Strabismus is a failure of the two eyes to maintain proper alignment and work together as a team. This occurs when there are neurological or anatomical problems that interfere with the control and function of the eye's coordination muscles. The problem may originate in the muscles themselves, or in the nerves or vision centres in the brain that control binocular vision.

Genetics also may play a role: If you or your spouse has strabismus, your children have a greater risk of developing strabismus as well.

When eyes don't line up together, the straight or straighter eye becomes dominant. The vision strength of the dominant eye stays normal because the eye and its connection to the brain are working as they should. The misaligned or weaker eye, though, doesn't focus as it should and its connection to the brain doesn't form correctly.

If a person's two eyes are not aligned properly, three different things can happen:

1. The person will see double because the two eyes are not aimed at the same point

2. One of the eyes can suppress or turn off (in the brain) to avoid double vision. This condition is called suppression, and if left untreated, leads to a 'lazy eye', technically referred to as amblyopia.

3. The brain can develop a new match with each eye so that fusion occurs even though the eyes are not aimed at the same spot. This last phenomenon is known as anomalous retinal correspondence.

Strabismus can be constant or intermittent. The misalignment also might always affect the same eye (unilateral strabismus), or the two eyes may take turns being misaligned (alternating strabismus). The primary sign of strabismus is a visible misalignment of the eyes, with one eye turning in (esotropia), out (exotropia), up (hypertropia), down (hypotropia) or at an oblique angle.

Sometimes, strabismus is very noticeable. Other times, it might only be noticed when a child is tired or looking at something very closely. Typically, constant large-angle strabismus does not cause symptoms such as eye strain and headaches because there is virtually no attempt by the brain to straighten the eyes. Because of this, large-angle strabismus usually causes severe amblyopia in the turned eye if left untreated.

Less noticeable cases of small-angle strabismus are more likely to cause disruptive visual symptoms, especially if the strabismus is intermittent or alternating. In addition to headaches and eye strain, symptoms may include an inability to read comfortably, fatigue when reading and unstable or "jittery" vision. If small-angle strabismus is constant and unilateral, it can lead to significant amblyopia in the misaligned eye. Both large-angle and small-angle strabismus can be psychologically damaging and affect the self-esteem of children and adults with the condition, as it interferes with normal eye contact with others, often causing embarrassment and awkwardness.

If an eye is constantly turned, and first-line treatments such as glasses and/or vision therapy have not been able to align the eye, surgery will likely be considered. The success of strabismus surgery depends on many factors, including the direction and magnitude of the eye turn. In some cases, more than one surgery may be required. In some cases of intermittent and small-angle strabismus, it may be possible to improve eye alignment non-surgically with glasses and/or vision therapy.