Crossed eyes, known medically as strabismus, happen when the muscles or nerves that control eye movement don’t work together properly, causing one or both eyes to turn inward, outward, up, or down. The condition affects 2% to 5% of the general population, with different causes depending on whether it appears in childhood or develops later in life.
How Eye Alignment Works
Each eye is controlled by six small muscles that work in three pairs: one pair moves the eye left and right, another moves it up and down, and a third handles diagonal and rotational movement. These muscles receive signals from three separate nerves branching from the brainstem. When any of these muscles or nerves malfunction, the affected eye drifts out of alignment with the other.
The most common nerve involved in crossed eyes is the one that controls the muscle responsible for turning the eye outward (toward the ear). When this nerve is damaged or weakened, the eye turns inward because the opposing muscle pulls unopposed. Damage to the nerve that controls most of the other eye muscles can cause more widespread problems, including a drooping eyelid and changes in pupil size alongside the misalignment.
The Most Common Cause in Children
Farsightedness is the leading trigger for crossed eyes in young children. When a child is significantly farsighted, their eyes have to work extra hard to focus on nearby objects. That focusing effort is linked to a reflex that also causes the eyes to converge inward. In a child with mild farsightedness, this system works fine. But when the farsightedness is more severe, the extra focusing effort pulls one or both eyes too far inward, creating a visible crossing. This type, called accommodative esotropia, typically appears between ages 2 and 3 and can often be corrected with glasses that reduce the need for that excessive focusing effort.
Some children are born with crossed eyes or develop them in the first six months of life. Infantile esotropia affects roughly 1 in 100 to 500 children and accounts for about 8% of all inward-turning cases. The exact cause isn’t always clear, but it tends to involve problems with how the brain coordinates eye movement rather than a simple muscle or nerve issue.
Premature Birth and Early Risk Factors
Babies born prematurely face a significantly higher risk. In one study comparing full-term and preterm infants, only 2% of full-term children developed strabismus, compared to 12% of moderately preterm babies and 22% of those born at 28 weeks or earlier. For the most premature babies who also developed a condition affecting the blood vessels in the back of the eye, the rate climbed to 26%. Lower gestational age and farsightedness were the strongest independent risk factors, meaning each additional week of prematurity measurably increased the odds.
Genetics and Family History
Strabismus runs in families, though it doesn’t follow a simple one-gene inheritance pattern. If you have a first-degree relative (parent, sibling, or child) with crossed eyes, your risk is roughly 3 to 5 times higher than average. About 19% to 31% of people with strabismus have a close relative with the same condition, depending on the study. Identical twins share the condition 54% to 82% of the time, while fraternal twins share it 14% to 47% of the time. That gap suggests genetics plays a strong role, but other factors clearly matter too, since even identical twins don’t always match.
Neurological Conditions That Affect Eye Alignment
Children and adults with certain neurological conditions develop crossed eyes at much higher rates. Cerebral palsy is one of the most studied examples. Researchers have proposed three possible explanations: spasticity in the eye muscles, damage to the brain’s motor circuits, or damage to the brain’s visual pathways. The eye muscle spasticity theory has largely been ruled out, since the eye muscles in people with cerebral palsy don’t actually behave like spastic limb muscles. Imaging studies point instead to disrupted visual pathways as the primary cause, particularly the nerve fibers that carry visual information from the back of the brain to processing areas. Damage to these deeper pathways, more common in children born prematurely, tends to cause inward turning, while damage to the visual cortex itself is more associated with outward turning.
Down syndrome, traumatic brain injury, and stroke can also disrupt the brain circuits responsible for coordinating eye movement, leading to misalignment at any age.
What Causes Crossed Eyes in Adults
When strabismus appears for the first time in adulthood, it usually signals an underlying medical problem. The most common culprits are conditions that damage the small blood vessels supplying the nerves to the eye muscles. Diabetes is a particularly strong risk factor. One population-based study found that people with diabetes were nearly 6 times more likely to develop palsy of the nerve controlling outward eye movement. When diabetes and high blood pressure occurred together, the risk jumped to more than 8 times the baseline.
Thyroid disease, specifically Graves’ disease, causes crossed eyes through a different mechanism. Rather than damaging nerves, the immune system triggers inflammation and swelling in the eye muscles themselves, physically restricting their movement. This is why people with Graves’ disease often have bulging eyes alongside their misalignment.
Other adult-onset causes include head trauma, complications from eye or facial surgery, tumors pressing on the nerves that control eye muscles, and stroke affecting the brainstem regions where those nerves originate. Outward-turning eyes become more common with age, which is why the overall prevalence of strabismus increases in people between 55 and 75.
Symptoms Beyond the Visible Misalignment
Crossed eyes cause more than a cosmetic difference. Adults and older children who develop strabismus suddenly almost always experience double vision, because the brain receives two conflicting images it can’t merge. Depth perception drops in all forms of the condition, making tasks like catching a ball, pouring liquid, or judging distances while driving more difficult. Some people unconsciously tilt or turn their head to compensate for the misalignment, which can lead to chronic neck discomfort over time.
Young children, however, rarely complain of double vision. Their brains adapt by suppressing the image from the misaligned eye, which prevents double vision but creates a different problem: amblyopia, commonly called lazy eye. The suppressed eye gradually loses visual sharpness because the brain stops developing the neural connections needed to process its input.
Why Early Treatment Matters
In children, untreated strabismus leads to worsening vision in the misaligned eye. A study tracking preschool-aged children who didn’t follow through with prescribed treatment found that none showed spontaneous improvement over the following year. In fact, 41% experienced further deterioration in the weaker eye, and three children who hadn’t previously had amblyopia developed it during that time. The brain’s visual system is most adaptable during early childhood, which means both the window for damage and the window for effective treatment are relatively short.
How Crossed Eyes Are Treated
Treatment depends on the cause and the person’s age. For children with farsightedness-driven crossing, prescription glasses alone can bring the eyes back into alignment by reducing the excessive focusing effort. Patching the stronger eye forces the brain to use the weaker one, helping reverse or prevent amblyopia.
When glasses and patching aren’t enough, surgery to adjust the tension in the eye muscles is the next step. The procedure involves loosening muscles that pull too hard or tightening those that are too weak. Long-term studies show success rates around 69% to 71% for moderate misalignment, with similar outcomes whether surgeons adjust one muscle or two. Some people need more than one surgery to achieve stable alignment.
For adults whose crossed eyes stem from a medical condition like diabetes or thyroid disease, treating the underlying cause sometimes resolves the misalignment on its own. Nerve palsies from blood vessel damage, for instance, often improve over several months as the nerve heals. Prism lenses, which bend light to compensate for the eye’s position, can relieve double vision during recovery or serve as a long-term solution when surgery isn’t appropriate.