Strabismus is any misalignment of the eyes. It is estimated that 2-4% of the population has strabismus.
There are many different types of strabismus. Strabismus is most commonly described by the direction of the eye misalignment; common types of strabismus are esotropia, exotropia, hypotropia, and hypertropia.
Strabismus can also be described by its cause. The 3 cranial nerves (III, IV, VI) responsible for eye movement can be weak or palsied and cause strabismus. Some examples of paralytic strabismus include third nerve palsy and superior oblique palsy.
Special patterns of strabismus can have unique names such as Brown syndrome, and Duane syndrome.
Esotropia is inward turning of the eyes (aka "crossed eyes"). Types of esotropia include infantile esotropia, accommodative esotropia, and sixth nerve palsy. Exotropia is the term used to describe outward turning of the eyes (aka "wall-eyed") [See figures 1 and 2].
The terms hypertropia and hypotropia are used to describe vertical misalignment. Hypertropia is an abnormal eye higher than the normal eye. Hypotropia is when the abnormal eye is lower than the normal eye. The terms can generally be interchanged.
Most strabismus is the result of an abnormality of the poorly understood neuromuscular (including brain) control of eye movement. Less commonly, a problem with the actual eye muscle causes strabismus.
Eye misalignment can cause amblyopia in children. When the eyes are oriented in different directions, the brain receives 2 different visual images. The brain may ignore the image from the misaligned eye to avoid double vision, resulting in poor vision development of that eye. Also, an eye that sees poorly tends to be misaligned.
Strabismus often occurs in children who are otherwise completely normal. However, disorders that affect the brain such as cerebral palsy, Down syndrome, hydrocephalus and brain tumor are more likely to develop strabismus.
Stroke is the leading cause of strabismus in adults. Trauma, neurological problems, and Graves disease (thyroid eye disorders) are other common causes of strabismus.
Trauma can cause strabismus by 1) brain damage that impairs control of eye movement, 2) damage of the nerves that control eye movement and/or 3) damage of the eye muscles either directly or secondarily from trauma to the eye socket.
The goal of strabismus treatment is to improve eye alignment which allows for better work together (binocular vision). Treatment may involve eye glasses, eye exercises, prism, and/ or eye muscle surgery. Problems associated with strabismus (including amblyopia, ptosis, and cataract) are usually treated prior to eye muscle surgery.