(Photo from “Handbuch der Augenheilkunde” (1883) via Flickr)
Strabismus is a condition that occurs when the eyes are not properly aligned with each other. It is often referred to as “crossed eye” or “turned eye.” One eye will appear to be either turned in or turned out. When the eye is turned in it is an esotropia; when it is turned out it is an exotropia. The eye can be turned all the time if it is constant, or occur some of the time if it is intermittent. Strabismus can result in permanent vision loss if amblyopia (lazy eye) develops. When a child has strabismus with amblyopia, they have a significantly higher risk of becoming blind by losing vision in the non-amblyopic eye from trauma or disease. When strabismus develops in young children, early identification and treatment may prevent amblyopia.
Alignment of the eyes is necessary for normal binocular vision, because it allows the two eyes to work together properly. This is needed for three-dimensional depth perception. Strabismus interferes with perceiving three-dimensional depth. Studies that assessed performance on a variety of tasks indicate that strabismus frequently leads to inefficient performance for educational, occupational, and avocational tasks. Someone who has an intermittent strabismus, may experience fatigue, eyestrain, headaches and double vision. When this is associated with near work it can interfere with reading comfortably and may cause the person to avoid reading. For a student, this can result in poor academic achievement. For an employee it can result in reduced productivity.
Strabismus may also be cosmetically concerning and can have a significant psychological impact. It is not uncommon for this to manifest as low self-esteem, and it can also have an adverse effect on family relationships. When the eyes aren’t properly aligned it becomes especially relevant socially when eye contact is necessary. This is especially important for adults who are employed in positions that involve substantial personal eye contact. Strabismus can also be concerning for infant development. Infants can be delayed in reaching developmental milestones such as walking and using single words. Young children with strabismus have also been found to have difficulty with tasks involving visual perception.
Strabismus has different causes, which can impact whether the eye is turned in or out and to what degree. It will also have a baring on when an eye turn will typically manifest. When an infant manifests a strabismus it is usually an esotropia. Infantile esotropia usually starts by 6 months of age, and it is usually a large constant turn. Infantile esotropia can involve one eye, or it can alternately affect each eye. Often a cross-fixation pattern can develop, in which objects off to the right side are viewed by the left eye and vice versa. This severely interferes with the development of normal binocular vision. Esotropia can be acquired after 6 months of age. In this case, the person usually had the benefit of binocular vision experience prior to the onset of the strabismus.
(From Cotter SA, Varma R, Tarczy-Hornoch K, McKean-Cowdin R, Lin J,et al. and The Joint Writing Committee for the Multi-Ethnic Pediatric Eye Disease Study and the Baltimore Pediatric Eye Disease Study Groups. Risk factors associated with childhood strabismus. Ophthalmology 2011;118:2251–2261.)
One of the most common types of acquired esotropia is known as accommodative esotropia. This type of strabismus is associated with the activation of accommodation, which is the focusing effort the eye exerts to make an image appear clear if the eye is farsighted or when looking close up. The eye turns inward in a response to the eye focusing. This tends to occur when someone has more farsightedness than usual. It also tends to occur when someone is using additional accommodative focusing effort to see close up. Often it is first noticed in children age 2 to 3 years old. Parents may notice the child’s eye turns in especially when they’re reading, doing near activities, or focusing intently. Children suspected of having accommodative esotropia should be examined immediately. There is an excellent chance they will achieve normal binocular vision when management and intervention is prompt. Half of all children with esotropia are expected to achieve either normal eye alignment or significantly reduced esotropia when wearing glasses for farsightedness.
An outward eye turn can also begin during the first 6 months of life. This is called an infantile exotropia, and tends to be more rare. A constant exotropia in infancy may be associated with neurological syndromes or defects, craniofacial syndromes, and structural abnormalities in the eye. An exotropia occurring after 6 months of age is considered to be acquired rather than infantile. Exotropias can manifest in one eye or alternate between both eyes, and they can also be there all the time (constant) or only sometimes (intermittent).
Most exotropias are intermittent. They typically develop in children between the ages of 1 and 4 years. In the United States, it is found in approximately 1 percent of children by the age of 7 years. Someone with intermittent exotropia can have variable vision. Sometimes they may see double images, or they may occasionally shut down the vision from an eye (suppress the eye), and at other times they may have normal binocular alignment and depth perception. The level of control a person has over their eye turn varies throughout the day and also between individuals.
There are different types of exotropia also based on whether the amount of the eye turn is greater when looking far or close. In basic exotropia, the eye turn is roughly the same amount when looking far or close. In the convergence-insufficiency type, the eye turns out farther when looking up close. Divergence-excess type exotropia occurs when the eye turns out more when looking far away. Basic intermittent exotropia is the most common type of intermittent exotropia, accounting for approximately 50 percent of all cases. Convergence insufficiency and divergence excess make up the balance of cases in approximately equal proportions. In younger children, the divergence-excess type is more common. In this case, parents may notice that the child is not looking at them properly or tends to close one eye when viewing at distance. It may only be apparent when the child is fatigued or inattentive.
Unlike other types of strabismus, if treatment of intermittent exotropia is delayed in young children, it is not likely to result in worsening the visual status. However, examination is still essential in the child, to rule out the types of exotropia that are likely to cause detrimental loss of normal binocular vision.
Strabismus can also result from a sensory deprivation in one eye that interferes with the brain’s ability to fuse the images of each eye. This is usually caused by severe vision impairment in one eye such as congenital cataract, or extensive retinal disease. It is critical that infants suspected of eye disease inducing strabismus be examined and treated accordingly to preserve as much sight as possible. Sensory exotropia and sensory esotropia are equally common in children under age 5; however, sensory exotropia predominates in persons older than 5 years. A strabismus that is acquired later in life, either an esotropia or an exotropia should be examined immediately due to the possibility of an underlying disease process that could be serious. Strabismus of this nature can be caused factors such as systemic disease, or neurological pathology.
Strabismus is estimated to affect 2 to 5 percent of the general population. There is also an increased prevalence in families such that 23 to 70 percent of family members are affected when a parent or sibling has strabismus. This indicates a genetic component to strabismus, although it’s not known whether the strabismus is inherited, or the conditions underlying the strabismus are inherited. Nevertheless, the evidence suggests that siblings of a strabismic child should be examined at an early age to rule out the possibility of strabismus.
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Featured image from 1916 publication, The Eyes of Our Children via Flickr