Correlation of optic retinal nerve fiber layer thickness and visual function in patients with nonproliferative diabetic retinopathy

Abstract

To investigate the change of optic retinal nerve fiber layer (RNFL) thickness in nonproliferative diabetic retinopathy (NPDR) and to evaluate the correlation between the optic RNFL structural change and visual function. A cross-sectional study. All cases came from ophthalmology department of Zhejiang Province Traditional Chinese Medical Hospital, and the First People's Hospital of Ningbo, and Zhongshan Ophthalmic Center, Sun Yat-Sen University from December 2009 to October 2012. All the disease and control cases were coming from the same hospitals at the same period. Subjects were divided into 3 different groups: patients with NPDR (n = 89, 89 eyes), patients with diabetic mellitus but without diabetic retinopathy (NRD) (n = 96, 96 eyes) and disease-free controls (n = 115, 115 eyes). One eye of each subject was randomly selected for study. Participants aged from 40 to 70 years at baseline and all diabetic patients had a diabetic history of at least 5 years. Optic RNFL thickness of each subject was measured by spectral domain optical coherence tomography(SD-OCT). Visual function examinations including contrast sensitivity test in spatial frequencies of 1.5, 3.0,6.0, 12.0, 18.0 cycles per degree (c/d), pattern electroretinograms (PERG) and best corrected visual acuity (BCVA) assay. The analysis of RNFL thickness in each group was performed at four preset locations of the optic disc (i.e., temporally, superiorly, nasally, and inferiorly). To assess the structure-function relationship, the BCVA,contrast sensitivity, PERG-P50 amplitude and latency to optic RNFL thickness at each quadrant were analyzed in both NPDR and NRD groups. The RNFL thickness at four preset locations of the optic disc (meanly, nasally, temporally, superiorly, and inferiorly) were (97.7 ± 13.0), (71.7 ± 10.3), (70.9 ± 13.3), (118.3 ± 19.7), and (123.1 ± 20.8) µm in the NPDR group; (98.6 ± 15.3), (74.8 ± 13.1), (71.8 ± 14.6), (119.5 ± 17.2), and (125.6 ± 19.9) µm in the NRD group;and (99.1 ± 11.8), (77.4 ± 12.6), (72.6 ± 13.2), (119.1 ± 18.1), and (127.1 ± 19.3) µm in the normal group. The nasally optic RNFL thickness was significantly different among the three groups (F = 8.56, P = 0.000). The thickness in the NPDR and in the NRD group were significantly thinner than that in the normal group (SNK-q test:q = 3.16, 3.11, respectively; both P < 0.05). In the NPDR group, the nasally optic RNFL thickness was significantly thinner than that in the NRD group (SNK-q test:q = 3.07, P < 0.05). The inferiorly optic RNFL thickness was also significantly different among the three groups (F = 3.841, P = 0.035). The thickness in the NPDR group was significantly thinner than that in the normal group (SNK-q test:q = 3.090, P < 0.05). Yet, the difference of inferiorly optic RNFL thickness between the NRD group and the normal or the NPDR group did not reach the level of statistical significance (SNK-q test:q = 2.101, 1.955, P > 0.05). The temporally or superiorly optic RNFL thickness,or the average thickness of optic RNFL did not differ significantly among the three groups (F = 0.985, P = 0.375; F = 0.333, P = 0.71; F = 0.975, P = 0.379, respectively). Contrast sensitivities (1.5, 3.0,6.0, 12.0, 18.0 c/d), PERG-P50 amplitude and latency were all significantly correlated with the RNFL thickness of the nasal quadrant (nasal = 0.28, 0.26, 0.3, 0.25, 0.45, 0.65, 0.48, respectively; P < 0.05) and the inferior quadrant (inferior = 0.25, 0.28, 0.27, 0.26, 0.28, 0.37, 0.71; P < 0.05) in the NPDR group. Contrast sensitivities in high spatial frequencies (6.0, 12.0, 18.0 c/d), PERG-P50 amplitude were also significantly correlated with the RNFL thickness of the nasal quadrant (nasal = 0.59, 0.45, 0.66, 0.33, respectively; P < 0.05) and the inferior quadrant (inferior = 0.46, 0.71, 0.52, 0.41, respectively; P < 0.05) in the NRD group. The optic RNFL thickness of the nasal and inferior quadrant have been already reduced in patients with NPRD and are significantly correlated with the change of vision function.