Morphometric Characterization of Limbal Vasculature using Ultra-high Resolution Optical Coherence Tomography
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Purpose: The aim of the present study was to compare and investigate morphometric characteristics of limbal vasculature within the superior and inferior limbal regions using ultra-high resolution optical coherence tomography. Method: Cross-sectional images of the human corneo-scleral limbus were acquired with a research grade ultra-high resolution optical coherence tomographer (UHR-OCT) from 14 healthy subjects after manual retraction of the upper and lower eyelid. The UHR-OCT provides an axial and lateral resolution in biological tissue of ~3μm and ~18μm, respectively. 3D stacks of OCT images (1000 x 1024 x 256) were acquired of the transition from cornea to bulbar conjunctiva at the superior and inferior limbal region. All visible vessels within the limbal region were measured using an Image J circle or ellipse tool. Vessel depth and size measurements were repeated for the same vessel and the concordance correlation coefficient was computed. Quantitative differences in vessel size and depth in the limbal region were analyzed using repeated measured ANOVA. R and SPSS were used for all data analysis procedures. Results: The average vessel size for superior and inferior limbus were 29.28µm ± 17.649 µm (SD) and 23.68µm ± 18.118µm (SD) respectively. The average vessel depth for superior and inferior limbus were 176.76µm ± 108.698µm (SD) and 205.62µm ± 131.991µm (SD) respectively. The concordance correlation coefficient for superior and inferior limbal vessel size were 0.95 (95% CI, 0.935 to 0.964) and 0.994 (95% CI, 0.991 to 0.995) respectively, while that of vessel depth were 0.998 (95% CI, 0.997 to 0.999) and 0.998 (95% CI, 0.997 to 0.998) respectively. The vessels within the superior limbus were larger than the vessels found in the inferior limbus (RM-ANOVA POS p = 0.004), and the vessels within the inferior limbus were on average deeper than the vessels found within the superior limbus (RM-ANOVA POS p = 0.042). There was a positive linear relationship between limbal vessel depth and size within the superior and inferior limbus with a Pearson correlation coefficient of 0.803 and 0.754, respectively. There were on average 9 vessels per subject within the superior limbal region as compared to 13 vessels per subject with the inferior limbal region. Conclusion: This study provided evidence that the UHR-OCT was capable of imaging (and therefore measuring) morphometric characteristics such as the size and depth of vessels within the limbus in a repeatable manner. A positive linear association between vessel depth and size was identified in the superior and inferior limbal regions. The results of this study suggest a definite difference in the size and depth of vessels across different positions of the limbus, and this may be indicative of adaptations to chronic hypoxia caused by the constant covering of the superior limbus by the upper eyelid.