Strabismus is a disorder in which, under normal circumstances, the eyes are not aligned in the same direction. Whereas one eye looks straight ahead, the other looks either upward, downward, inward or outward.
The condition may affect just one eye (unilateral) or may take turns between both eyes (alternating strabismus). It may also be present all the time (constant) or may occur intermittently, especially when the individual is tired or sick.
Strabismus can be present from birth (congenital) or can appear at around 2-4 months (infantile). In a few cases, the condition can develop in adulthood. Crossed eyes that is seen in infants before six months is in most cases normal. It is referred to as pseudostrabismus. The condition occurs as the infant learns how to use the eyes together. It becomes a cause for alarm when it persists thereafter.
Also Known As
- Crossed eyes
- Deviating eyes
- Wandering eyes
Strabismus can classified depending on the direction in which the misaligned eye deviates. The different types include:
- Esotropia – This is the most common form of strabismus. It is a misalignment of one of the eyes in the inward direction.
- Exotropia – Unlike in esotropia where the eye faces inwards, the eye deviates in the outward direction in exotropia.
- Hypotropia – In this type, one of the eyes looks downward while the other looks straight forward.
- Hypertropia – This is where the deviated eye looks upward.
Causes and Risk Factors
The eyes have six muscles which allow them to move in the various directions. The binocular system in the brain transmits signals to the muscles for movement and coordination. Strabismus may occur due to abnormalities in the development of this binocular system. The disorder can also arise with improper development of the muscles or problems with the nerves.
Strabismus can be associated with other factors such as:
- Family history
- Refractive errors – A significant amount of uncorrected farsightedness can cause a strain in focusing. This can result in accommodative esotropia.
- Medical conditions – This can include Cerebral palsy, Down’s syndrome, stroke or head injuries.
Signs & Symptoms
The most obvious sign of strabismus is the misalignment of the eyes. The eyes do not move in the same direction. Other signs and symptoms can include:
- Double/distorted vision
- Inability to gauge depths
- Straining of the eyes
- Tilting of the head to one side
- Squinting of one eye
The diagnosis may include assessments such as:
- Patient history – The eye care professional will ask about the current symptoms and any medical conditions that the patient could be having. The family history may also be noted.
- Alignment and focusing test - This tests for how well the eyes can work and focus together
- Visual acuity – It is used to check for clarity in vision
- Refraction – This test involves use of a phoropter to determine the lens power that is required to correct any refractive error
Treatment is aimed at correcting vision. The treatment can involve medical or surgical procedures.
- Eyeglasses/prisms - The doctor may prescribe eyeglasses or prism lenses for some forms of strabismus. An individual with accommodative esotropia, will, for instance need glasses that help correct the focusing power to restore proper alignment.
- Botulinum toxin (Botox) – The drug is injected into one of the eye muscles that control movement. It works to paralyze the muscle by inhibiting the nerve impulse. The weakening of the muscles for a while helps the eyes to realign themselves.
- Eye drops and patches – Where vision is suppressed in one eye, patches and drops are used on the ‘stronger’ eye to block its vision. As a result, the weaker eye is utilized, and vision is balanced.
Strabismus surgery is done to shorten or to correctly position the eye muscles. The patient may still need vision therapy after the operation to exercise the muscles.
An alternative approach to medical or surgical treatment is the vision therapy. This involves a variety of techniques that enables the patient to learn how to minimize deviation of the eye. It can include exercises that strengthen the muscles. The approach is particularly effective in exotropic cases.
Treatment is successful in strabismus if detected early. When delayed, an individual may develop double vision (where the eyes do not work as a team). The brain responds to the confusion by picking up images from only one eye and ignoring the other (suppression). Eventually, about a third of individuals develop amblyopia, otherwise known as lazy eye. It is a condition where there’s permanent loss of vision in one eye.
It is not entirely possible to prevent strabismus. However, the loss of vision that results can be prevented. This is by going for regular eye screening, especially from a young age.