Comparison of Whole Eye versus First-Surface Astigmatism in Down Syndrome

Abstract

Purpose Individuals with Down syndrome (DS) have structural differences in the cornea and lens, as compared to the general population. This study investigates objectively measured refractive and corneal astigmatism, as well as calculated internal astigmatism in subjects with and without DS. Methods Refractive (Grand Seiko autorefraction) and anterior corneal astigmatism (difference between steep and flat keratometry obtained with Zeiss Atlas corneal topography) were measured in 128 subjects with DS (mean age = 24.8±8.7 yrs) and 137 controls without DS (mean age = 24.9±9.9 yrs), with 1 eye randomly selected for analysis per subject. Refractive astigmatism and corneal astigmatism were converted to vector notation (J0, J45) to calculate internal astigmatism (Refractive – Corneal) and then converted back to minus cylinder form. Results Mean refractive astigmatism was significantly greater in subjects with DS than controls (−1.94±1.30DC vs −0.66±0.60DC, t=−10.16, p<0.001), as was mean corneal astigmatism (1.70±1.04DC vs 1.02±0.63DC, t=6.38, p<0.001) and mean internal astigmatism (−1.07±0.68DC vs −0.77±0.41DC, t=−4.21, p<0.001). A positive linear correlation between corneal and refractive astigmatism was observed for both study populations for both the J0 and J45 vectors (p<0.001 for all comparisons, R2 range = 0.31 to 0.74). The distributions of astigmatism orientation differed significantly between the two study populations when compared across all three types of astigmatism (Chi-Square, p<0.001). Conclusions This study demonstrates that corneal astigmatism is predictive of overall refractive astigmatism in individuals with DS, as it is in the general population. The greater magnitudes of astigmatism and wider variation of astigmatism orientation in individuals with DS for refractive, corneal, and calculated internal astigmatism is likely attributable to previously reported differences in the structure of the cornea and internal optical components of the eye from that of the general population.

Simulated Keratometry Repeatability in Subjects with and without Down Syndrome

“…A comparison of corneal powers and astigmatism between subjects with DS and controls has already been reported from this dataset 16 and thus it is known that the corneal parameters differed overall between these populations, although whether those are stable structural differences or the result of potentially progressive corneal disease remains unknown.…”

“…The difference in trends between the two groups is likely attributed to the much larger range of astigmatism and steep K power in the subjects with DS, which has been previously reported. 16 …”

“…As described in a previous publication of this dataset 16 , a cornea and contact lens specialist with over 40 years of clinical experience who was masked to the purpose of the study and the study population assessed the quality of each topography image obtained from subjects with DS. This step was taken due to concerns about the potential for poor quality due to poor fixation or subject movement during capture in this population.…”

“…This decision was justified by demonstrating that the masked specialist and unmasked examiner graded a subset of control images similarly, as described in a previous publication of this same dataset 16 .…”

Variability in Objective Refraction for Persons with Down Syndrome

“…The population of individuals with Down syndrome exhibit both increased refractive cylinder 13 and higher order ocular aberration 14 which has the potential to further compound the problems associated with subjective refraction as it relies entirely on patient feedback concerning perceived image quality. For typical eyes, where perceived image quality is predominantly a function of residual sphere and cylinder error, the optical parameters at the control of the clinician largely compensate for those impacting the subject’s perceived visual quality.…”