The word strabismus comes from the Greek "strabismus" that means squint. It is the deviation of one or both eyes where the eyeballs may not move simultaneously in the same direction.

Considering that in humans the development of normal vision in both eyes with the appropriate alignment of both eyes (orthophoria) and the ability to integrate the images from both eyes are essential for an optimum sense of depth perception (stereopsis), early detection and treatment in children is critical.

The condition may be alternating, affecting both eyes, or monocular (affecting a single eye), depending on the vision and fixation patterns. In alternating strabismus, the patient may use either eye to fix or to see, while the other deviates. Since in that situation, the eyes take turns to see, vision develops more or less the same in both eyes, leading the patient’s brain to learn to suppress the image of the deviating eye (non-fixing eye) in order to avoid double vision (diplopia).

When only one eye is used for seeing (the preferred eye) and the other is permanently deviated, there is a monocular deviation (right or left strabismus). In those cases, the child is highly prone to develop amblyopia (“lazy eye”) as a result of hardly ever using the deviating eye.

Based on the direction of the deviation, strabismus is classified as horizontal (esotropia or exotropia), and vertical (hypertropia or hypotropia). The latter may occur alone or in combination with the more frequently occurring horizontal deviations.

Depending on the cause, the most important types of strabismus are: paralytic, non-paralytic, accommodative and non-accommodative.

True strabismus must be distinguished from the false impression of a deviation (pseudo-strabismus) created as a result of certain anatomical variants. For example, children with prominent epicantal folds and broad flat nasal bridges may simulate esotropia when in fact their eyes are perfectly aligned. Likewise, a child with no ocular deviation may appear to have exotropia due to a large inter-pupillary distance or a disparity in the position of the corneal light reflex and the pupillary axis. Several types of facial asymmetries may also contribute to the false impression of a vertical strabismus.

It is a condition characterized by lower vision in one eye that is impossible to improve with spectacles or contact lenses. In general, these are non-diseased eyes. Amblyopia develops in childhood because of poor visual learning due to factors such as the following:

Misalignment of the eyes, or preference for one eye over the other by the brain during its maturation process.

Power difference: a myopic, hyperopic or astigmatic eye that is not adequately corrected with spectacles or contact lenses.

A large difference in refractive power between the two eyes; one of the eyes is selected.

After birth, a child has to learn detail vision. When a defect is not corrected promptly, the weakest eye will have poorer vision. Vision develops in the first six years of age and the sooner a defect is found to exist, the easier it is to correct it.

The most common causes of amblyopia are strabismus and anisometropia. With misalignment of the eyes in strabismus, one eye is preferentially used over the other. The weakest eye is not stimulated adequately and the visual cells in the brain do not develop normally. The same is true in anisometropia – power difference – where one eye is hyperopic and the other is myopic. For the developing brain, it is very difficult to balance out the difference that, if not corrected adequately, preference is given to the stronger better-focusing eye.

Amblyopia does not produce obvious symptoms that can be observed by the parents, and it may go unnoticed for year until a visual examination is performed. For this reason, it is frequently identified when the child first enters school, when it is already too late to solve it. In strabismic amblyopia, the deviated eye is an obvious sign, but some children may complain of headaches or may develop the habit of shutting one eye when performing certain activities. These may all be signs of poor vision and warrant a visit to the ophthalmologist.