Relation of myopia and retinal nerve fiber layer thickness: An ocular coherence tomography based study

Aim: To evaluate the relationship between the axial length and refractive error and peripapillary retinal nerve fiber layer (RNFL) thickness measured using Stratus optical coherence tomography (OCT) in subjects with high myopia. Materials and Methods: It was a prospective comparative observational study. Eighty eyes of 40 non glaucomatous subjects aged between 18 to 51 years with spherical equivalent ≥ -6 dioptres (D) were examined during a period of 3 years. All subjects underwent a full ophthalmic examination. OCT was performed with OCT model 3000 version 48 (Stratus OCT, Carl Zeiss Meditec Inc.) Total average, quadrantic & mean clock hour RNFL thickness was measured with the fast RNFL (3.4) (768 A-scans in 1.92 seconds) scanning protocol. The same procedure was performed for 80 emmetropic eyes of 40 age matched subjects of the control group. Statistical analysis was performed with SPSS (version 11.5, SPSS, Chicago). Results: Mean age of myopes was 28.4 years (SD +/-10.3). Mean spherical equivalent was -9.48125 (SD +/- 1.73), with mean axial length of 28.72 (SD +/-1.70). In the emmetropic control group, mean age was 26.9 years (SD 9.9) and mean axial length was 23.8 (SD +/-0.4). There was significant difference between axial length (P<0.0001) of the two groups. Mean average RNFL thickness in myopes was 73.27µm with standard deviation (SD) +/-19.33 µm. Mean average RNFL thickness in emmetropes was 98.5µm (SD +/- 12.9 µm). RNFL thickness decreased with higher axial length (overall r2=0.016, P> 0.05), but the correlation was not found to be statistically significant. No statistically significant associations were noted between mean RNFL thickness and age (r2 = 0.017, p > 0.05). Conclusion: High myopic subjects tend to have thin peripapillary RNFL as measured by Stratus OCT. This phenomenon should be considered when interpreting a glaucoma suspect's RNFL measurements compared with the normative database which may be misleading in such cases resulting in a substantial proportion of false positive errors. The variation in the normal population needs to be taken into account to distinguish a pathologically from a physiologically thin RNFL. Stratified normal databases are required for accurate diagnosis of conditions resulting in nerve fiber loss such as chronic glaucoma.