Rates of change and longitudinal variability of using multiple OCT retinal nerve fibre layer ring sizes in glaucoma

Abstract

Objective

To compare the proportion of segmentation errors, rates of change and longitudinal variability of optical coherence tomography (OCT) circumpapillary retinal nerve fibre layer (cp-RNFL) global and sectoral thickness measurements when using three different ring sizes in a cohort of healthy and glaucomatous eyes.

Design

Retrospective cohort study.

Subjects

205 eyes of 105 patients with normal, suspect or manifest glaucoma.

Methods

Subjects underwent cp-RNFL scans on four or more consecutive follow up visits (mean follow up 4.5 years). Scans were taken using the Spectralis® OCT (Heidelberg Engineering, Heidelberg, Germany). Global thickness and six sectoral thickness values in the three ring sizes (3.5, 4.1, 4.7 mm) were extracted. Longitudinal variability was quantified by mean absolute regression residuals (MAR).

Main outcome measures

Rates of change (RoC) and longitudinal variability of global and sectoral cp-RNFL thickness across three ring sizes.

Results

The rate of global cp-RNFL thinning in the 3.5 mm ring was -0.40 µm/year (CI -0.58 to -0.22, p < 0.001), while the inferotemporal sector displayed the greatest rate of thinning (-0.97 µm/year, CI -1.26 to -0.68, p < 0.001). There was no significant difference in the RoC of thickness values in the 4.1 mm and 4.7 mm compared to the 3.5 mm ring size in the global or any of the six sectors. Longitudinal variability was consistently lower in the 4.7 mm compared to the smaller ring sizes, with a significant difference observed in the inferonasal, inferotemporal and superonasal sectors.

Conclusions

While there was no significant difference in the global and sectoral RoC across ring sizes, the larger RNFL ring sizes demonstrated significantly lower longitudinal variability for inferonasal, inferotemporal and superonasal sectors compared to the smaller ring sizes. Using larger RNFL ring sizes may therefore provide more accurate estimates of the true rate of change overtime.