Strabismus is an eye disorder where the eyes are misaligned and point in different directions. In the general population, children are the most affected where one eye turns down, up, or out, while the other one looks straight ahead.
Strabismus occurs when there is a dysfunction in the six eye muscles. All six muscles must work together to focus on one image for clear vision. Four muscles help to move the eye up, down, and at an angle. The fifth muscle causes the eye to move to the left, while the last muscle helps it move to the right. Without eye alignment as the child develops, amblyopia sets in. The eye that is not aligned has a weaker vision, and the child’s vision may deteriorate due to amblyopia. Strabismus may be intermittent (eyes don’t turn all the time) or constant (eyes turn throughout) and mild or severe. Children may develop strabismus a few months after birth or later in childhood.
Pseudostrabismus can develop in infants below one year where the eyes look crossed. The condition often develops in children with broad, flat noses or skin folds at the inner eyelid. Pseudostrabismus may disappear on its own, but not strabismus.
- Hypertropia (upward eye turn)
- Hypotropia (downward eye turn)
- Infantile esotropia where the child’s eyes cross inwards
- Esotropia/accommodative esotropia where one eye turns inward when the focus is on a distant or near object. This type is the most common in children aged above two years
- Exotropia where the child’s eye turns outward when viewing a close-up or distant object. When the sun is bright, the affected child may squint. Exotropia also occurs in ill, daydreaming, or tired children
Classification can also be done according to the cause. There are 12 cranial nerves responsible for eye movement. Three of these can suffer weakness and lead to strabismus. Examples include superior oblique (IV) palsy and third nerve (III) palsy. Some conditions, such as Duane syndrome and Brown syndrome, have special strabismus patterns.
Causes & Risk Factors
Brain disorders can cause strabismus in children under six months old. They include brain tumors, hydrocephalus, cerebral palsy, premature birth, and Down syndrome. Prenatal exposure to alcohol and specific congenital disabilities can also lead to strabismus. Other strabismus causes may include a cataract (cloudy lens) or eye injury, although these are rare causes. Inherited patterns or family history increase the risk of a child getting strabismus.
In children aged six months and above, strabismus is due to a refractive error where the light does not bend properly when refracted. There is no balance in the pull of muscles controlling eye position. If the refractive error is severe, it interferes with the brain’s function of maintaining eye alignment leading to strabismus. If the socket fractures, it can block eye movement and result in strabismus.
Signs & Symptoms
Strabismus signs and symptoms include:
- Loss of vision
- Closing one eye
- Diplopia or double vision
- Squinting in bright sunlight
- Tilting the head to view an object
- Eyes looking in different directions at the same time
Diagnosis of strabismus may be made through:
- Cover and Hirschberg tests which check for eye misalignment
- Family and medical history which are vital in establishing if some hereditary patterns are present
- Pupil examination where infants are supposed to have equal and reactive pupils from the time of birth
- Eye examination or inspection using a penlight to reveal any anomalies with the eyelids and adnexa. Other structures such as the sclera, iris, pupil, and cornea are also examined
- The red reflex test where a direct ophthalmoscope is used to examine the two eyes simultaneously. It is the most effective and essential screening examination for infants and young children
- Visual acuity where children aged two to four months should be able to follow objects with their eyes. By the time a child is two years old, they should demonstrate good alignment like reaching for toys
- Observation for ocular motility and nystagmus (shaking of the eyes). Parents and caregivers can watch for abnormal eye movements and children showing symptoms of nystagmus referred for a low vision evaluation
Treatment is aimed at aligning the eyes to focus on the same object. Treatment options depend on the age and type of misalignment.
For children more than two years old, special eyeglasses can assist with focussing and strengthening the eye. Prisms are also helpful in aligning the eyes and can be fixed as part of the lens or be attached to glasses. Another method is to strengthen the weaker eye by patching the stronger one or using blurring eye drops.
Infants and very young children with esotropia need strabismus surgery to enable proper eye alignment, leading to good vision development. Such children can also do with glasses. During the procedure, the surgeon makes a tiny cut in the tissue to access the eye muscles, which s/he repositions. Once repositioned, the muscles will point in the same direction.
Strabismus surgery can be performed in one or both eyes, and some children may need a repeat procedure. It takes about three days for the child to resume normalcy after the surgery. Although strabismus surgery is effective, the child may still experience blurry vision and wear patches or eyeglasses. If the child has other ocular conditions like amblyopia and cataract, they are treated before the eye muscle surgery.
Exercising eye muscles can help to align the eyes.
Prognosis & Long-Term Outlook
If strabismus is diagnosed early and treated, it can result in excellent vision and may also prevent vision loss.
Prevention & Follow Up
Strabismus cannot be prevented. Nevertheless, it's possible to prevent complications that may arise, with early detection and regular screening. There is a need for follow-up to determine if treatment was effective.