Strabismus, commonly known as being cross-eyed, is a condition where a person’s eyes do not align properly. This misalignment prevents both eyes from focusing on the same object simultaneously. Understanding the causes of strabismus, especially the role of genetics, is important for those seeking information about this eye condition.
The Basics of Strabismus
Strabismus is a misalignment of the eyes. One eye may deviate while the other remains focused straight ahead. This deviation can manifest as an inward turn (esotropia), an outward turn (exotropia), an upward turn (hypertropia), or a downward turn (hypotropia). The condition can be constant or intermittent, appearing sometimes, such as when a person is tired or ill.
Six muscles surround each eye, working in coordination to control its movement and ensure proper alignment. In individuals with strabismus, these muscles do not work together effectively, or there is a problem with the nerves or brain centers that control them. This can lead to the brain receiving two different images, which it may learn to ignore from the misaligned eye, potentially causing reduced vision in that eye over time.
Is Strabismus Inherited?
Strabismus often has a genetic component and can run in families. While the inheritance pattern is complex, a family history of strabismus significantly increases an individual’s likelihood of developing the condition. Studies show that approximately 30% to 55% of individuals with strabismus have a close relative who also has the condition.
Twin studies further support a strong genetic influence, particularly for inward-turning strabismus (esotropia). Identical (monozygotic) twins, who share nearly identical genetic material, show a higher rate of both developing strabismus compared to fraternal (dizygotic) twins, who share about half their genes.
How Genes Influence Strabismus
The genetic influence on strabismus is complex, involving multiple genes rather than a single gene. Researchers are still working to define the precise genetic mechanisms involved, as different forms of strabismus may have distinct genetic underpinnings. These genes can affect various aspects of eye function, including the development and control of eye muscles, the nerves that transmit signals to these muscles, or the brain’s ability to coordinate eye movements.
For example, mutations or variations in certain genes can lead to improper positioning of the eyes. One gene, WRB on chromosome 21, has been identified as potentially playing a role in esotropia, with a higher likelihood of inheritance from the father. This connection is notable because individuals with Down syndrome (Trisomy 21) have a higher incidence of strabismus. While genetics increase an individual’s susceptibility, having a genetic predisposition does not guarantee the condition will develop, as other factors also contribute.
Non-Genetic Factors in Strabismus Development
While genetics play a role, strabismus can also arise from factors that are not directly inherited. Certain medical conditions significantly increase the risk of developing strabismus. These include neurological disorders such as cerebral palsy, hydrocephalus (fluid buildup in the brain), and brain tumors, which can disrupt the brain’s control over eye movements.
Other health issues, like uncorrected farsightedness (hyperopia), can lead to a type of strabismus called accommodative esotropia, where the eyes turn inward due to excessive focusing effort. Conditions present at birth or acquired later, such as cataracts, retinopathy of prematurity, or significant vision differences between the eyes, can contribute to misalignment. Premature birth and low birth weight are also recognized as risk factors for childhood strabismus.
Head injuries or stroke can cause strabismus by damaging the nerves that control eye muscles or the brain areas responsible for eye coordination. Maternal smoking during pregnancy has also been linked to an increased risk of strabismus in children. These varied non-genetic factors show that strabismus can result from a combination of influences.