[Reliable parameters for assessing the birefringence of retinal nerve fiber layer under various conditions of corneal birefringence].
OBJECTIVE
To find a reliable parameter for the assessment of parafoveal nerve fiber layer using scanning laser polarimetry.


METHODS
The retinal nerve fiber layer was imaged using a Nerve Fiber Analyzer II (NFA II) along the parafoveal circle with a 90-pixel diameter in 4 dilated eyes of 4 normal subjects. The average thickness was measured in the superior(S), temporal(T), inferior(I), and nasal(N) quadrants. The image was obtained by means of a laser beam passing through 4 different locations in the cornea: the center, 2 mm temporal from the center, 2 mm lower temporal from the center, and 2 mm lower from the center.


RESULTS
The circumference profile of the nerve fiber layer around the fovea showed a biphasic pattern similar to that obtained around the optic disc. However, the peaks of the pattern varied according to the corneal location through which the laser beam passed. Accordingly, the average thickness of the nerve fiber layer in each quadrant showed great fluctuation when using a displaced laser beam. The coefficient of variance was large (0.265-0.313) for the nerve fiber layer thickness itself; however, it was far less for the ratio of the nerve fiber layer thickness in the opposite direction, such as temporal-to-nasal (0.082) or superior-to-inferior (0.089).


CONCLUSIONS
Although the measurement of the retinal nerve fiber thickness itself depends largely on the location in the birefringent cornea through which the laser beam passes, the ratios of nerve fiber layer thickness such as temporal-to-nasal or superior-to-inferior are reliable and show less fluctuation in the measured values.
Kogure
Satoshi
S
Department of Ophthalmology, Faculty of Medicine, University of Yamanashi, 1110 Shimokato, Tamaho-cho, Nakakoma-gun, Yamanashi 409-3898, Japan.
Chiba
Tatsuya
T
Iijima
Hiroyuki
H
Kohwa
Hiroyuki
H
Tsukahara
Shigeo
S
D000293:Adolescent; D000328:Adult; D001718:Birefringence; D002648:Child; D003315:Cornea / Q000033:anatomy & histology*; D003941:Diagnostic Techniques, Ophthalmological; D006801:Humans; D007834:Lasers; D009412:Nerve Fibers; D009898:Optic Disk / Q000033:anatomy & histology; D012160:Retina / Q000033:anatomy & histology*; D012165:Retinal Ganglion Cells / Q000166:cytology