Retinal Never Fiber Layer Thickness in Chronic Renal Failure : Analysis by Optical Coherence Tomography

Methods: This was a prospective, analytical, cross-sectional casecontrol study. The study sample comprised 22 eyes from 11 patients and 33 eyes from 17 patients in the case and control groups respectively. RNFLT was measured with a model 3000 OCT unit (Stratus OCT-3TM, Carl Zeiss Meditec Inc., Dublin, CA). The fast RNFL protocol was used, which consists of three consecutive 3.4 mm-diameter circular scans centered on the optic nerve. Measured parameters included overall mean RNFL and mean RNFL at the temporal (31645°), superior (46-135°), nasal (136-225°), and inferior (226-315°) quadrants. The Mann-Whitney U was employed to assess possible betweengroup differences in mean overall RNFL and RNFL at the superior, temporal, nasal, and inferior quadrants. The null hypothesis was rejected when p-values were smaller than the set significance level of <0.05.


Introduction
Chronic kidney disease (CKD) is defined as a reduction in glomerular filtration rate (GFR) and implies irreversible, progressive loss of renal function, rendering patients dependent on renal replacement therapy (dialysis or kidney transplant) to prevent development of potentially fatal uremia [1].
Optical coherence tomography (OCT) is a highresolution, noninvasive, non-contact diagnostic technique that allows acquisition of two-dimensional image representations of pathological conditions in the epi-, intra-or subretinal layers, choroid, and optic nerve (ON).OCT permits assessment of several relevant aspects, including changes in reflectivity secondary to inflammatory infiltrates, atrophy, and fibrosis of the neurosensory retina, of the retinal pigment epithelium-choriocapillaris complex, and of the vitreoretinal interface, and also provides a tool for analysis of the retinal nerve fiber layer (RNFL) [10][11][12][13].The hypothesis that chronic kidney disease and its treatment with dialysis could induce changes in the retinal microvasculature, leading to ischemia and RNFL damage, provides the rationale for this study.
The objective of this study was to analyze the retinal nerve fiber layer thickness (RNFLT) in CKD patients by means of optical coherence tomography, ascertaining mean overall RNFLT and mean RNFLT in the nasal, temporal, superior, and inferior quadrants and comparing these measurements to those obtained from a control group.

Methods
Patients were recruited from the hemodialysis clinic of the Hospital das Clínicas de Pernambuco and examined at the hospital Ophthalmology department and at Fundação Altino Ventura.
This was a prospective, analytical, cross-sectional case-control study.
Patients with hepatitis, alcoholism, malnutrition, diabetes, or opacity of the transparent media of the eye preventing OCT were excluded from the sample, as were those who declined to provide informed consent for participation in the study.
All participants underwent a comprehensive eye examination, which consisted of uncorrected highcontrast visual acuity testing with an ETDRSTM chart (Lighthouse Inc., New York, NY); thorough biomicroscopy; measurement of intraocular pressure with a Goldmann tonometer (Haag-Streit, Bern, Switzerland); and funduscopy (including assessment of the optic disc and peripapillary nerve fiber layer) with a Volk 90D lens (Volk Optical Inc., Cleveland, OH).After this examination, participants were referred to Fundação Altino Ventura for OCT.
Quantitative variables were expressed as means and standard deviations.Qualitative variables were expressed as absolute and relative frequencies.
The Mann-Whitney U was employed to assess possible between-group differences in mean overall RNFLT and RNFLT at the superior, temporal, nasal, and inferior quadrants.The null hypothesis was rejected when p-values were smaller than the set significance level of <0.05.
The study project was approved by the Hospital das Clínicas da UFPE Research Ethics Committee.All participants provided informed consent prior to inclusion in the sample.

Results
Twenty-two and 33 eyes were assessed in the case and control groups respectively.RNFLT measure-ments for the retina as a whole and at the superior, temporal, nasal, and inferior quadrants in healthy controls are shown in Table 1.Table 2 shows these measurements as obtained in the case group (CKD patients).
Table 3 shows that mean RNFLT overall and at the superior, nasal, and inferior quadrants was greater in the control group, and that no significant between-group differences were detected in RNFLT at the temporal quadrant.Figure 1 provides a clearer visualization of these measurements.

Table 1 .
Overall retinal nerve fiber layer thickness (RNFLT) and RNFLT at the superior, temporal, nasal, and inferior quadrants in healthy controls (in micrometers).

Table 2 .
Overall retinal nerve fiber layer thickness (RNFLT) and RNFLT at the superior, temporal, nasal, and inferior quadrants in participants in the case group (in micrometers).

Table 3 .
Comparison of means in the case and control groups.