A clinical description of anterior segment variables measured using optical coherence tomography in a healthy South African young adult population: the development of normal reference intervals.
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Date
2018
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Abstract
Background: Assessment of anterior segment variables is important to screen, diagnose and monitor ocular anomalies. Previous studies, which have focused exclusively on Caucasian and Asian sub-populations with limited attention to South African sub-populations, suggest that anterior segment variable (corneal and anterior chamber angle) measurements vary with demographic and/or ocular factors. This study investigated anterior segment variables, measured using optical coherence tomography, in a healthy South African young adult population and develop a clinical biometric guideline with normal reference intervals.
Methods: A quantitative cross-sectional research design was used. Multistage random sampling was used to select 700 participants from a university population. Anterior segment variables were measured using the Fourier-domain iVue100 Optical Coherence Tomographer. The Oculus Keratograph, Goldmann applanation tonometer and Nidek US-500 ultrasonographer were used to measure corneal topography, intraocular pressure (IOP) and axial biometry respectively. Data were analysed by descriptive and inferential statistics. The reference intervals were computed using the non-parametric method recommended by the Clinical and Laboratory Standards Institute.
Results: The mean age of the sample, which consisted of 350 males and 350 females, was 20.4 ± 1.8 years. The anterior segment variable measurements of the right and left eyes showed high levels of interocular symmetry with intraclass correlation coefficients greater than 0.933 and marginal mean interocular differences. Accordingly, data from only the right eyes were analysed because of the high levels of interocular symmetry. The mean central corneal thickness (CCT) was 501.91 ± 33.74 μm and significantly thinner than the mean corneal thickness in each quadrant of the paracentral and peripheral cornea (p < 0.001). The mean minimum corneal thickness was 495.73 ± 33.89 μm and 1.23% thinner than the
mean CCT measurement (p < 0.001). The thinnest point on the cornea was central for 94% of participants (n = 659). The anterior chamber angle (ACA) width variables, which included the angle-opening distance taken at 500 μm (AOD500) and trabecular-iris angle (TIA), were ~553 μm and ~37° respectively. The majority of participants showed ACA width variable measurements associated with open non-occludable ACAs. The temporal ACA had slightly higher variable measurements than the nasal ACA. The corneal thickness measurements in the different zones were normally distributed (p ≥ 0.095) whereas the ACA width variable measurements were asymmetrically distributed (p < 0.001).
Black participants had significantly thinner mean corneal thickness measurements than Indian participants (range between 29.10 μm between 36.38 μm) for all zones (p < 0.001). For both the nasal and temporal ACAs, Black participants had 10 μm to 22 μm lower median AOD500 measurements (p ≥ 0.031) and slightly higher (less than 1°) median TIA measurements (p ≥ 0.068). The mean corneal thickness in males were 0.35 μm to 3.93 μm thicker compared with females (p ≥ 0.137). Female participants had higher median ACA width variable measurements than male participants for both the nasal and temporal ACAs (p ≥ 0.029). Emmetropes and hyperopes had the lowest corneal thickness and ACA width variable measurements respectively. The anterior segment variables were inversely correlated with spherical equivalent refraction (p ≤ 0.003) although the correlation coefficients were relatively weak (range between 0.111 and 0.222).
The CCT was the most important anterior segment variable, with a cut-off value of 527 μm, to influence IOP in the unpruned and pruned regression tree models. The other important variables included the average peripheral corneal thickness, axial anterior chamber depth and average paracentral corneal thickness. The clinical biometric guideline presents the normal reference intervals as well as the associated 95% confidence intervals for the corneal thickness and ACA width variables in a healthy South African young adult population. The normal reference interval for the CCT measurement ranged from 434 μm to
566 μm. In the present study, the mean, range and normal reference interval for the CCT measurement differed when compared with the measurements reported in other studies involving healthy African samples living within the African continent.
Conclusion: This study demonstrated that anterior segment variable measurements in a South African young adult population differ when compared with studies involving Caucasian, Asian and other African sub-populations globally. Consequently, the clinical biometric guideline with normal reference intervals therein should be used by eye care personnel when examining South African individuals. Moreover, the possible influences of demographic and/or ocular factors should be considered when evaluating anterior segment variable measurements.
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Doctoral Degree. University of KwaZulu-Natal, Durban.