Squint (Strabismus)

Squint is a condition in which the eyes are misaligned. It is also known as strabismus, tropia or crossed eyes. In squint, when one eye focuses on an object, the other deviates away from the point of focus and either turns out, in, up or down. Squint can occur at any age but it is more common in children.

A person with squint usually does not experience binocular vision and depth perception is poor. Vision in the affected eye is also poorer than that of the other eye. Squint is easily observable, though in mild cases it may not be obvious.

The appearance of squint is most common in babies; though sometimes, it may not be true squint but rather an illusion. If the skin fold at the inner corner of the eyelids is broad or the nasal bridge is broad and flat, the space between the iris and the inner corner of the eyelids appear smaller in size giving an impression that the child has a squint. This false squint is known as pseudo-squint or pseudo-strabismus. It is more obvious when a child who has it looks at you from an angle.

If squint is constant in an eye, the eye may develop amblyopia, a condition characterized by reduced vision that is not correctable with lenses. Hence, children with squint need to have their eyes examined and treatment given to prevent the development of amblyopia.

Types of squint

There are different types of squint, classified according to the direction the eye turns, its causes, nature of occurrence and more. Here are some of the ways squint can be classified.

Types of squint based on the direction the eye turns

Exotropia: The eye turns out in relation to the straight eye
Esotropia: The eye turns in in relation to the straight eye
Hypertropia: The eye turns up in relation to the straight eye
Hypotropia: The eye turns down in relation to the straight eye
Cyclotropia: The eye turns into a tilted position in relation to the other eye

Types of squint based on the nature of occurrence of the deviation

Constant squint: One eye deviates away from the point of focus at all times.
Intermittent squint: One eye deviates sometimes and at other times it is straight.
Alternating squint: In this type of squint, the two eyes take turns to deviate. When the left eye deviates, the right eye becomes straight and when the right eye deviates, the left eye becomes straight. The alternation of squint between the two eyes usually depends on the distance the eyes are focusing. One may turn when the person is looking at far objects while the other turns when the person changes focus to near objects.

Types of squint based on whether the degree of deviation is same or not when the eye looks in different various

Concomitant (comitant) squint: This is when the degree of squint is the same in all directions of gaze.
Non-concomitant (incomitant) squint: This is when the degree of squint varies from one direction of gaze to another.

Description based on the whether the squint is caused by a refractive error or not

Refractive squint: This is squint that occurs as a result of a refractive error.
Non-refractive squint: This is a squint that is not due to a refractive error.

Causes of squint

Refractive errors

Squint can occur as a result of refractive errors; particularly if there is a significant difference in the errors in both eyes (a condition known as anisometropia). In anisometropia, images from the two eyes are so dissimilar that in order to avoid double vision, the brain may ignore the image from one eye (usually the eye with the poorer image). Continuous disuse of the image from that eye can cause it to deviate (turn).

Out of all the refractive errors, long-sightedness is the most common cause of squint. This is because the eyes try to compensate for this refractive error through accommodation. During accommodation, the eyes converge (turn inwards). Excessive and constant accommodation may cause the eye to turn inward permanently resulting in esotropia (also known as accommodative esotropia).

Accommodative squints are usually noticed when a child plays with a toy or while reading or colouring. Generally, refractive squints usually do not show up until the age of 2.

Eye muscle problem

The movement of the eyes is controlled by six extraocular muscles. Any problem that interferes with the movement and function of these muscles can cause squint. Hence, eye diseases, tumours and nerve problems that affect the extraocular muscles can cause squint.

Genetics /Heredity

Squints may be inherited as it has been observed to run in families. However, it is not clear how genetics play a role in squint.

Illness

Some illnesses like cerebral palsy, Down’s syndrome, hydrocephalus, brain injury, tumours and cancers like retinoblastoma can cause squint.

Idiopathic origin

Sometimes the cause of squint is not known. A child, for instance, may be born with squint (congenital squint) without any identifiable cause.

Signs and symptoms of squint in children

Squints are easily noticeable as one eye is straight and the other is not. Minor squints, however, may not be very obvious except when the eyes are tired or stressed. Some behaviours that may suggest that a child has squint include:

Closing one eye: If a child tends to close one eye while looking at an object; it may be that he sees double when his two eyes open. In order to eliminate the discomfort of seeing two images, he closes one eye, sometimes unconsciously.
Head tilt or neck turn: A child may tilt his head or turns his neck to a particular position when looking at an object. This also suggests he may be having double vision which is commonly associated with squint.

squint, strabismus, tropia, crossed eyes