All you need to know about: squint eye

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An estimated 1.81% of the global population has horizontal strabismus, according to a recent systematic review and meta-analysis. Image used for representational purposes only | Photo Credit: RAJARATHINAM RM

Squint, or strabismus, is a condition in which the eyes are not aligned and do not look in the same direction at the same time. One eye may turn inwards towards the nose (esotropia), outwards (exotropia), upwards or downwards, while the other eye remains focused on an object. This misalignment may be constant or may occur intermittently, such as when a person is tired.

A recent study estimated that horizontal strabismus affects 1.81% of the global population, or about 138.5 million people. Exotropia affects an estimated 1.14% (87.5 million), compared with 0.67% (60 million) for esotropia. The pattern has also shifted across generations, with exotropia now more common globally. The study found regional differences, but noted that parts of South Asia remain under-represented in research.

The World Health Organization (WHO) recognises strabismus as one of the eye conditions that can require long-term monitoring and treatment. WHO’s guidance on vision screening in preschool children notes that strabismus is one of the causes of amblyopia, or reduced vision resulting from abnormal visual development.

The most obvious sign is that the eyes appear to point in different directions. Some children may close one eye in bright light, tilt or turn their head to see, or have difficulty judging distances.

Children may not always complain about poor vision. When the eyes are misaligned, the brain can suppress the image from the affected eye to avoid double vision. If this happens during the period when vision is developing, the child can develop amblyopia, commonly called lazy eye .

Adults who develop a squint may experience double vision, blurred vision, eye strain, headaches or difficulty with depth perception. A new squint that appears suddenly in an adult should be assessed promptly because it can sometimes be associated with conditions affecting the eye muscles, nerves or brain.

Not every apparent squint is true strabismus. Some children appear to have crossed eyes because of the shape of the eyelids or the bridge of the nose. This is known as pseudostrabismus and can be distinguished from true strabismus through an eye examination.

Strabismus occurs when the systems that control eye movement and alignment do not work properly together. The problem may involve the eye muscles, the nerves that control them, or the parts of the brain that coordinate eye movements.

In many children, the exact cause is not known. Genetic factors can play a role, and children with a family history of strabismus are at higher risk.

Refractive errors can also contribute. Squint is also more common in children with certain neurological or developmental conditions.