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).
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