Diseases & Conditions
Strabismus (also called squint, cross-eyes, lazy eye, or wandering eye) is an intermittent or constant misalignment of an eye so that its line of vision is not pointed at the same object as the other eye. If untreated, strabismus can cause amblyopia (a decrease in vision) and permanent loss of vision. Strabismus is treated with correction of any refractive error, a patch to equalize vision, and, in some cases, surgery.
The causes of strabismus are varied and include an imbalance in the pull of muscles that control the position of the eyes or poor vision in one eye. Although not usually caused by a general medical or neurologic disorder, strabismus is a serious problem that should be evaluated and treated and not ignored or "watched." Prompt examination by an eye doctor, either an ophthalmologist or an optometrist, is essential.
There are several types of strabismus. Some types are characterized by inward turning of the eye (esotropia) and some by outward turning of the eye (exotropia). Others are characterized by upward turning of the eye (hypertropia) or downward turning of the eye (hypotropia). The defect in alignment may be constant or intermittent and may be mild or severe.
Phoria is a tendency for misalignment of the eyes. The tendency is a minor defect that is easily corrected by the brain to maintain apparent alignment of the eyes and allow fusion of the images from both eyes. Thus, phorias usually do not cause symptoms and do not need treatment unless they are large and decompensate, producing double vision.
Tropia is a constant, visible deviation or misalignment of the eyes. An intermittent eye deviation that is frequent and poorly controlled by the brain is termed intermittent tropia.
Strabismus: A Misaligned Eye
There are several types of strabismus. In the most common types, an eye turns inward (esotropia or cross-eye) or outward (exotropia or walleye). In this illustration, the child's right eye is affected.
Strabismus may cause double vision (diplopia) in an older child or amblyopia in a younger child.
Parents sometimes notice strabismus because the child squints or covers one eye. The defect may be detected by observing that the child's eyes appear to be positioned abnormally or do not move in unison.
Children should be examined periodically to measure vision and to detect strabismus starting at a few months of age. To examine an infant, a doctor shines a light into the eyes to see whether the light reflects back from the same location on each pupil.
Older children can be examined more thoroughly. Children may be asked to recognize objects or letters with one eye covered and to participate in tests to assess alignment of the eyes. All children with strabismus require examination by an eye doctor (ophthalmologist or optometrist).
If the defect is minor or intermittent, treatment may not be needed. However, if strabismus is severe or is progressing, treatment is required. Treatment depends on the characteristics of the strabismus (see below).
Infantile Esotropia: Infantile esotropia is a constant inward turning of the eyes that develops before 6 months of age; it often runs in families and tends to be severe. The eyes often begin to turn inward by 3 months of age. The degree of turning is large and easily noticeable.
Surgery, which is accomplished by altering the pull of the eye muscles, is usually needed to realign the eyes. Repeated operations may be necessary. Even with the best possible treatment, strabismus may not be fully corrected. Occasionally, amblyopia develops but usually responds to treatment.
Accommodative Esotropia: Accommodative esotropia is inward turning of the eyes that develops between the ages of 6 months and 7 years, most often in children aged 2 to 3 years and is related to optical focusing (accommodation) of the eyes.
The misalignment is the result of how the eyes move when focusing on nearby or distant objects. Children with accommodative esotropia are farsighted. Although everyone's eyes turn inward when focusing on very close objects, eyes that are farsighted also turn inward when looking at distant objects. In mild cases, the eyes may turn too far inward only when looking at nearby objects. In more severe cases, the eyes turn too far inward all the time. With treatment, accommodative esotropia can usually be corrected. Eyeglasses can help the child focus on objects, reducing the tendency for the eyes to turn inward when viewing those objects. Many children outgrow farsightedness and eventually do not need eyeglasses.
Occasionally, drugs (such as echothiophate Some Trade Names PHOSPHOLINE eye drops) are used to help the eyes to focus on nearby objects. If eyeglasses and eye drops fail to properly align the eyes, surgery may help. Amblyopia often develops in children with accommodative esotropia and sometimes in children with infantile esotropia.
Intermittent Exotropia: Intermittent exotropia is outward turning of the eyes that occurs intermittently, usually when the child is looking at distant objects or when the child is tired or ill. Intermittent exotropia is detectable after the age of 6 months.
Intermittent exotropia that is of small magnitude, occurs infrequently, and does not cause symptoms may not require treatment because amblyopia does not usually develop. If symptoms of eye strain from an uncorrected refractive error become troublesome or if attempting to bring the eyes into alignment becomes troublesome, eyeglasses may be used. In more severe cases, surgery may be needed.
Paralytic Strabismus: In paralytic strabismus, one or more of the eye muscles that move the eye in different directions become paralyzed. As a result, the muscles no longer work in balance. The eye muscle paralysis is usually caused by a disorder that affects the nerves to the eye muscles, such as certain viral illnesses, brain injuries, or brain tumors that increase pressure within the skull and compress these nerves.
In children with paralytic strabismus, movement of the affected eye is impaired only when the eye tries to move in a specific direction, not in all directions. Amblyopia or double vision may develop. The double vision is made worse by looking in directions normally controlled by the paralyzed eye muscles.
Paralytic strabismus may resolve by itself over time. However, it may need to be corrected with eyeglasses and covering of the unaffected eye. Sometimes eyeglasses with prisms are used. Alternatively, surgery may be needed. If paralytic strabismus results from another condition affecting the nerves, such as a brain tumor, the other condition also needs to be treated.
Last full review/revision April 2006 by Albert W. Biglan, MD
Source: The Merck Manual Home Edition