Strabismus (from Greek: στραβισμός strabismos, from στραβίζειν strabizein "to squint," from στραβός strabos "squinting, squint-eyed) is a condition in which the eyes are not properly aligned with each other. It typically involves a lack of coordination between the extraocular muscles that prevents bringing the gaze of each eye to the same point in space and preventing proper binocular vision, which may adversely affect depth perception. Strabismus can be either a disorder of the brain coordinating the eyes or a disorder of one or more muscles, as in any process that causes a dysfunction of the usual direction and power of the muscle or muscles.
A simple screening test for strabismus is the Hirschberg test. A flashlight is shone in the patient's eye. When the patient is looking at the light, a reflection can be seen on the front surface of the pupil. If the eyes are properly aligned with one another, the reflection will be in the same spot of each eye. Therefore, if strabismus is present, the reflection from the light will not be in the same spot of each eye.
Whereas amblyopia, if minor and detected early, can often be corrected with use of an eyepatch on the dominant eye and/or vision therapy, the use of eyepatches is unlikely to change the angle of strabismus. Advanced strabismus is usually treated with a combination of eyeglasses or prisms, vision therapy, and surgery, depending on the underlying reason for the misalignment. Surgery attempts to align the eyes by shortening, lengthening, or changing the position of one or more of the extraocular eye muscles and is frequently the only way to achieve cosmetic improvement. Glasses affect the position by changing the person's reaction to focusing. Prisms change the way light, and therefore images, strike the eye, simulating a change in the eye position.
Early treatment of strabismus and/or amblyopia in infancy can reduce the chance of developing amblyopia and depth perception problems. Eyes that remain misaligned can still develop visual problems. Although not a cure for strabismus, prism lenses can also be used to provide some comfort for sufferers and to prevent double vision from occurring.
Strabismus can be an indication that a cranial nerve has a lesion. Particularly Cranial Nerve III (Occulomotor), Cranial Nerve IV (Trochlear) or Cranial Nerve VI (Abducens). A strabismus caused by a lesion in either of these nerves results in the lack of innervation to eye muscles and results in a change of eye position. A strabismus may be a sign of increased intracranial pressure, as CN III is particularly vulnerable to damage from brain swelling.
More commonly however, squints are termed concominant (i.e. non paralytic). This means the squint is not caused by a lesion reducing innervation. The squint in this example, is caused by a refractive error in one or both eyes. This refractive error causes poor vision in one eye and so stops the brain from being able to use both eyes together.
"Cross-eyed" or "boss-eyed" means that when a person with strabismus looks at an object, one eye fixates the object and the other fixates with a convergence angle less than zero, that is the optic axes overconverge. "Wall-eyed" means that when a person with strabismus looks at an object, one eye fixates the object and the other fixates with a convergence angle greater than zero, that is the optic axes diverge from parallel.