Comparison of retinal nerve fibre layer thickness and visual field loss between different glaucoma groups.

Background/aims: Scanning laser polarimetry (SLP) uses a confocal scanning laser ophthalmoscope with an integrated polarimeter to evaluate the thickness of the retinal nerve fibre layer (RNFL). The aim of this study was to verify the ability of the SLP to detect differences in RNFL thickness between normal and glaucomatous eyes and between glaucomatous eyes subdivided in groups by the severity of visual field damage.
Methods: This was a cross sectional retrospective study. The charts of 40 healthy subjects and 68 glaucoma patients who underwent complete ophthalmological examination, optic disc stereophotography, peripapillary, and macular SLP imaging were reviewed. The right eye of subjects eligible for the study was enrolled. Only eyes with SLP examinations indicating a minimised effect of anterior segment birefringence based on macular image were included. The ability of retardation parameters to discriminate between healthy and glaucomatous eyes was evaluated. Based on visual field loss, glaucoma patients were subdivided in three subgroups (early, moderate, and severe). RNFL thickness between healthy control group and glaucoma subgroups was compared. RNFL thickness and visual field loss correlation was evaluated.
Results: There was a significant difference in superior and inferior maximum RNFL thickness between normal and glaucomatous eyes (p<0.001). With these two parameters, the area under receiver operator characteristic curve was 0.75 and 0.74, respectively. Superior and inferior RNFL thickness was significantly different between healthy control group and all glaucoma subgroups (p<0.001) and between glaucoma subgroups (p<0.05), except for early and moderate glaucoma subgroups (p>0.05). Linear regression showed a weak correlation between RNFL thickness and visual field loss.
Conclusion: These results suggest that once visual field loss is established, smaller reductions in the RNFL thickness detected by SLP are necessary for a given reduction of mean defect value.