The term “cross eye,” or strabismus, refers to a condition where the eyes do not align with each other simultaneously when looking at an object. This misalignment means one eye focuses on the target while the other turns in a different direction. Strabismus is one of the most common eye conditions in children, affecting approximately 2 to 4 percent of the population. This condition often runs in families, suggesting a strong genetic influence.
Understanding Strabismus and Its Forms
Strabismus is fundamentally a problem with the neuromuscular control of eye movement, involving either the six muscles surrounding the eye or the brain centers that coordinate them. Under normal circumstances, these muscles work in perfect synchrony to ensure both eyes point in the same direction, but strabismus disrupts this alignment.
The condition is categorized by the direction of the eye turn. When an eye turns inward, toward the nose, it is called esotropia, which is the most common type and what is often described as “crossed eyes.” Conversely, an eye that turns outward, away from the nose, is known as exotropia. Vertical misalignments also occur, with an eye that is higher than the other called hypertropia, and one that is lower called hypotropia.
This misalignment has significant consequences for vision development, especially in children. When the eyes are not aligned, the brain receives two different images, which can cause double vision. To correct this, a child’s brain may learn to ignore the image from the misaligned eye, a process called suppression. If this suppression continues, it can lead to a permanent reduction in vision in the turned eye, a condition known as amblyopia, or “lazy eye.”
The Role of Genetics in Strabismus
Research confirms that strabismus has a substantial hereditary component, with family studies noting its tendency to cluster in relatives. The condition follows a pattern of multifactorial inheritance, meaning it results from a complex interaction between multiple genetic predispositions and various environmental factors. No single gene is responsible for all cases of strabismus; rather, it is influenced by a combination of many genes, each contributing a small amount to the overall risk.
The risk of developing strabismus is significantly increased for individuals who have a first-degree relative—a parent or sibling—with the condition. The risk is three to five times higher than that of the general population. Studies have shown that approximately 30 percent of children with strabismus have another family member affected by the same problem.
Genetic factors influence the condition by altering the structure or function of the eye muscles or the neurological pathways that control them. For instance, certain genes may affect the development of eye movement control centers in the brain or the nerves that innervate the six extraocular muscles.
The genetic link is particularly pronounced in certain types, such as accommodative esotropia. This form is often associated with significant farsightedness, which causes the eyes to turn inward as the child attempts to focus. In families where accommodative esotropia is present, the prevalence of strabismus among first-degree relatives can be as high as 26 percent. This suggests an inherited tendency for both the refractive error and the visual system’s response to it.
Non-Inherited Causes of Strabismus
While genetics plays a large role in many cases, especially those that present in early childhood, strabismus can also be acquired later in life due to factors unrelated to heredity. These non-inherited causes primarily involve damage to the nerves or muscles responsible for eye movement or to the brain’s control centers.
Physical trauma, such as a head injury, can directly damage the nerves that control the eye muscles, leading to paralytic strabismus. Similarly, underlying medical conditions can trigger the onset of eye misalignment. Neurological problems like cerebral palsy, hydrocephalus, or a stroke—the leading cause of strabismus in adults—can disrupt the delicate coordination required for binocular vision.
Strabismus can also develop secondary to other eye problems, such as severe vision loss from a cataract. The lack of clear visual input causes the affected eye to drift out of alignment. Non-genetic risk factors also include developmental issues like low birth weight and premature birth.