Strabismus

There are several types of strabismus, many of which are treatable. In childhood, recognizing treatable forms of strabismus early is particularly important because young children may develop decreased vision in one eye (amblyopia) and/or poor depth perception.

Both strabismus and amblyopia are referred to as lazy eye. However, it is important to make the distinction between strabismus versus amblyopia. Strabismus refers to the misalignment of the eyes. Amblyopia refers to decreased vision in one or both eyes that may be associated with strabismus.

The definition of strabismus is misalignment of the eyes. One or both eyes might turn inward toward the nose (crossed-eyes, convergent) or outward (walleyes, divergent), or one eye can turn up or down relative to the other eye (vertical). The misalignment may be intermittent (transient) or constant. The amount of misalignment may vary depending on the direction of gaze or whether one is focusing at distance or near.

The definition of amblyopia is decreased vision due to a failure of the normal neural pathways between the eye and the brain to develop in infancy or early childhood. In amblyopia, the structures in the eye are usually normal. Amblyopia is a result of either failure of the eye to be used or the absence of a clear image being provided to the brain by that eye. Amblyopia may be secondary to strabismus, an uncorrected refractive error or something blocking the eye from seeing, such as a droopy lid, a cataract, or a corneal opacity. Amblyopia usually affects only one eye but, occasionally, in cases where an infant has severe farsightedness or astigmatism in both eyes, amblyopia may be present in both eyes.

The misalignment is called esotropia when an eye turns in toward the nose, exotropia when an eye turns out, and hypertropia when an eye turns up.

Strabismus can be intermittent or constant.

A phoria is a type of strabismus that is intermittent and usually only present when a person is fatigued, or when the two eyes are not working together, as when one eye is covered. Convergence insufficiency is a common form of exophoria that is present when focusing up close.

Many types of strabismus are further categorized by their age of onset or underlying cause. For example, congenital or infantile esotropia presents in the first year of life. Accommodative esotropia is due to farsightedness (hyperopia) and is usually first noticed around 2 ½ years of age. Certain acquired forms of strabismus may appear at any age and may be due to conditions that interfere with the normal function of the extraocular muscles that move the eye (for example, restrictive, paralytic, secondary strabismus).

Pseudo-strabismus is a term used when the eyes appear to be misaligned but are in fact properly aligned. This illusion can be due to eyelid skin folds or certain ocular or orbital bone disorders.

Newborn babies often have brief, intermittent eye misalignment that resolves, but if the misalignment is frequent and/or pronounced to a large degree, your ophthalmologist should perform an eye exam to rule out strabismus.

In most cases of congenital or infantile esotropia, there is no known cause. Some forms may be hereditary, and some are associated with premature birth. Farsightedness (hyperopia) causes accommodative esotropia. Other forms of strabismus develop when vision in one eye is poor relative to the other eye, for example when one eye has a cataract or a large refractive error. In these cases, the child will favor using the eye with better vision, and the eye with blurry vision will wander out of alignment. Less common causes include congenital syndromes such as Duane's syndrome and Brown syndrome.

In adults, strabismus may develop if an underlying medical condition affects the muscles that move the eyes, for example when there is loss of blood supply to the muscles, problems with nerves that control the muscles, trauma, or a lesion or swelling of orbital tissue that interferes with muscle function. The underlying disease might affect one eye (unilateral) or both eyes (bilateral) to varying degrees.

Presumably even young children will experience some degree of blurry or double vision when the eyes are misaligned. The symptom usually disappears because the child rapidly neuro-adapts by using just one eye to see. If one eye is consistently favored over the other, amblyopia (lazy eye, poor vision) can develop in the non-favored eye. In some cases, the child may cross-fixate, that is, alternate use of each eye one at a time. In both cases, because the two eyes are not working together, the child will often develop poor depth perception, also known as poor binocularity.

If strabismus is worse in certain gazes, the child may turn their head to minimize the symptoms. For example, if the strabismus is present in straight ahead gaze and worse when the gaze is toward the right, the child will turn their head to the right so the eyes are positioned in left gaze where the strabismus is minimal or absent. Over time, this can lead to changes in posture and neck pain.

Convergence insufficiency is a common condition that is present when focusing at something nearby and can result in fatigue and headache when trying to read or engage in up-close tasks.

In adults who have equal vision in each eye, any newly acquired strabismus will typically cause blur and/or double vision (diplopia) and decreased depth perception, regardless of the cause.

An eye doctor (optometrist or ophthalmologist) will perform a thorough examination to detect decreased vision and the presence of strabismus. The type and amount of misalignment is measured using tools such as lenses and prisms. There are techniques to assess vision and strabismus in a baby or any nonverbal individual.

In children, if amblyopia is present or if there is risk of developing amblyopia, time is of the essence. The earlier that treatment is undertaken, the better the odds of success.

The eye doctor will look for any eye or orbital condition that may contribute to strabismus (for example, large refractive errors, droopy eyelid, cataracts, scar tissue, retinal lesions, orbital abnormalities, etc.). If an underlying medical condition is suspected, further testing (such as blood tests and imaging) and consultation with other health care specialists may be needed.

It's possible to correct some forms of strabismus without surgery. Children with accommodative esotropia are prescribed glasses to correct farsightedness, and in some cases, this alone will fully realign the eyes. Convergence insufficiency is often treatable with eye exercises or vision therapy that strengthen the muscles that converge the eyes (for example, the medial rectus muscles that move the eyes toward the nose).

In a child who has an opacity blocking vision to one or both eyes (for example, droopy eyelid, corneal scar, cataract, or retinal lesion), treatment should be offered as soon as feasible to prevent amblyopia. If amblyopia has developed, therapy to strengthen the vision in the amblyopic eye (for example, patching eye therapy) should be offered as soon as possible.

If strabismus is still present after correcting refractive error and addressing any underlying disease, medical or surgical treatment that change the balance of the eye muscles' forces can realign the eyes. In some cases, an eye care provider may inject Botox into an overactive eye muscle. An eye doctor will surgically reposition the eye muscles' insertions on the eye to correct strabismus, and in some cases, ophthalmologists use adjustable sutures to fine tune the alignment. The surgery cost depends on insurance coverage, and the risks and benefits of surgery vary from person to person. Weigh these factors against the possible problems associated with uncorrected strabismus, such as poor vision, poor depth perception, and psychological and cosmetic considerations.

In adult-onset strabismus, any underlying causes need to be addressed, such as diseases of the circulatory system, nervous system disorders, or pathology within the orbits or brain. In many cases, the degree of strabismus is variable and may come and go with severity of the underlying disease. In these situations, prisms can be added to glasses to relieve the symptom of double vision while undergoing treatment for the underlying condition.

Surgery recovery time varies from patient to patient. Close postoperative follow-up is important to look for any signs of complication as well as to assess the success of alignment. If amblyopia is present, start therapy as soon as possible.

The prognosis of strabismus varies depending on the underlying cause. Correction may require a combination of nonsurgical and surgical treatments. In all cases of childhood strabismus, assessing for and treating any amblyopia is important as the prognosis is generally much better if started early.

In most cases, there is no known way to prevent strabismus. Premature birth is a risk factor for strabismus. Therefore, proper prenatal care to reduce the chances of prematurity can help lower the risk. Adults can also take steps to reduce the risk by addressing preventable and treatable associated medical conditions (for example, by maintaining normal blood sugars and blood pressure to keep the circulatory system in good health). Using proper eye protection when engaging in activities where there is risk of injury to the orbits can prevent strabismus due to traumatic injury of the eye or orbit.