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

AIMS To investigate the effects 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 offsets 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 effects of the rotated eye and the measurement of the opposite eye position were significant. The eyes rotated 90 degrees showed quite a different 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) microm and 122.5 (11.2) microm, respectively (p<0.0001, Scheffe 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.