Corneal ectasia in mothers of Down syndrome children

In this study, corneal findings regarding keratoconus (KC) and early KC among mothers with Down syndrome children (MDS) and a group of age-at-delivery-matched mothers with normal children (MNC) were compared. KC was diagnosed based on the presence of a clinical sign and at least one abnormal tomographic or biomechanical criterion. Early KC was defined as having no clinical sign in the presence of at least one abnormal tomographic or biomechanical criterion. The normal subgroups in each group were compared in terms tomographic and biomechanical parameters. In MDS and MNC, the prevalence rates were 6.5% and 1.6% for KC (P = 0.047), and 30.9% and 14.3% for early KC (P = 0.014), respectively. Comparison between the two normal subgroups showed significant differences in mean index of height asymmetry, irregularity index, anterior asphericity, pentacam random forest index, corneal stiffness parameters at first applanation, deformation amplitude ratios, integrated radius-1 mm, highest concavity deflection amplitude, biomechanical corrected IOP, peak distance, and radius (all P < 0.05). This study showed that MDS are more likely to have KC and also to have thinner, steeper and softer corneas compared to MNC. This results support the need for further work for determining the risk of delivering a child with DS.


Methods and materials
Patients and sampling. This comparative study was performed in 2020 at Noor Ophthalmology Hospital in Tehran. The cases were mothers with children with Down syndrome (MDS); they were selected from a study of DS children, whose methodology has already been described 13 . Women in the control group were mothers with normal children (MNC); these were selected from the hospital staff and visitors to match the MDS group in terms of maternal age at delivery. Any case with a history of eye surgery was excluded from the study.
Definitions. In both groups, corneas were categorized into KC, "mild or fruste" KC-which can be interpreted as with high susceptibility for ectasia progression, and normal subgroups based on ranges of ectasia indices as follows: Statistical analysis. Statistical analysis was performed using SPSS version 21 (IBM Corp., Armonk, NY, USA). The sample size was determined 56 cases in each group using n = 2(Z α/2 + Z β ) 2 × σ 2 /d 2 where the α = 0.05, β = 0.01, σ = 1.0 D for Zonal Kmax-3 mm, and d = 1.5 D. Bonferroni's correction was not applied to maintain a power of 99% for comparing indices between groups. In the analysis, assuming that the abnormality would be unilateral or asymmetrically bilateral 19 , both right and lefts eyes were entered in the analysis. Binary generalized estimating equations (GEE) were used to compare the prevalence of KC and early KC between the two groups (MDS and MNC). For normal subgroups of MDS and MNC, linear GEE was used to compare mean values of the indices. In the analysis, fellow-eye correlations were applied using an unstructured correlation matrix.
Ethical consideration. The Ethics Committee of Tehran University of Medical Sciences approved the study (ID: IR.TUMS.MEDICINE.REC.1399.772). The aims and methods of the study were explained to the participants and written informed consent was obtained. The study adhered to the Helsinki Declaration at all stages.

Results
In this study, after applying the inclusion and exclusion criteria, 140 participants were enrolled; 136 eyes of 77 MDS cases were compared with 126 eyes of 63 MNC controls. Mean maternal age at birth was 31.34 ± 6.86 KC was diagnosed in 6.5% (5 cases) in the MDS group and 1.6% (1 case) in the MNC group; the prevalence of KC was significantly higher among MDS (P = 0.047). Mild or fruste KC was identified in 30.9% (21 cases) in the MDS group and 14.3% (9 cases) in the MNC group (P = 0.014). Table 1 summarizes results of the comparisons between the normal MDS subgroup (n = 84 eyes) and the normal MNC subgroup (n = 106 eyes). As presented in this table, mean intergroup differences were statistically significant for IHA (P = 0.

Discussion
This is the first clinical study comparing corneal tomography and biomechanical properties between MDS and MNC. The results indicated that KC and mild, fruste (or subclinical) KC were more prevalent in MDS, which also had thinner, steeper, and softer corneas compared to MNC even in the absence of definitive KC. KC is a multifactorial disorder that is instigated by a combination of genetic and environmental factors. However, the exact cause is unknown 20 . To date, several studies have shown and confirmed the role of genetics in the pathogenesis of KC 6,20,21 , and several loci on various chromosomes and single nucleotide polymorphisms responsible for KC have been identified 21 . Such evidence suggest genetic heterogeneity and complex pathogenesis for this disease. One of the genes that have been investigated as a candidate gene for KC in several studies is the superoxide dismutase 1 gene (SOD1) [21][22][23] . This gene encodes a cytoplasmic enzyme that detoxifies superoxide radicals, a form of reactive oxygen species, and thus reduces the level of oxidative stress in cells 21 . Increased levels of oxidative stress in corneal cells appear to be a predisposing factor for KC, so it has been proposed that a mutation in SOD1 increases oxidative stress and is involved in the pathogenesis of ectatic disorders 21 . Nevertheless, there is no consensus on the role of SOD1 in KC, and there are different pieces of evidence about it 20,21 . SOD1 is the only candidate gene for KC located on chromosome 21, and it could explain the association between KC and DS in some way 21 . In 95% of cases, DS occurs when a gamete with two chromosomes 21 (due to nondisjunction of chromosome 21 pairs in meiosis) with a normal gamete produces a zygote cell with three chromosomes 21 (trisomy 21) 24 . The relationship between DS and KC has been investigated and confirmed in numerous studies 25 . Although eye rubbing due to blepharitis is one of the main causes of KC in DS patients, genetic defects in the structure and composition of the cornea of these patients can be another cause of ectatic disorders 8,26 . Thus, the genes responsible for the defect in corneal biomechanical properties, refractive errors, and the development of ectatic diseases such as KC in DS patients may be located on chromosome 21. On the other hand, it is possible that age-related defects in these particular genes make mothers more susceptible to chromosome 21 nondisjunction in meiosis, thereby increasing the chances of DS births. SOD1 is an example of these genes, which have also been linked to KC; there may be other genes yet unidentified. In this regard, the results of this study indicate that abnormal KC indices are more common in MDS than MNC, but these abnormalities point more to mild KC.
The study has limitations, but open a new horizon of research for the associations between KC and the risk of a mother deliver a child with DS. Following the first anecdotal report, this is the first clinical study that comprises sample sizes large enough to detect statistical significant differences between MDS and MNC. The participants were recruited from different sources from Iran, which makes the findings more generalizable. This study is the lack of genetic testing for KC-related genes (e.g. SOD1) in the participants. The rare nature of KC and DS limited the study design to a correlation study, otherwise we would able to conduct their association with a higher power. Conducting a cohort study would have required enrolling a cohort of KC women throughout their childbearing age and monitoring pregnancy outcomes in term of DS birth; instead, we conducted a case-control study with a lower power to test this hypothesis. Despite the lower power, differences between MDS and MNC were statistically and clinically significant.
The current study detected that MDS are more likely to have KC and also to have thinner, steeper and softer corneas compared to MNC. Such findings support the need for further work but are not to be considered at this point for determining the risk of delivering a child with DS. Indeed, multimodal corneal imaging including corneal tomography and corneal biomechanical assessments should be considered in future larger retrospective studies so that a major prospective study can be defined.

Data availability
Data supporting the findings in this report are available from the corresponding author (SA) upon reasonable request.