Strabismus is a condition where the eyes do not align properly and point in different directions. This misalignment can involve one eye turning inward, outward, upward, or downward while the other remains focused. Commonly known as “crossed eyes” or “wall eyes,” strabismus means that both eyes do not look at the same object simultaneously. This condition affects the coordinated movement of the six muscles surrounding each eye, which normally work together to ensure both eyes focus on the same point.
Genetic Predisposition to Strabismus
Strabismus often has a significant genetic component, meaning it can run in families. Research consistently supports this familial clustering, with studies indicating that approximately 20% to 30% of individuals with strabismus have a close relative who also has the condition. This suggests a strong hereditary influence, though the precise genetic risk can be complex and influenced by various factors.
The likelihood of a child developing strabismus increases if a parent or sibling is affected. For instance, if one parent has a common form of childhood strabismus, their child is 3 to 5 times more likely to develop it compared to the general population. Twin studies further reinforce the genetic link, showing much higher concordance rates for strabismus in identical twins (54%–82%) compared to fraternal twins (14%–47%), highlighting the role of shared genetics. While a strong genetic predisposition exists, it is not a simple, single-gene inheritance pattern, but rather a complex interaction.
How Genes Influence Strabismus
Strabismus is typically considered a complex, multifactorial, or polygenic condition, meaning that multiple genes contribute to its development rather than a single gene. The combined variations in these numerous genes can influence various aspects of eye development and function. These genetic influences might affect the precise development of eye muscles, the intricate nerve control pathways that coordinate eye movements, or even the visual processing centers within the brain.
Specific genes are currently being investigated for their roles in strabismus. For example, research has identified genes that, when mutated, can disrupt the development of cranial nerves responsible for innervating eye muscles. The full genetic picture remains under investigation, but these findings suggest that genetic variations can lead to subtle or significant abnormalities in the neural networks and muscular structures responsible for proper eye alignment.
Non-Genetic Factors in Strabismus
While genetics play a significant role, other factors can also cause or contribute to strabismus, demonstrating that it is not exclusively inherited. Prematurity and low birth weight are recognized risk factors, with studies showing an increased likelihood of strabismus in infants born weighing less than 2000 grams or with lower gestational ages. For instance, the prevalence of strabismus in very preterm infants with low gestational age and low birth weight has been reported to be as high as 42%.
Certain medical conditions are also associated with a higher incidence of strabismus. Children with Down syndrome, for example, have an increased risk, with about one-third of affected children also having strabismus. Other neurological conditions like cerebral palsy can also contribute to eye misalignment. Additionally, significant uncorrected refractive errors, such as farsightedness (hyperopia), are known ocular risk factors for strabismus. Eye injuries or other health problems can also lead to strabismus later in life, and these non-genetic factors can interact with existing genetic predispositions to influence the overall risk and presentation of the condition.