EVects of artefacts on scanning laser polarimetry of retinal nerve fibre layer thickness measurement

Aims—To investigate the eVects of artefacts on scanning laser polarimetry of the retinal nerve fibre layer. Methods—Six eyes of six normal volunteers and an artificial nerve fibre layer were examined using the nerve fibre analyser II. The retinal nerve fibre layer thickness (RNFLT) was measured in each of four 90 degree quadrants, superior (S), temporal (T), inferior (I), and nasal (N), at 1.5 disc diameters from the disc margin. Study 1: Measurement in normal eyes. The amount of maximum error in RNFLT measurements was investigated as follows: (1) the intensity setting of the laser beam was changed to be as weak as possible or to be as strong as possible; (2) the intentional oVsets of the laser beam axis in relation to the pupil were made in four directions; (3) the eye was rotated by shifting the head 45 or 90 degrees; (4) the right eye was measured by moving it to the left eye position on the head rest. Study 2: Measurements on an artificial nerve fibre layer. The birefringence measurements were confirmed with a plastic disc, which has a radial arrangement of birefringence. The plastic disc with black paper was fixed at the right eye position or the left eye position on the head rest. The retardation of the laser beam by the plastic disc on the black paper was measured. The retardation of the plastic disc was checked by an automatic birefringence evaluation system (ABR-10A, Uniopt Co, Ltd, Shizuoka). Results—Study 1: The eVects of the rotated eye and the measurement of the opposite eye position were significant. The eyes rotated 90 degrees showed quite a diVerent pattern in which the thicker and thinner locations of the RNFLT are switched. The nasal RNFLT of the baseline and the 90 degree rotated eye are 41.9 (SD 6.0) μm and 122.5 (11.2) μm, respectively (p<0.0001, ScheVe multiple comparison test). Study 2: The uniform retardation of the plastic disc was observed with the ABR-10A. The NFA detects the retardation of the plastic disc which the retardation map showed as a double humped pattern. Conclusions—Study 2 indicated that the amount of corneal compensation was not small. The cause of significant influences by the rotated eyes and right eyes measurement in left eye position were thought to be incorrect corneal compensation. To increase the diagnostic ability of SLP, an improved compensation of the cornea is thought to be important. (Br J Ophthalmol 2000;84:1013–1017) Scanning laser polarimetry (SLP) has been proposed as an objective and quantitative method for assessing the thickness of the retinal nerve fibre layer (RNFLT) in vivo. This technique estimates RNFLT based on the retardation of the laser beam which is caused by the birefringence of the nerve fibre layer. 6–8 The high sensitivity, specificity, and reproducibility of this method has been reported. 9–11 Further development of this technique made it possible to detect abnormalities of RNFLT based on a normal database. This technique would seem to be a strong candidate as an eVective diagnostic procedure for early glaucoma. However, the RNFLT which were estimated with SLP were correlated with glaucomatous change in the superior and the inferior area of the optic disc, but not in its the temporal area. 13 Morgan et al report the comparison of the SLP measurement and histological measurement in which the nasal side of optic disc is not correlated and the temporal side of optic disc had a lower correlation coeYcient than the superior or the inferior one. To improve the temporal and nasal measurements we need to investigate the properties of the SLP. In this report, we have investigated the eVects of artefacts on scanning laser polarimetry of the RNFLT. Study 1: Measurements on normal eyes MATERIALS AND METHODS Six right eyes of six normal volunteers who were free from ophthalmic and systemic disease were studied. They ranged in age from 25 years to 36 years. The refractive error of the subjects ranged from −2.75 dioptres to +0.25 dioptres. Nerve fibre thickness profiles were obtained using the nerve fibre analyser II (NFA II, Laser Diagnostic Technologies, San Diego, CA, USA) Version 2.1.17 beta, as described elsewhere in detail. 6–10 15 Using the NFA, a polarisation image, which is a topographic image representing the RNFLT at each image pixel, was obtained in an array of 256 × 256 pixels. The RNFLT was measured within a 10 pixel-wide band located at a distance of 1.5 disc diameters from the disc margin. The mean thickness in each of the four 90 degree quadrants, superior (S), temporal (T), inferior (I), and nasal (N), were generated from each Br J Ophthalmol 2000;84:1013–1017 1013 Department of Ophthalmology, Yamanashi Medical University, Japan