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 lay er 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 diamet ers from the disc margin. Study 1: Measurement in normal eyes. The amount o f 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 o r to be as strong as possible; (2) the intentional offsets of the laser bea m axis in relation to the pupil were made in four directions; (3) the eye w as rotated by shifting the head 45 or 90 degrees; (4) the right eye was mea sured by moving it to the left eye position on the head rest. Study 2: Meas urements on an artificial nerve fibre layer. The birefringence measurements were confirmed with a plastic disc, which has a radial arrangement of bire fringence. The plastic disc with black paper was fixed at the right eye pos ition or the left eye position on the head rest. The retardation of the las er beam by the plastic disc on the black paper was measured. The retardatio n 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 rota ted eye are 41.9 (SD 6.0) mu m and 122.5 (11.2) mu m, respectively (p<0.000 1, Scheffe multiple comparison test). Study 2: The uniform retardation of t he plastic disc was observed with the AER-10A. The NFA detects the retardat ion 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 n ot 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 compe nsation of the cornea is thought to be important.