Bilateral exotropia is a form of strabismus where both eyes turn outward, drifting away from the nose. This misalignment can affect individuals of any age, though it is frequently identified during childhood. It represents a divergent deviation of the visual axes, differing from other forms of strabismus where eyes might turn inward or vertically.
Identifying the Signs
Observing the outward drift of one or both eyes is often the first indication of bilateral exotropia. This deviation can be intermittent, appearing when an individual is tired, unwell, or focusing on distant objects, or it can be constant. Children may frequently squint one eye in bright sunlight or rub their eyes, which can be a compensatory behavior for the misalignment. Adults might experience symptoms like double vision (diplopia), eye strain, headaches, or difficulty with depth perception. Some individuals may instinctively close one eye to alleviate visual confusion caused by the misaligned images.
Underlying Causes and Risk Factors
The primary issue in exotropia often stems from the brain’s inability to effectively coordinate the eye muscles, rather than a problem with the muscles themselves, leading to an imbalance in eye movement control. Genetic predisposition plays a role, with a family history of strabismus increasing the likelihood. Uncorrected refractive errors, such as nearsightedness or farsightedness, can also contribute. Certain neurological or developmental conditions, including cerebral palsy, Down syndrome, hydrocephalus, or brain tumors, are associated with a higher risk. Additionally, poor vision in one eye, perhaps due to conditions like cataracts, can cause that eye to drift outward.
The Diagnostic Process
A comprehensive eye examination performed by an optometrist or ophthalmologist confirms a diagnosis of bilateral exotropia and includes visual acuity tests to assess the sharpness of vision in each eye. Key diagnostic tools include the cover-uncover test, which helps differentiate between a constant misalignment (tropia) and a latent deviation (phoria) by observing eye movement when one eye is briefly covered. The alternate cover test is then performed to reveal the full extent of the deviation. A cycloplegic refraction is also performed, where eye drops temporarily relax focusing muscles for an accurate measurement of the full refractive error. This detailed examination provides the necessary information to determine the type and magnitude of the exotropia and to formulate an appropriate treatment strategy.
Corrective Treatment Approaches
Corrective treatment for bilateral exotropia aims to improve eye alignment, enhance binocular vision, and prevent long-term complications like amblyopia, also known as “lazy eye,” or permanent vision loss. Non-surgical options are often considered first, especially if a refractive error is present. Corrective lenses, such as eyeglasses or contact lenses, can help align the eyes by reducing the effort needed for focusing, particularly in cases linked to farsightedness or astigmatism. Prism lenses can also be incorporated into eyeglasses to bend light and reduce double vision, though they do not correct the underlying eye movement problem.
Vision Therapy
Vision therapy, also referred to as orthoptics, involves a series of eye exercises designed to train the eyes and brain to work together more effectively, including “pencil push-ups” for convergence, fusion exercises, and base-out prism exercises. This therapy is particularly effective for intermittent exotropia, aiming to increase control over the eye deviation rather than solely minimizing its size. For cases where amblyopia develops due to one eye being consistently misaligned, patching therapy or atropine eye drops may be used. Patching involves covering the stronger eye to encourage the weaker, amblyopic eye to develop better vision. Atropine eye drops can achieve a similar effect by temporarily blurring vision in the stronger eye.
Surgical Intervention
Surgical intervention is recommended for larger or constant deviations that do not respond sufficiently to non-surgical treatments. The procedure involves adjusting the tension of the eye muscles to improve alignment. This involves weakening muscles that pull eyes outward (lateral rectus) or strengthening muscles that pull eyes inward (medial rectus), often on one or both eyes. The goal of surgical treatment is to achieve better ocular alignment, restore or preserve binocular vision, and improve the cosmetic appearance of the eyes.