The relationship between structure, observed by different imaging techniques, including scanning laser polarimetry (SLP), and function, determined by standard automated perimetry (SAP) has been investigated before.
9 10 11 12 13 14 15 16 17 With SLP with variable corneal compensation (VCC; commercially available in the GDx Nerve Fiber Analyzer; Carl Zeiss Meditec, Inc., Dublin, CA), the structure-function relationship has been shown to be curvilinear when VF sensitivity is expressed in a decibel scale.
9 10 11 12 However, when VF sensitivity is expressed in an antilog (1/Lambert) scale, this relationship appears to be linear.
10 11 12 SLP is a noninvasive, noncontact diagnostic technique that indirectly quantifies the RNFL thickness. It is based on the principle that polarized light passing through the presumed form birefringent RNFL undergoes a measurable phase shift, known as retardation, that is linearly related to histologically measured RNFL thickness.
18 Because the anterior segment (mostly the cornea) can also exhibit birefringence, VCC was developed to obtain the true RNFL retardation by subtracting the eye-specific anterior segment retardation from the total retardation.
19 However, atypical birefringence patterns (ABPs), as seen in a subset of normal and glaucomatous eyes, may confound the RNFL thickness measurement by VCC. VCC images with ABPs are characterized by an abnormal retardation map (i.e., with variable areas of high retardation arranged in a spokelike peripapillary pattern, or splotchy areas of high retardation nasally and temporally).
20 Quantitatively, images with a typical scan score (TSS) of 80 or less have been reported to be atypical.
21 ABPs may be related to age, myopia, and blond fundi. Hypothetically, ABPs are caused by a low signal-to-noise ratio resulting from loss or attenuated reflectivity of the retinal pigment epithelium.
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