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Published on: March 29, 2022
Birth order and myopia
Jeremy A Guggenheim1, George McMahon, Kate Northstone
1School of Optometry & Vision Sciences, Cardiff University , Cardiff , UK .
Insights
First-born individuals have a slightly increased risk of myopia, with the association strongest in larger studies and for more severe cases. This link appears independent of known risk factors like outdoor time.
Area of Science:
- Ophthalmology
- Genetics
- Epidemiology
Background:
- Previous observations suggest a link between birth order and reduced unaided vision, a potential indicator of myopia.
- Understanding factors influencing myopia development is crucial for public health interventions.
Purpose of the Study:
- To investigate the direct association between birth order and myopia.
- To analyze this association across diverse international cohorts.
Main Methods:
- Four large-scale studies were analyzed: ALSPAC (UK), SCORM (Singapore), REHS (Australia), and IDFC (Israel).
- Myopia prevalence in first-born individuals was compared to non-first-born individuals.
- Odds ratios (OR) were calculated, adjusting for potential confounding risk factors.
Main Results:
- A numerically higher prevalence of myopia was observed in first-born individuals across all groups.
- Adjusted ORs for myopia in first-borns ranged from 1.04 to 1.31.
- In the largest cohort (IDFC), the association was stronger for first-born vs. fourth-born and increased with myopia severity.
Conclusions:
- The increased risk of myopia in first-born individuals is generally low (OR < 1.3).
- Statistically significant evidence was primarily found in studies with over 4000 participants.
- The observed association may stem from mechanisms other than established risk factors like time spent outdoors or reading.
Purpose:
An association between birth order and reduced unaided vision (a surrogate for myopia) has been observed previously. We examined the association between birth order and myopia directly in four subject groups.
Methods:
Subject groups were participants in (1) the Avon Longitudinal Study of Parents and Children (ALSPAC; UK; age 15 years; N = 4401), (2) the Singapore Cohort Study of Risk Factors for Myopia (SCORM; Singapore; age 13 years; N = 1959), (3) the Raine Eye Health Study (REHS; Australia; age 20 years; N = 1344), and (4) Israeli Defense Force Pre-recruitment Candidates (IDFC; Israel; age 16-22 years; N = 888,277). The main outcome was odds ratios (OR) for myopia in first-born versus non-first-born individuals after adjusting for potential risk factors.
Results:
The prevalence of myopia was numerically higher in first-born versus non-first-born individuals in all study groups, but the strength of evidence varied widely. Adjusted ORs (95% confidence intervals, CIs) were: ALSPAC, 1.31 (1.05-1.64); SCORM, 1.25 (0.89-1.77); REHS, 1.18 (0.90-1.55); and IDFC, 1.04 (1.03-1.06). In the large IDFC sample, the effect size was greater (a) for the first-born versus fourth- or higher-born comparison than for the first-born versus second/third-born comparison (p < 0.001) and (b) with increasing myopia severity (p < 0.001).
Conclusions:
Across all studies, the increased risk of myopia in first-born individuals was low (OR < 1.3). Indeed, only the studies with >4000 participants provided strong statistical support for the association. The available evidence suggested the relationship was independent of established risk factors such as time outdoors/reading, and thus may arise through a different causal mechanism.
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