Myopia, commonly known as nearsightedness, is a vision condition where close-up objects appear clear, but distant objects look blurry. This occurs because the eye focuses light in front of the retina instead of directly on it, often due to an elongated eyeball or an overly curved cornea. The development of myopia is a complex process influenced by multiple factors.
Understanding Genetic Predisposition
Myopia frequently runs in families, indicating a significant genetic influence on its development. Research consistently shows that children with one or both parents who are myopic have a higher likelihood of developing the condition themselves. For instance, children with two myopic parents had a 47.5% prevalence of myopia, compared to 15.9% for those with one myopic parent and 7.3% for those with no myopic parents.
Twin studies provide further evidence, comparing identical (monozygotic) twins to fraternal (dizygotic) twins. Identical twins share nearly all their genes, while fraternal twins share about half. These studies reveal that identical twins exhibit significantly higher rates of concordance for myopia compared to fraternal twins, with heritability estimates for refractive error ranging from approximately 60% to 90%. This suggests genetic factors account for a substantial portion of myopia variation, even when twins grow up in similar environments.
Identified Genes and Inheritance
Myopia is a polygenic condition, meaning its development is influenced by the combined effects of multiple genes rather than a single one. Over the past two decades, researchers have identified over 400 genetic loci associated with myopia and refractive errors. These genetic markers are involved in functions including eye development, growth regulation, synaptic transmission, and extracellular matrix components.
Many identified genes are light-dependent and related to cell cycle and growth pathways, suggesting their involvement in how the eye grows and adapts to visual stimuli. While most genetic variants carry a low individual risk, they are prevalent in the general population, and their collective interaction can significantly increase susceptibility to myopia. For example, individuals with high myopia often inherit more of these myopia-predisposing genetic variants.
The Role of Environment
While genetic factors contribute to myopia, environmental influences also play a role in its onset and progression. Two key environmental factors are extended near work and insufficient time outdoors. Near work, such as reading, writing, or using electronic devices, requires the eyes to focus on close objects for prolonged durations. Spending more time on activities at less than 20 cm is associated with an increased risk of myopia.
Conversely, increased time outdoors has a protective effect against myopia, particularly preventing its onset. This benefit may be linked to natural light exposure, which is thought to trigger dopamine release, a neurotransmitter that can inhibit eyeball elongation. Spending at least two hours per day outdoors can significantly reduce incident myopia. These environmental factors can interact with genetic predispositions; individuals with genetic susceptibility may be more affected by prolonged near work or limited outdoor exposure.