A squint, known medically as strabismus, is a condition where the eyes don’t line up with each other. One eye looks directly at what you’re focusing on, while the other points in a different direction. It affects roughly 2 to 4% of children and can also develop in adults. While a squint is sometimes obvious to spot, it can also be subtle or come and go, making it easy to miss without a proper eye exam.
How the Eyes Normally Work Together
Your eyes sit slightly apart in your skull, so each one captures a slightly different image of the world. Your brain fuses those two images into a single three-dimensional picture, giving you depth perception. This is called binocular vision, and it’s what lets you judge distances accurately, especially for objects close to you.
Six small muscles attached to the outside of each eyeball control its movement. These muscles are directed by signals from several areas of the brain. A squint can develop when something goes wrong at any point in this chain: the muscles themselves, the nerves that supply them, or the brain regions that coordinate eye movement and process visual information.
Types of Squint
A squint is classified by which direction the misaligned eye turns:
- Inward (esotropia): the eye turns toward the nose, sometimes called being “cross-eyed.” This is the most common type in young children.
- Outward (exotropia): the eye drifts toward the ear, sometimes called a “wall eye.” This often starts as an intermittent problem that worsens over time.
- Upward (hypertropia): the eye points higher than the other.
- Downward (hypotropia): the eye points lower than the other.
Inward and outward squints are grouped as horizontal strabismus, while upward and downward are vertical strabismus. Some people have a combination. A squint can also be constant or intermittent, meaning it only appears some of the time, often when you’re tired, unwell, or focusing at a particular distance.
What Causes a Squint
There is no single cause. A squint has both genetic and environmental risk factors, and in many cases, especially in children, the exact trigger is never identified. About 31% of people with a squint have a close relative who also has one, a pattern first noted by Hippocrates, who observed that “children of parents having distorted eyes squint also for the most part.”
Farsightedness is a well-established risk factor. When a child has to work harder to focus, particularly on nearby objects, the extra effort can pull one eye inward. Correcting the vision with glasses sometimes resolves the squint entirely. A significant difference in prescription between the two eyes also raises the risk.
Non-eye-related risk factors include low birth weight, premature birth, maternal smoking during pregnancy, and neurological conditions. In adults, a squint can appear after a stroke, head injury, or conditions that damage the nerves supplying the eye muscles.
Symptoms and How It Affects Vision
The most visible sign is eyes that don’t appear to look in the same direction. But the effects go deeper than appearance. Because the two eyes are pointing at different things, the brain receives conflicting images. How it handles that conflict depends on the person’s age and how long the squint has been present.
Adults who develop a sudden squint typically see double. The image from the misaligned eye appears blurred and offset from the image produced by the straight eye. This can be disorienting and may cause nausea or difficulty with balance.
Children’s brains respond differently. Rather than tolerating double vision, a child’s brain will often suppress the image from the turned eye, effectively switching it off. This solves the double vision problem in the short term but creates a bigger one: the suppressed eye doesn’t develop normal visual connections. Over time, this leads to amblyopia, commonly called lazy eye, where vision in that eye becomes permanently weaker.
In all cases, depth perception is reduced. Judging distances becomes harder, which can affect activities like catching a ball, pouring liquid into a glass, or driving. Some people with a constant squint also tilt or turn their head into an unusual position to try to keep their eyes aligned, which can cause neck and shoulder discomfort.
Why Early Detection Matters
The brain’s visual pathways develop most rapidly in the first seven years of life, with the earliest months being especially critical. This is the window during which a squint is most likely to cause amblyopia, but it’s also when treatment is most effective. Children aged 7 to 12 still show some benefit from treatment. After age 13, improvement tends to be minimal.
This is why routine vision screening in young children is so important. A squint isn’t always obvious, particularly if it’s intermittent or the angle of misalignment is small. One simple screening method involves shining a small light toward both eyes: in a child with straight eyes, the light reflects from the center of each pupil, while in a child with a squint, the reflection sits off-center in one eye.
A more formal test, called the cover test, involves covering one eye and watching what happens to the other. If the uncovered eye shifts to pick up focus, that indicates a misalignment was present. Eye care professionals use variations of this test to measure the size and direction of the squint.
Treatment Options
Treatment depends on the type of squint, its severity, and the person’s age. The general approach follows a logical sequence: first correct any vision problems, then address amblyopia if present, and finally realign the eyes if they’re still misaligned.
Glasses
If a refractive error like farsightedness is contributing to the squint, corrective glasses are the first step. For some children with inward-turning eyes, the right prescription is enough to bring the eyes into alignment. Bifocal lenses, which have different strengths in the upper and lower portions, are sometimes used when the squint is worse during close-up work like reading.
Patching and Eye Exercises
When amblyopia has developed, the stronger eye is patched for set periods each day. This forces the brain to use the weaker eye, stimulating its visual pathways and gradually improving its acuity. The length of patching depends on the child’s age and how severe the amblyopia is, with follow-up exams typically scheduled every two to three months.
Orthoptic exercises, which involve training both eyes to focus on the same target simultaneously, can help improve coordination between the eyes. These exercises work on the brain’s ability to fuse images from both eyes and are most useful for intermittent squints where some degree of binocular vision is already present.
Surgery
When glasses and other non-surgical methods don’t fully correct the alignment, surgery on the eye muscles is the most effective option. The procedure adjusts the tension in one or more of the muscles that control eye movement, either tightening or loosening them to bring the eyes into line. It’s performed under general anesthetic in children and sometimes under local anesthetic in adults.
Success rates vary by the type of squint. For intermittent outward squints, studies report surgical success in 70 to 92% of patients, depending on the specific criteria used and the length of follow-up. Recurrence can happen, with some studies finding that up to 50% of cases of intermittent outward squint may recur within three years. This is one reason why ongoing monitoring matters. Children who are well-aligned after treatment and don’t have amblyopia are typically reviewed every 6 to 12 months.
Squint in Adults
Although squint is most commonly associated with childhood, adults develop it too. Causes range from poorly treated childhood squint that worsens over time, to new onset after a stroke, head trauma, thyroid eye disease, or nerve damage. Adults with a new squint almost always experience double vision, since their brains never learned to suppress one eye’s image the way a child’s brain does.
Treatment follows similar principles. Prism lenses, which bend light to compensate for the misalignment, can reduce or eliminate double vision without surgery. These are often applied as temporary stick-on prisms first, to test whether they help before committing to permanent lenses. Surgery remains an option for adults and can improve both alignment and quality of life, though restoring full binocular depth perception is less likely than in children treated early.