Bilateral strabismus is a vision condition where both eyes fail to align properly, preventing them from working together when focusing on an object. The condition can be present from early infancy or develop later in life. While it is often referred to as “crossed eyes,” this term only describes one form of the condition.
Signs and Types of Bilateral Strabismus
This condition is categorized based on the direction of the eye turn. Bilateral esotropia occurs when both eyes turn inward, toward the nose, while bilateral exotropia is when both eyes turn outward, away from each other. Other, less common forms include hypertropia, where an eye turns upward, and hypotropia, where it turns downward.
Beyond the visible eye turn, other signs include frequently tilting the head to see more clearly, squinting often, or closing one eye to reduce visual confusion. Complaints of double vision (diplopia) and eye strain are also common. In many cases, the misalignment is alternating, meaning the person switches which eye they are using for fixation. This ability to alternate can protect against developing amblyopia, or “lazy eye,” because both eyes continue to receive visual input.
Underlying Causes of Misalignment
Bilateral strabismus stems from a problem with how the brain controls the eye muscles, not an issue with muscle strength. The condition can be congenital, appearing within the first six months of life, or acquired. A family history is a risk factor, as about 30% of children with strabismus have a family member with the condition.
A common cause in children is a significant, uncorrected refractive error like farsightedness (hyperopia), where the extra effort to focus leads to an inward turning known as accommodative esotropia. Certain medical conditions are also associated with a higher risk, including cerebral palsy, Down syndrome, and premature birth. In adults, strabismus can be triggered by a stroke, head injury, or conditions like Graves’ disease.
Diagnosis and Treatment Pathways
Diagnosing bilateral strabismus involves a comprehensive eye examination. This includes the cover-uncover test, where a doctor observes how each eye moves when the other is covered to identify the misalignment. The Hirschberg test, which observes light reflection from the pupils, is another method used to check for proper eye alignment.
For some, especially those with accommodative esotropia, corrective eyeglasses or contact lenses may be sufficient to straighten the eyes. Special lenses called prisms can also be incorporated into glasses. These bend the light entering the eye to help merge two images into one, reducing double vision.
Vision therapy is another non-surgical approach. This structured program of visual exercises is designed to improve eye coordination and strengthen the connection between the eyes and the brain.
If other treatments are not sufficient, eye muscle surgery may be recommended. This procedure adjusts the length or position of the eye muscles to improve alignment. Early diagnosis and intervention are important to prevent amblyopia, where the brain starts to ignore the input from the misaligned eye, leading to reduced vision in that eye.