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Editors Selection IGR 9-2

Examination methods: Laser scanning: RNFL thickness

Comment by Michael Kook on:

17604 Glaucoma, Tezel G; Wax MB, Chemical immunology and allergy, 2007; 92: 221-227

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One prerequisite for quantitative imaging technique software to detect the progression of glaucoma is for the imaging device to provide reliable and robust reproducibility. When testing reproducibility, it is important to select a cohort of subjects that are tested over longer intervals than when tests are repeated over the course of just a few days or weeks, since clinicians are interested in using imaging instruments to longitudinally follow patients suspected of having glaucoma or glaucoma patients over months to years. Medeiros et al. (589) addressed the long-term variability of GDx VCC retinal nerve fiber layer (RNFL) thickness measurements in an appropriate clinical environment. Thirty-one subjects without any evidence of visual field damage or change in the optic nerve underwent repeated GDX VCC testing at approximately 12-month intervals during follow-up of 9.1 years.Intraclass correlation coefficients was in the good to excellent range overall. Long-term variability was slightly greater than short-term variability for all parameters, although RNFL measurements were still highly reproducible. This finding has an important clinical implication in that GDx VCC is acceptable in the use of longitudinal assessment of the RNFL.

GDx VCC is acceptable in the use of longitudinal assessment of the RNFL

In a subset of eyes, GDx VCC scans have atypical birefringence patterns (ABP) that can generate spurious RNFL thickness measurements. Although eyes with typical scan score greater than 25 were only included in the study by Medeiros et al., no information was described as to the proportion of eyes that had some degree of atypical image. Clinicians and investigators should be aware of this phenomenon and cautious in that long-term reproducibility can be theoretically influenced by this phenomenon as ABP might occur or change over the course of long follow-up. New software, such as enhanced module, might be expected to improve long-term reproducibility in a cohort of glaucoma suspects that contains a subset of eyes with ABP.

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