Crossed eyes, also called strabismus, occurs when the eyes appear to be misaligned and point in different directions. Strabismus can occur at any age, but is most common in infants and young children. It can be seen in up to 5 percent of children, affecting boys and girls equally.
Strabismus can occur part of the time (intermittent) or all of the time (constant). Intermittent strabismus may worsen when the eye muscles are tired — late in the day, for example, or during an illness. Parents may notice their infant's eyes wandering from time to time during the first few months of life, especially when the infant is tired. This occurs because the infant is still learning to focus his or her eyes and to move them together. Most babies outgrow this intermittent strabismus by the age of 3 months.
Strabismus can be caused by problems with the eye muscles, with the nerves that control the eye muscles or with the brain, where the signals for vision are processed. Strabismus can accompany some illnesses such as diabetes, high blood pressure, multiple sclerosis, myasthenia gravis or thyroid disorders.
Strabismus is classified according to the direction of misalignment. When one eye is looking straight ahead, the other eye may turn inward toward the nose (esotropia or convergent), outward toward the ear (exotropia or divergent), downward (hypotropia) or upward (hypertropia).
Esotropia is the most common type of strabismus and appears in several variations:
Infantile esotropia is present at birth or develops within the first six months of life. The child often has a family history of strabismus. Although most children with infantile esotropia are otherwise normal, there is a high incidence of this disorder in children with cerebral palsy and hydrocephalus.
Many infants appear to have strabismus, but do not. Rather, they have a condition known as pseudostrabismus (or pseudoesotropia), in which a widened nasal bridge or an extra fold of skin makes the white sclera less visible on the nose side of the eye. This gives the appearance that the eyes are crossed. This usually goes away as the infant grows and the facial structures change.
Accommodative esotropia is seen in children who are very farsighted. Their eyes cross because of difficulty focusing on nearby objects. Parents notice the child's eyes turning in sometimes, usually when he or she is concentrating on something up close. Accommodative esotropia typically is diagnosed between ages 2 and 3 years. A family history of this condition is common.
Strabismus has mistakenly been called lazy eye or amblyopia, which refers to diminished vision in one or both eyes beyond what is expected after correcting any eye problem as fully as possible. However, strabismus can lead to amblyopia. When the eyes are not aligned, the brain receives two different images, resulting in double vision. In young children the visual system has not reached full maturity and the brain is able to suppress the image from one eye to avoid double vision. Amblyopia results if vision from one eye is consistently suppressed and the other eye becomes dominant. Among children with strabismus, one-third to one-half develop amblyopia. Although strabismus may be obvious to the observer, only an eye doctor can confirm the diagnosis of amblyopia.