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Editors Selection IGR 8-4
Computerized image analysis: SLP, ABP & ECC
Comment by Nic Reus & Hans Lemij on:
16867 Scanning laser polarimetry with variable and enhanced corneal compensation in normal and glaucomatous eyes, Sehi M; Guaqueta DC; Feuer WJ et al., American Journal of Ophthalmology, 2007; 143: 272-279
Scanning laser polarimetry (SLP) with variable corneal compensation (VCC) allows an evaluation of the thickness of the retinal nerve fiber layer (RNFL) in glaucoma by measuring the amount of retardation exhibited by the RNFL which corresponds to its thickness. This technique is commercially available in the GDx VCC. In approximately 10% of subjects, atypical patterns of birefringence occur despite adequate corneal compensation. These atypical birefringence patterns (ABP) are characterized by variable areas of high retardation that do not match the expected distribution of the RNFL and may confound the detection of glaucoma.
Sehi et al. (170) investigated a method called enhanced corneal compensation (ECC), that has been described earlier to improve the signal-to-noise ratio of SLP measurements and eliminate artifacts associated with ABP. To this end, they measured 84 healthy subjects and 45 glaucoma patients at 4 study sites. The severity of ABP was assessed with the typical scan score, a classifier that has been shown to be highly predictive of the presence of ABP.
The frequency of mild and moderate-severe ABP, expressed as the typical scan score, was significantly lower with ECC compared with VCC. In eyes with moderate-severe ABP, the diagnostic performance of SLP improved when ECC was used. However, the assessment of the diagnostic accuracy may have been biased to some extent because of a difference in age (with glaucoma patients being older) despite a statistical correction. In general, SLP measurements correlated better with visual field loss when assessed with ECC than with VCC.
In conclusion, the ECC algorithm may improve the clinical use of SLP, especially in eyes with moderate to severe ABP.
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