Causes and Risk Factors of Strabismus
Strabismus refers to a misalignment of the eyes in which both eyes are not directed at the same object. One eye deviates from the normal direction of gaze, which disrupts binocular vision. Strabismus may be present at birth or develop over the course of life, in both children and adults.
Key facts
- Frequency and occurrence: Strabismus is relatively common—about 2–6% of the population are affected; among children the figure is around 6%. In more than half of cases, strabismus appears before the age of 3.
- Congenital predisposition: Genetic factors play a major role. Strabismus often runs in families: around 20–30% of affected individuals have relatives with strabismus. A child’s risk is about 3 to 5 times higher if a parent or sibling has strabismus.
- Refractive error as a cause: Untreated farsightedness (hyperopia) or differing refractive power between the two eyes are among the most common causes of acquired strabismus in childhood.
- Other risk factors: Premature infants have a markedly increased risk—about one in five children with a birth weight under 1250 g later develops strabismus.
- Strabismus in adulthood: If strabismus appears suddenly in adulthood, it usually presents with double vision. In such cases, potentially serious causes must always be considered—for example, paralysis of an eye muscle due to cranial nerve palsy.
Congenital and early-childhood causes of strabismus
Congenital or early-onset strabismus (e.g., infantile esotropia/strabismus syndrome) typically begins in the first months of life. In these cases, the exact cause often remains unclear. In most instances there is no visible disease of the eyes themselves; instead, an inborn neurological predisposition is assumed, which makes coordination of the two eyes more difficult.
The eye muscles and nerves function normally, but control by the brain is immature or disrupted. As a result, an infant’s visual axes cannot be kept steadily aligned in parallel.
Genetic predisposition
An important early-childhood risk factor is genetic predisposition. A positive family history is common: if close relatives have strabismus, the likelihood increases that a baby will also begin to squint early. About 20% of strabismus patients have a parent or sibling who also has strabismus—significantly more than in the general population. Today, strabismus is considered a complex inherited condition: multiple genes in combination with environmental factors likely influence the regulation of eye alignment.
Premature birth as a risk factor
There are also circumstances in babies that are associated with increased risk. Prematurity is a major factor. Among premature infants with very low birth weight (< 1.25 kg), about one in five develops strabismus over time. Birth complications such as temporary oxygen deprivation in the newborn are also considered a risk factor for early-childhood strabismus.
Congenital eye problems
Lastly, congenital eye problems themselves can lead to strabismus (secondary strabismus). For example, a child with congenital cataract (lens opacity) or a retinal malformation may start to squint with one eye because that eye has poor vision from the outset.
The brain then suppresses the blurred input from that eye, causing it to deviate inward or outward.
In very rare cases, congenital malformations of the eye muscles or nerves are the cause—certain syndromes such as Duane syndrome (abnormal innervation of an eye muscle), for example, lead to limitations of eye movements. However, such cases account for less than 5% of all strabismus disorders.
Acquired causes of strabismus in childhood
Not every child is born with strabismus. Acquired forms in toddlers and school-age children often develop only after the first year of life—sometimes not until preschool age. Here, visual/refractive errors are the main causes. Particularly common is strabismus due to unrecognized or untreated farsightedness. The child’s eye tries to compensate for hyperopia through accommodative focusing. This constant effort is associated with a strong inward turning of the eyes—if this exceeds the child’s ability to compensate, manifest esotropia develops. With early spectacle correction (correcting hyperopia), this accommodative strabismus can often be completely prevented or resolved.
Unequal refractive power as a cause
Unequal refractive power in the two eyes (anisometropia) during childhood can also cause strabismus. If, for example, one eye is significantly more farsighted than the other, the better-seeing eye “takes over” fixation. The weaker eye is suppressed by the brain and gradually deviates. In such cases, amblyopia (reduced vision) of the unused eye often develops as well, because it cannot provide sharp visual input. It is therefore important to correct differing refractive errors early with glasses or contact lenses to enable equal visual input from both eyes.
Organic eye diseases
In addition to refractive errors, organic eye diseases in childhood can trigger strabismus (secondary strabismus). Examples include lens opacities or corneal scars after injuries. These impair vision in the affected eye, so the child effectively “switches it off.” A rare but important example is retinoblastoma (a malignant retinal tumor) in childhood, which can sometimes first become noticeable through sudden esotropia. For this reason, any newly occurring strabismus in a child should be thoroughly examined by an ophthalmologist to rule out serious causes.
The nervous system as a cause
The nervous system can also play a role. Eye muscle paralysis in children (paralytic strabismus) is less common than in adults, but it does occur. Causes can include inflammation of the brain or meninges.
It is known, for example, that measles viruses can rarely affect the brain and damage cranial nerves.
Severe accidents with traumatic brain injury can also lead to eye muscle palsies in children. In such cases, strabismus often appears suddenly, associated with double vision and a tilted head posture (the child tries to compensate for the visual disturbance).
Latent strabismus
Finally, a previously latent (hidden) alignment problem can be triggered by external stress. Children with a predisposition may show it only when they are physically weakened—for example, after severe infections with high fever or during phases of intense growth-related stress.
Emotional stress or crises are also discussed as potential triggers. In such moments, it becomes harder for the visual system to compensate for small deviations, and previously controlled strabismus becomes visibly apparent (normosensory late-onset strabismus).
Causes in adulthood
Although strabismus is often viewed as a childhood problem, adults can also develop eye misalignment for the first time. When new-onset strabismus occurs in adulthood, those affected almost always notice double vision immediately, because the adult brain can no longer simply suppress the input from the deviating eye.
Sudden-onset strabismus in adults is an alarm signal and should be taken seriously. Often, it is due to paralysis of an eye muscle caused by failure of the supplying nerves (e.g., abducens nerve palsy with newly occurring esotropia). Such palsies frequently arise from circulatory disturbances in the small arteries supplying the cranial nerves. This occurs more often in people with diabetes mellitus or high blood pressure.
Other serious conditions may also underlie the problem, such as strokes, brain tumors, or—rarely—an aneurysm (vascular outpouching) in the brain.
Therefore: if an adult with no known history suddenly develops strabismus, prompt and careful evaluation is essential (including a neurological examination and imaging such as MRI).
Recurrence of strabismus
Another situation is recurrence of earlier strabismus. Many adults were successfully treated for strabismus in childhood (e.g., with strabismus surgery) and had straight eyes for years. Nevertheless, a relapse can occur decades later.
The reason is that the underlying predisposition—a neurological weakness of coordination—remains lifelong, even if the eye muscles were surgically adjusted. Over time, the effect of the earlier surgery can diminish, especially if the demands on binocular vision increase or refractive power changes.
The brain then can no longer compensate for the misalignment, and the strabismus becomes visible again.
It is important to know that a single strabismus surgery does not always provide a permanent “cure,” because while it corrects eye position, it cannot change the neurological predisposition.
Nevertheless, recurrences can usually be treated well: often a repeat (or even third) eye muscle operation is performed to realign the eyes. Such procedures are relatively low-risk and can still be successful in adulthood.
Current research and research gaps
Myopia management in children and adolescents
Worldwide, nearsightedness (myopia) is increasing among children and adolescents. While myopia itself rarely directly causes strabismus (in contrast to hyperopia), extreme differences between the eyes or very high myopia can disrupt binocular balance.
Above all, high myopia carries other risks for eye health. Therefore, intensive research is focused on how to slow myopia progression during youth.
Innovative approaches include special spectacle lenses with Defocus Incorporated Multiple Segments (DIMS) as well as low-dose atropine eye drops. Both methods have shown a significant slowing of myopia progression in studies.
DIMS spectacle lenses (e.g., MiYOSMART) deliberately project defocused areas onto the retina to inhibit elongation of the eyeball. Atropine at very low concentrations (0.02–0.05%) relaxes the lens and also slows myopia development—without notable side effects.
These myopia control measures are already used in many eye practices. If they help prevent extreme refractive errors, they could indirectly reduce the occurrence of strabismus, especially forms promoted by large refractive differences.
Digital therapies for amblyopia and binocular deficits
A classic consequence and accompanying feature of childhood strabismus is amblyopia (reduced vision) in one eye. Traditionally, it is treated with patching (occlusion): the good eye is covered for certain hours to force the weaker eye to see. This method is effective, but children often accept it reluctantly.
This is where a newer research direction comes in: therapeutic computer games and apps that enable playful training of the weaker eye. Using special binocular tablet games or VR applications, children can, for example, play a game or watch a movie while each eye is shown a slightly different image.
This forces the amblyopic (weaker) eye to participate in order to succeed in the game, training it subconsciously.
Such “dichoptic” amblyopia therapies (i.e., dual-channel therapies for both eyes) are still in their early stages and usually do not replace occlusion, but they represent an additional new treatment approach. Independent studies suggest that effectiveness is not better than occlusion therapy, but in some cases adherence (cooperation) is better because children accept “playing on a tablet” more readily than the visible and often disruptive patching of an eye. Since dichoptic therapy is not more effective than occlusion therapy and involves significantly higher costs, it is currently not covered by health insurers in Switzerland.
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Do you have further questions about strabismus and possible treatments, or would you like to schedule a consultation? Our team at FIRST SIGHT, directly at Zurich Main Station, will be happy to assist you.
Further Information
Strabismus Surgery in Children: Everything About Pediatric Strabismus Operations
Conservative Treatment: Non-Surgical Treatment for Strabismus
After the Strabismus Operation: Recovery Process, Complaints & Follow-up
