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Editors Selection IGR 17-4

Anatomical Structures: RNFL thinning rate topography

Comment by Andrew Tatham on:

71410 Trend-Based Progression Analysis for Examination of the Topography of Rates of Retinal Nerve Fiber Layer Thinning in Glaucoma, Lin C; Mak H; Yu M et al., JAMA ophthalmology, 2017; 135: 189-195

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Trend-based progression analyses can be used to quantify rates of change in visual field or structural measurements in glaucoma, with perimetry and imaging devices incorporating software for this purpose. By performing linear regression of measurements over time it is possible to obtain annualized rates of change and estimate the risk of future visual disability.

By examining rates of RNFL change throughout the OCT scan it may be possible to improve accuracy of progression detection

Progression analysis using optical coherence tomography (OCT) has largely relied on measurements of the circumpapillary retinal nerve fiber layer (cpRNFL) and several studies have shown faster rates of cpRNFL loss to be associated with increased risk of worsening visual field loss.^1,2 Routinely performed OCT scanning protocols often capture information about a much larger area of RNFL than the cpRNFL and the premise of Lin and colleagues' work was that restricting progression analysis to evaluation of cpRNFL alone may reduce the ability to detect localized changes and to detect progression outside the circumpapillary region. In other words, by examining rates of RNFL change throughout the OCT scan it may be possible to improve accuracy of progression detection and perhaps better predict visual field changes.

A 'rates of change RNFL thickness map' was developed and used to examine the topography of rates of RNFL thinning. The authors used the Cirrus OCT (Carl Zeiss Meditec) to obtain a 6 x 6 mm² optic disc region RNFL thickness map and examined rates of RNFL change in individual 50 x 50 superpixel regions. The initial study examined rates of change in RNFL thickness in 240 eyes of 139 patients with POAG over a follow-up period of at least five years.³ One hundred seventeen eyes were found to have progressive RNFL loss using the RNFL thickness change map, which was associated with an eight-fold increased risk of worsening visual field loss during follow-up.³

The current study examined the topography of RNFL thinning in the 117 eyes progressing on the RNFL thickness change map and scrutinized the mean and peak rates and the location of faster rates of loss. During the follow up period, a median of 18 OCT scans was acquired for each eye. The rates of change RNFL thickness map showed wide variation in rates of RNFL thinning between eyes and at different locations, with faster loss observed in the inferotemporal and superotemporal regions, particularly closer to the optic disc margin. Faster mean and peak rates of RNFL thinning were significantly associated with visual field progression, even after accounting for confounding factors such as age, IOP, CCT, axial length and average cpRNFL thickness at baseline. Over a follow-up period of 4.9 to 7.2 years, 23% and 24% of eyes had visual field progression defined by event-based analysis (based on Early Manifest Glaucoma Trial (EMGT) criteria) and pointwise trend analysis respectively. Each 1 µm/year faster rate of mean RNFL loss was associated with a 39% increased risk of visual field progression by the EMGT criteria and 18% by pointwise trend analysis. The rates of change RNFL thickness map seemed to perform well even in eyes with moderate to advanced glaucoma at baseline, with faster rates of change remaining indicative of visual field progression. However, the area of progressive RNFL thinning was not indicative of visual field worsening.

Further work is needed, particularly to compare the ability of measures of rates of change in cpRNFL and change in RNFL thickness map to predict visual field loss, and to examine whether adjustment for age-related RNFL loss will improve performance. However, the rates of change RNFL thickness map shows promise as a tool for assessing progression as it maps the rate and location of RNFL loss and provides further information compared to assessments based solely on cpRNFL. This could be particularly important given the variation in rates of change in RNFL over time at different locations between and within eyes.

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