of MRW, MRA and peripapillary RNFLT against complete orbital optic nerve axon counts in non-human primates (NHP) with unilateral EG.

 

Methods

 

Longitudinal measurements of MRW, MRA and RNFLT were acquired by SDOCT (Spectralis) under manometric IOP control (10 mmHg) during biweekly baseline imaging sessions (mean ± SD: 5.0 ± 1.4) and at a final session just prior to euthanasia in 35 NHP. MRW and MRA parameters were derived from SDOCT scans consisting of 80 radial B-scans centered on the ONH; RNFLT was derived from a single peripapillary circular B-scan with 12 deg diameter. After baseline scans were completed in each NHP, laser photocoagulation was applied to the trabecular meshwork of one eye to induce chronic unilateral IOP elevation. At the study endpoint (pre-defined for each NHP), euthanasia occurred under deep anesthesia; optic nerves were sampled ~2 mm behind the globe to obtain thin sections for histological processing and automated axon counting across 100% of the optic nerve cross-sectional area.¹

 

Results

 

For SDOCT parameters at baseline, the correlation between the two eyes of each animal was strongest for RNFLT (R=0.97) and MRW (R=0.97), but lower for MRA (R=0.77). At the final imaging session, there was no difference from baseline in fellow control eyes for RNFLT (p=0.95), MRW (p=0.94), or MRA (p=0.89). The magnitude of SDOCT parameter change in EG eyes was similar when expressed relative to baseline or relative to control eye values at the final session. To enable comparison with axon counts the latter is reported; loss in EG eyes was: 20 ± 19% for RNFLT, 37 ± 22% for MRW and 36 ± 23% for MRA; optic nerve axon loss was 35 ± 26%. The correlation to axon counts was strongest for RNFLT (R=0.81), as compared to MRW (R=0.71) or MRA (R=0.71), p=0.001 for each comparison (Steiger’s test, raw data Fig1; see also relative loss Fig2). At 99% specificity, diagnostic sensitivity was 66% for RNFLT, 43% for MRW and 60% for MRA.

 

Conclusions

 

Peripapillary RNFLT was more closely correlated with total orbital optic nerve axon count than were the ONH parameters MRW or MRA. This is likely because connective tissue deformations have a greater influence on the ONH parameters MRW and MRA than on RNFLT.<br /> ¹Reynaud et al, PMC3382379