Strabismus

Signs and symptoms

The misalignment of the eyes may be quite apparent in a person with a constant eye turn of significant magnitude, but a small or intermittent strabismus can easily be missed on casual observation. An eye care professional can use tests such as cover testing to determine the full extent of the condition. Symptoms include double vision and eye strain; to avoid double vision, the brain may adapt by ignoring one eye. In children, prolonged suppression of the misaligned eye can lead to amblyopia, which may result in permanent vision loss if not treated early.

Often no noticeable symptoms are present other than a minor loss of depth perception, and this deficit may not be noticeable in someone who has had strabismus since birth or early childhood, as they have likely learned to judge depth using monocular cues. A constant unilateral strabismus causing constant suppression is a risk for amblyopia in children. Small-angle and intermittent strabismus are more likely to cause disruptive visual symptoms, including headaches, eye strain, inability to read comfortably, fatigue when reading, and unstable or jittery vision.

People of all ages with noticeable strabismus may experience psychosocial difficulties, and attention has been drawn to potential socioeconomic impact. One study showed that strabismic children commonly exhibit behaviors marked by higher degrees of inhibition, anxiety, and emotional distress, often leading to emotional disorders related to negative perception by peers. Strabismus interferes with normal eye contact, often causing embarrassment, anger, and feelings of awkwardness, with a possible negative effect on self-esteem.

Pathophysiology

Strabismus can result from problems with the extraocular muscles or the cranial nerves that control them. These muscles are governed by cranial nerves III, IV, and VI. Damage to cranial nerve III makes the affected eye point downward and outward, and it may or may not alter pupil size. A congenital issue with cranial nerve IV causes the eye to drift upward, sometimes with a slight inward turn.

Sixth nerve palsy, which has numerous triggers because the nerve has a long path, makes the eye turn inward; increased pressure inside the skull can pinch the nerve where it runs between the clivus and the brain stem. Another possible cause involves the visual cortex. When the images from the two eyes differ greatly in clarity, the brain may not receive enough input to properly realign the eyes. Conditions that create such a visual difference—like uneven cataracts, refractive errors, or other eye diseases—can also bring on or worsen strabismus.

Diagnosis

To diagnose strabismus and assess its effect on vision, eye doctors use tests like cover testing or the Hirschberg test. Retinal birefringence scanning can help screen young children for eye misalignment. One Cochrane review that looked at different diagnostic methods found just one study, which tested a photoscreener; it was good at correctly identifying people without the condition but poor at spotting those who actually had it. Strabismus is classified in several ways.

It can be manifest, called a tropia or heterotropia, where the misalignment is present when both eyes are open and the person cannot align their gaze to fuse images. Or it can be latent, called a phoria or heterophoria, which only appears when binocular vision is interrupted, like when one eye is covered; these people can usually maintain fusion despite the misalignment. Intermittent strabismus is a mix of both, where fusion is possible but sometimes fails, leading to a manifest deviation.

Strabismus is also classified by when it starts: congenital, acquired, or secondary to another condition. Many infants have slightly misaligned eyes at birth, but this usually corrects itself by six to twelve months. Acquired and secondary forms develop later. Accommodative esotropia, an inward turning caused by focusing effort, mostly appears in early childhood.

Management

Strabismus is usually treated with a combination of eyeglasses, vision therapy, and surgery, depending on the underlying reason for the misalignment. The primary goal is comfortable, single, clear, normal binocular vision at all distances and directions of gaze. In accommodative esotropia, the eyes turn inward due to the effort of focusing far-sighted eyes, and treatment necessarily involves refractive correction via corrective glasses or contact lenses; surgical alignment is considered only if such correction does not resolve the eye turn.

In cases of strong anisometropia, contact lenses may be preferable to spectacles because they avoid visual disparities due to size differences (aniseikonia) caused by spectacles with very different refractive power for the two eyes. In a few cases of strabismic children with anisometropic amblyopia, balancing refractive error via refractive surgery has been performed before strabismus surgery. Early treatment when the person is a baby may reduce the chance of developing amblyopia and depth perception problems, but a review of randomized controlled trials concluded that the use of corrective glasses to prevent strabismus is not supported by existing research.

Glasses affect eye position by changing the person's reaction to focusing; prisms change the way light strikes the eye, simulating a change in eye position. Strabismus surgery attempts to align the eyes by shortening, lengthening, or changing the position of one or more extraocular muscles, typically performed in about an hour with about six to eight weeks for recovery. Adjustable sutures may be used to permit refinement of eye alignment in the early postoperative period, but it is unclear if there are differences between adjustable and non-adjustable sutures as it has not been sufficiently studied. An alternative is minimally invasive strabismus surgery (MISS), which uses smaller incisions.

Did You Know?

• Siamese cats and related breeds are prone to crossed eyes, likely a behavioral compensation for developmental abnormalities in nerve routing in the optic chiasm.
• Strabismus may occur in dogs, most often due to imbalanced muscle tone of the muscles surrounding the eye; some breeds such as the Shar Pei are genetically predisposed.
• One study reported 85% of adults with strabismus had problems with work, school, and sports because of their strabismus, and 70% said it had a negative effect on their self-image.