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Editors Selection IGR 11-1

Optical Coherence Tomography: Trend-based analysis of RNFL by OCT

Comment by David Greenfield on:

27755 Trend-Based Analysis of Retinal Nerve Fiber Layer Thickness Measured by Optical Coherence Tomography in Eyes with Localized Nerve Fiber Layer Defects, Lee EJ; Kim T-W; Weinreb RN et al., Investigative Ophthalmology and Visual Science, 2011; 52: 1138-1144

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Various reference standards exist for assessing the sensitivity and specificity of RNFL imaging technology. Many studies have used standard automated perimetry, yet changes in structure and function are often poorly correlated. A more appropriate methodology is to compare two structural methods; the standard reference for assessment of glaucomatous structural damage is optic disc photography. Although, cross-sectional studies have compared agreement between RNFL loss detected using redfree photography and ocular imaging, prospective data to date has been sparse.

In the present study, Lee et al. (182) examine the rate of change in RNFL thickness measured using trend-based analysis with Stratus OCT in 153 eyes with stable and progressive localized RNFL defects judged using red-free photography. The rates of progressive RNFL thinning (global, affected quadrant, and affected clock-hour sectors) were significantly faster in progressors vs non-progressors. The rate of RNFL thinning in affected clock- hour sectors had the highest ability to discriminate between stable and progressive RNFL thinning with a sensitivity of 62%.

Serial analysis of RNFL thickness measurements may complement standard automated perimetry for the assessment of glaucoma progression over time. Recently a new RNFL analysis algorithm has been incorporated with Stratus OCT to judge glaucomatous change using a trend-based approach. Trend-based analyses use all available RNFL data and plot the RNFL thickness changes over time using linear regression analysis to generate a slope (rate of change) and its significance (p-value). As demonstrated in the present study, glaucoma progression often occurs in a region of pre-existing RNFL atrophy. Trend-based analysis may be used to quantify how fast, on average, a particular localized region has changed in order to judge the efficacy of IOP lowering therapy.

Glaucoma progression often occurs in a region of pre-existing RNFL atrophy

The authors employ an intuitive and novel approach by comparing quantitative RNFL measurements obtained with OCT to qualitative RNFL assessments using serial photography, although limitations exist. It is difficult to assess the RNFL using red-free photography in eyes with cloudy media and this technique is variably reproducible depending upon focus and illumination. In addition, as noted by the authors, red-free photography is unable to detect isolated progressive deepening of RNFL progression in which widening of the defect does not occur. OCT detection of RNFL progression in such cases would be incorrectly classified as a false positive. Other determinants of RNFL thickness such as signal strength should be controlled for as wide fluctuation may bias the assessment of change over time.

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