Retinal Nerve Fiber Layer Thickness Measurements by Stratus OCT. What is Normal?

Abstract

Purpose: : To determine normal values of retinal nerve fiber layer (RNFL) thickness as measured by Stratus OCT with respect to age, race, gender, and optic disc size.

Methods: : One eye was studied from two hundred and twenty three normal patients undergoing Stratus OCT imaging. Patients were excluded with a previous diagnosis of glaucoma, visual acuity worse than 20/40, intraocular surgery excluding cataract surgery, any retinal treatment in the studied eye, or evidence of significant retinal pathology in the studied eye. If both eyes were valid the eye with the higher signal strength by OCT was selected. Stratus OCT using Peripapillary Fast RNFL scan protocol with a set diameter of 3.46mm centered on the optic disc was used. Mean RNFL thickness and the RNFL thickness of each quadrant were collected from each patient’s scan. Multiple linear regression analysis was performed to observe the effects of age, sex, gender, optic disc size and ethnicity (African American, Caucasian, and Other [Hispanic, Asian, Indian]) on the mean RNFL thickness. Each RNFL quadrant was compared between races using ANOVA tests with race as the independent variable and RNFL thickness as the dependent variable.

Results: : The mean RNFL thickness of the entire study group was 95.6 ±12.0µm. Mean RNFL thickness decreased an average of 2.97µm per decade (r=0.38, p <0.0001). Mean RNFL thickness increased by 3.97µm for each square millimeter increase in optic disc size (r=0.18, p=0.007). When looking at each RNFL quadrant individually, the only quadrant that showed statistical difference between races was the temporal quadrant, which was thinner in African Americans (62.5µm) compared to Caucasians (68.9µm) and others (71.1µm) (p=0.003). There was no statistically significant difference between gender and mean RNFL thickness after adjusting for age. The mean RNFL thickness was not statistically different between races.

Conclusions: : Mean RNFL thickness measurements by Stratus OCT show thinning with increasing age and decreasing optic disc size. Gender and race do not seem to significantly affect mean RNFL thickness; however the African American population did have a thinner temporal RNFL. It should be noted that differences in RNFL thickness between races should be interpreted with caution due to small sample size of Caucasians and Others compared to African Americans. It should also be noted that the increasing RNFL thickness with increasing optic disc size may be due to use of a fixed scan diameter, producing a technical artifact reflecting a shorter distance between the scan and optic nerve head rim.

Keywords: nerve fiber layer • imaging/image analysis: clinical • imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound)