Scanning Laser Polarimetry and Optical Coherence Tomography for Detection of Retinal Nerve Fiber Layer Defects

Early detection of glaucomatous damage is important for preventing irreversible optic neuropathy. Retinal nerve fiber layer (RNFL) abnormalities have been shown to precede the development of visual field defects in glaucoma patients.1-3 It has been reported that changes in the RNFL are a better predictor of glaucoma deterioration than are changes in the optic disc cup configuration.4-7 Although RNFL photography using red-free fundus photographs (red-free photographs) has been a useful method for the detection of RNFL defects, the interpretation of the photographs is sometimes qualitative and subjective.6,8 Several instruments have been introduced to quantitatively assess RNFL defects. Scanning laser polarimetry (SLP) is based on the birefringence properties of ganglion cell axon neurotubules that alter the laser scanning beam polarization according to the RNFL thickness.9 A SLP version with variable corneal compensation (GDx-VCC, Laser Diagnostic Technologies, Inc., San Diego, CA, USA) has resulted in improved diagnostic accuracy as compared with an earlier version of this instrument that used fixed corneal compensation. 10,11 Optical coherence tomography (OCT) uses a scanning interferometer to obtain a cross-section of the retina.12 The topographic representation is based on the reflectivity of the different retinal layers.13 The third-generation machine software, Stratus OCT (Carl Zeiss Meditec, Inc., Dublin, CA, USA), can analyze both RNFL thickness and the optic nerve head.14 The majority of reports that have compared the use of GDx-VCC and OCT indicate a similar ability to diagnose glaucoma in both instruments.15-19 Most previous studies on SLP and OCT have used software-provided global parameters, including NFI (SLP) and the average of RNFL thickness (SLP, OCT), to compare their abilities to discriminate glaucomatous eyes. However, Brusini et al.16 stated that software-provided parameters are based on data from a large region of the RNFL, thus limiting the detection of certain localized RNFL defects. Jeoung et al.20 evaluated the ability of OCT to detect localized RNFL defects using the OCT RNFL analysis map containing a normative database. They showed that in most cases a localized RNFL defect by red-free photography overlapped the same defect observed with OCT. GDx-VCC has a similar statistical analysis system using a normative database called the deviation map.21 Because the deviation map uses a grayscale fundus image of the eye as a background and displays abnormal grid values as colored squares over this image, users are able to determine the precise location of the abnormality. Kook et al.22 and Choi et al.23 suggest that the deviation map algorithm with a severity score calculation might enhance the understanding of GDx-VCC in the detection of glaucoma. Although RNFL OCT analysis maps correlate well with localized RNFL defects seen in RNFL photography, few reports show agreement between photographic RNFL defects and GDx-VCC deviation maps. This study was designed to compare the ability of GDx-VCC and OCT to detect diffuse or focal RNFL defects seen in redfree photography with the GDx-VCC deviation maps and OCT RNFL analysis maps. We did not intend this study to address the ability of these instruments to detect glaucoma.