Strabismus (crossed eyes) is a condition in which the eyes do not line up with one another. In other words, one eye is turned in a direction that is different from the other eye.
Eye misalignment, or strabismus, results in a departure from the typical gaze’s parallelism. A cover test and observation of the corneal light reflex are used in the clinical diagnosis. Treatment options include surgical correction, alignment with corrective lenses, and patching for vision impairment.
Several varieties of strabismus have been described, based on direction of deviation, specific conditions under which deviation occurs, and whether deviation is constant or intermittent. Description of these varieties requires the definition of several terms:
- Eso: Nasal deviation
- Exo: Temporal deviation
- Hyper: Upward deviation
- Hypo: Downward deviation
A tropia is a manifest deviation, detectable with both eyes open (so that vision is binocular). A tropia can be constant or intermittent and may involve one eye or both eyes.
A phoria is a latent deviation, detectable only when one eye is covered so that vision is monocular. The deviation in a phoria is latent because the brain, using the extraocular muscles, corrects the minor misalignment.
A comitant deviation has the same amplitude or degree of misalignment in all gaze directions.
An incomitant deviation varies in amplitude or degree of misalignment depending on gaze direction.
Infantile esotropia is a different kind of strabismus. Infants with this syndrome have significant inward bending of both eyes, which often begins before the age of six months. Typically, there isn’t much farsightedness, and wearing glasses won’t make the crossing go away. The tendency to turn inward may begin sporadically but eventually takes on a consistent character. Both when the youngster is gazing far away and up close, it is there. Surgery to realign the muscles of one or both eyes is the therapy for this kind of strabismus.
Strabismus can also affect adults. Ocular misalignment in adults is most frequently caused by stroke, although it can also be brought on by physical trauma or untreated childhood strabismus that has returned or worsened. Adults with strabismus may benefit from a range of treatments, including prism glasses, patching, monitoring, and/or surgery.
The majority of strabismus is caused by abnormalities in the neuromuscular system that govern eye movement. These brain’s control areas are currently becoming better understood. Less frequently, there is an issue with the eye muscle itself. Around 30% of children with strabismus have a family member who also has the condition, proving that strabismus is a common genetic condition.
Other conditions associated with strabismus include:
- Uncorrected refractive errors
- Poor vision in one eye
- Cerebral palsy
- Down syndrome (20-60% of these patients are affected)
- Hydrocephalus (a congenital disease that results in a buildup of fluid in the brain)
- Brain tumors
- Stroke (the leading cause of strabismus in adults)
- Head injuries, which can damage the area of the brain responsible for control of eye movement, the nerves that control eye movement, and the eye muscles
- Neurological (nervous system) problems
- Graves’ disease (overproduction of thyroid hormone)
An infant’s eyes should be able to focus on small objects and be straight and well-aligned by the time they are 3 to 4 months old. A baby that is six months old should be able to focus on both close-up and distant things.
Infants and young children are most commonly affected by strabismus, which typically manifests by the time a kid is 3 years old. Yet, strabismus can also occur in older kids and even adults. In an older child or adult, the rapid onset of strabismus, particularly with double vision, may be a sign of a more severe neurologic condition. Make a quick call to your doctor if this occurs.
A disorder known as pseudostrabismus, sometimes known as false strabismus, can make it look as though a baby’s eyes are crossed when they are actually pointing in the same direction. A flat nasal bridge and/or excess skin covering the inner corners of the eyes can also contribute to pseudostrabismus. The eyes will no longer seem crossed as the baby’s face matures and expands.
At well-child visits, strabismus can be found through a history and eye exam. If family or caregivers have noted a deviation in gaze, questions regarding when it started, when or how often it is present, and whether there is a preference for utilizing one eye for fixation should be asked as part of the evaluation. The scope of extraocular movements, pupil responsiveness, and visual acuity should all be evaluated during a physical exam. Slit-lamp examination is used to look for cataracts, while funduscopy is used to look for structural flaws or the pathology of diseases like retinoblastoma. It is crucial to have a neurologic evaluation, especially of the cranial nerves.
The corneal light reflex test is a good screening test, but it is not very sensitive for detecting small deviations. The child looks at a light and the light reflection (reflex) from the pupil is observed; normally, the reflex appears symmetric (ie, in the same location on each pupil). The light reflex for an exotropic eye is nasal to the pupillary center, whereas the reflex for an esotropic eye is temporal to the pupillary center. Vision screening devices such as photoscreeners may be used by trained lay personnel to identify children at risk.
When doing the cover test, the child is asked to fixate on an object. One eye is then covered while the other is observed for movement. No movement should be detected if the eyes are properly aligned, but manifest strabismus is present if the uncovered eye shifts to establish fixation once the other eye, which had fixed on the object, is covered. The test is then repeated on the other eye.
In a variation of the cover test, called the alternate uncover test, the child is asked to fixate on an object while the examiner alternately covers one eye and then the other, back and forth. An eye with a latent strabismus shifts position when it is uncovered. In exotropia, the eye that was covered turns in to fixate when the cover is removed; in esotropia, it turns out to fixate when the cover is removed. Deviations can be quantified by using prisms positioned such that the deviating eye does not need to move to fixate. The power of the prism is used to quantify the deviation and provide a measurement of the magnitude of misalignment of the visual axes. The unit of measurement used by ophthalmologists is the prism diopter. One prism diopter is a deviation of the visual axes of 1 cm at 1 m.
Pseudostrabismus, which appears as esotropia in a kid with adequate visual acuity in both eyes but a wide nasal bridge or extensive epicanthal folds that cover most of the white sclera nasally while looking laterally, should be separated from strabismus. In a kid with pseudostrabismus, the light reflex and cover tests are both normal.
Finding the root of acquired cranial nerve palsies may require neuroimaging. Moreover, a genetic analysis may be helpful for some ocular abnormalities.
Treatment options include the following:
- Eyeglasses or contact lenses: Used in patients with uncorrected refractive errors. With corrective lenses, the eyes will need less focusing effort and may remain straight.
- Prism lenses: Special lenses that can bend light entering the eye and help reduce the amount of turning the eye must do to look at objects.
- Orthoptics (eye exercises): May work on some types of strabismus, especially convergence insufficiency (a form of exotropia).
- Medications: Eye drops or ointments. Also, injections of botulinum toxin type A (such as Botox) can weaken an overactive eye muscle. These treatments may be used with, or in place of, surgery, depending on the patient’s situation.
- Patching: To treat amblyopia (lazy eye), if the patient has it at the same time as strabismus. The improvement of vision may also improve control of eye misalignment.
- Eye muscle surgery: Surgery changes the length or position of eye muscles so that the eyes are aligned correctly. This is performed under general anesthesia with dissolvable stitches. Sometimes adults are offered adjustable strabismus surgery, where the eye muscle positions are adjusted after surgery.
What can happen if strabismus is not treated?
Some believe that children will outgrow strabismus or that it will get better on its own. In truth, it can get worse if it is not treated.
If the eyes are not properly aligned, the following may result:
- Lazy eye (amblyopia) or permanent poor vision in the turned eye. When the eyes are looking in different directions, the brain receives two images. To avoid double vision, the brain may ignore the image from the turned eye, resulting in poor vision development in that eye.
- Blurry vision, which can affect performance in school and at work, and enjoyment of hobbies and leisure activities
- Eye strain
- Double vision
- Poor 3-dimensional (3-D) vision
- Low self-esteem (from embarrassment about one’s appearance)
It is also possible that by not diagnosing strabismus, a serious problem (such as a brain tumor that is causing the condition) may be overlooked.