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Editors Selection IGR 19-3
Examination methods: Stratus OCT and intra-subject variability
Comment by Jose-Maria Martinez de la Casa on:
19579 Factors associated with variability in retinal nerve fiber layer thickness measurements obtained by optical coherence tomography, Wu Z; Vazeen M; Varma R et al., Ophthalmology, 2007; 114: 1505-1512
In the last few years, measurement of the thickness of the retinal nerve fibre layer by optical coherence tomography has become an essential diagnostic tool for use in patients with glaucoma. Several studies have reported the reproducibility of its measurements in normal subjects and patients with glaucoma, and this reproducibility has successively increased with each new version of the instrument. Opacities of the ocular media, pupil diameter, the resolution of the scan and the type of scan are some of the factors identified to affect the quality of the image acquired and the variability of OCT measurements. The study by Wu et al. (953) complements these works by trying to identify the factors associated with intrasubject variability in measurements taken with the Stratus OCT. To this end, they retrospectively selected a group of 150 patients with glaucoma or suspect glaucoma, who had been examined with the Fast Nerve Fiber Layer Thickness (FNFLT) protocol in two consecutive sessions, because the result of the first exam had been suboptimal. Through multivariate analysis, the authors examined the possible variables related to the change in nerve fibre layer thickness produced between the first and second exams. The factors identified to produce a high variability of measurements were: signal intensity, a low analysis confidence (a measure of the quality of data reported by the OCT software) and the thickness of the nerve fibre layer. An increase in signal intensity in the second exam translated to an increased nerve fibre layer thickness. Measurement variability was higher in the patients showing a thinner fibre layer.
Despite improvement in the second exam, the two sets of images obtained included a large number of scans with a signal strength below 5, the threshold currently suggested by the manufacturer for a reliable Stratus OCT result. In future studies, it will have to be demonstrated whether the factors identified in this report will still emerge as relevant when images that fulfil the latest quality standards established are prospectively assessed.
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