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Editors Selection IGR 22-4

Structure and function: ECC versus VCC

Comment by Linda Zangwill on:

20048 Diagnostic accuracy of scanning laser polarimetry with enhanced versus variable corneal compensation, Mai TA; Reus NJ; Lemij HG, Ophthalmology, 2007; 114: 1988-1993

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Evaluation of the retinal nerve fiber layer is important for the clinical management of glaucoma. Scanning laser polarimetry provides an indirect, objective quantification of the nerve fiber layer by measuring the amount of retardation, or phase shift of polarized light as it passes through the birefringent RNFL. As described by Mai et al.,¹ the anterior segment, particularly the cornea, also exhibits form birefringence. The GDx VCC, designed to neutralize the influence of corneal birefringence has been shown to have better diagnostic accuracy than the original scanning laser polarimeter that assumed the same fixed magnitude and axis of corneal polarization for each eye. Unfortunately, atypical birefringence patterns (ABPs) appear in a proportion of GDx VCC scans. Scanning polarimetry with enhanced corneal compensation (GDx ECC) was introduced to improve the RNFL measures by reducing the prevalence of ABPs. It should be noted that GDx ECC obtained scans cannot be used interchangeably with GDx VCC scans. Mai et al. (1304) evaluated five RNFL parameters, and found that the area under the receiver operator curve of inferior average, and temporal, superior, nasal, inferior, temporal (TSNIT) average and TSNIT standard deviation were higher when obtained with the GDx ECC compared to the GDx VCC.

GDX ECC RNFL measures show 1) improved diagnostic accuracy of GDx ECC particularly in eyes with ABPs; and 2) stronger correlation to visual field damage

As the Nerve Fiber Indicator (a machine learning classifier that indicates the probability of glaucoma) was developed using GDx VCC data, it is not surprising that no difference in this parameter was found. The authors appropriately conclude that 'A future automated classifier, similar to the current NFI, may perform better if it is trained on data obtained with ECC.' It should be noted that the sensitivities (at 95% specificity) of the 5 parameters obtained with the GDx ECC were similar to those obtained with the GDxVCC. The excellent diagnostic accuracy reported (ranging between .93 and .987) is likely due to the severity of glaucoma included in the study; over 35% of the glaucoma eyes had severe visual field defects; the mean (+ SD) pattern standard deviation was 8.0 + 3.9 dB.The results of this study are consistent with other recent publicationsby Mai and others that highlight the advantages of GDxECC over GDxVCC. ^2-8 Specifically, these studies indicate that compared to RNFL measurements with GDx VCC, GDX ECC RNFL measures show 1) improved diagnostic accuracy of GDx ECC particularly in eyes withABPs; and 2) stronger correlation to visual field damage.

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