The effect of myopia on retinal nerve fibre layer measurement: a comparative study of spectral-domain optical coherence tomography and scanning laser polarimetry

Background/aim To evaluate the effect of myopia on retinal nerve fibre layer (RNFL) thickness measurement by using spectral domain optical coherence tomography (Cirrus HD OCT) and scanning laser polarimetry with enhanced corneal compensator (GDx ECC). Methods One hundred and forty-nine eyes of 149 myopic subjects were recruited and subdivided into three groups according to their refractive errors: high myopia group (spherical equivalent (SE)# 6 D); moderate group ( 3 D$SE> 6 D); low group ( 0.5 D$SE> 3 D). RNFL thickness was measured by Cirrus High Definition (HD) OCT and GDx ECC. Associations between RNFL thickness and axial length/ SE were evaluated by using Pearson correlation analysis. Results Average RNFL thickness measured with Cirrus OCT correlated significantly with axial length (r1⁄4 0.322, p<0.001) and SE (r1⁄40.291, p<0.001). No significant correlation was detected between axial length (r1⁄4 0.068, p1⁄40.407)/SE (r1⁄40.109, p1⁄40.187) and RNFL thickness measured with GDx ECC. Conclusion Average RNFL thickness measured with Cirrus HD OCT decreases as the degree of myopia increases while no such correlation was detected in GDx ECC. Histological studies are warranted to further our understanding of the relationship between RNFL thickness and myopia. INTRODUCTION Since myopia is a worldwide common ocular disorder that can increase the risk of primary open angle glaucoma (POAG) twoto threefold, the diagnosis of POAG in myopic patients is important. Myopic eye is associated with tilted optic disc, increased intraocular pressure (IOP) and visual field defects. All these factors make the clinical diagnosis of POAG in myopic patients challenging. It has been shown that retinal nerve fibre layer (RNFL) thinning may be an early sign of glaucomatous damage. Measurement of RNFL thickness has been emerging as an important diagnostic technology for glaucoma. It is therefore important to establish the relationship between RNFL thickness and myopia. Both optical coherence tomography (OCT) and scanning laser polarimetry (SLP) have been developed to measure in vivo RNFL thickness objectively. Studies with the instruments based on these two technologies have been performed to evaluate the relationship between myopia and RNFL thickness. Using OCT-1, Hoh et al did not find any significant associationbetweenRNFL thickness andaxial length/ SE, while recent studies based on OCT 3 (StratusOCT; Carl Zeiss Meditec Inc, Dublin, CA, USA; the timedomain technology) have reported a negative relationship between RNFL thickness and axial length. However, no such evident correlation between RNFL and myopia was found in a recent studybasedonSLPwithvariable corneal compensator (GDx VCC). Thus, the association between RNFL thickness and myopia is controversial. It has been shown that spectral domain optical coherence tomography (Cirrus High Definition (HD) OCT; Carl Zeiss Meditec, Dublin, California, USA) and SLP with enhanced corneal compensation (GDx ECC), which are the latest commercially available versions of OCTand SLP, respectively, have superior performance over conventional OCT and SLP with variable corneal compensation (GDx VCC). 11 The aim of this studywas to evaluate the relationship between myopia and RNFL thickness using Cirrus HD OCT and GDx ECC. To date, no study has used SLP and OCT concurrently to evaluate the relationship between RNFL thickness and myopia. It is unclear whether myopia can lead to the discrepancy of RNFL measurements between SLP and OCT. Simultaneous use of these two technologies will be helpful for further understanding of the relationship between RNFL thickness and myopia. MATERIALS AND METHODS Subjects One hundred and sixty-six Chinese subjects were recruited from June 2009 to October 2009. One eye from each subject was randomly selected. All subjects received complete ophthalmic examinations in the Joint Shantou International EyeCenter,which included visual acuity, IOP measured by Goldmann applanation tonometry, axial length measured by IOLMaster (Carl ZeissMeditec Inc, Jena, Germany), refraction and dilated fundus stereoscopic examination. For each subject, RNFL thicknesswasmeasured with Cirrus HD OCTand GDx ECC by three experienced technicians. All subjects were subdivided into three groups according to refractive error: high myopia group (SE# 6 D); moderate group ( 3 D$SE> 6 D); low group ( 0.5 D$SE> 3 D). Inclusion and exclusion criteria All the included eyes had an SE of less than 0.5 D and no other concurrent diseases. Subjects with Joint Shantou International Eye Center of Shantou University and The Chinese University of Hong Kong, Shantou, Guangdong Province, The People’s Republic of China