Diplopia is double vision caused by a defective function of the extraocular muscles or a disorder of the nerves that innervate (stimulate) the muscles.
A transient episode of diplopia is usually of no clinical significance, indicating only a brief relaxation of the fusion mechanism of the central nervous system that maintains straightness.
Double vision is usually a symptom of strabismus (deviation or misalignment of the two eyes), although not all strabismus produces double vision. In this condition, movement of the eye in a particular direction is impaired due to paralysis of one or more muscles. Tilting or turning the head can sometimes overcome the double vision.
A growth in the eyelid pressing on the front of the eyeball can also cause temporary image separation by distorting the shape of the front of the eye and by causing a slight displacement in the path of light rays entering that eye (and thus variation in the points at which they are focused on the retina). Double vision may also be caused by a tumor or blood clot behind the eye that prevents the normal motion of the eyeball.
In endocrine-related exophthalmos, protrusion of the eyeballs is the result of an underlying hormonal disorder and double vision results from swelling and scarring in the eye muscles, causing abnormal alignment and motion of the eyes.
Rarely, double vision arises because of an abnormality within the eye. For example, a dislocation of the lens in the eye may result in some light rays passing through the lens and others around it, so that separate images fall on the retina of one eye.
The brain of a young child with strabismus (congenital, accommodative, or paralytic) learns to suppress the second, unwanted image seen by the misaligned, strabismic eye so as not to see double. Continued misalignment of the child's eyes may eventually lead to poor vision in the affected eye. The poor vision is called amblyopia, or lazy eye. Patching of the better-seeing eye may improve the vision in the poorly seeing, deviated eye.
The strabismus may need to be corrected early (by means of glasses and/or surgery) in children to prevent amblyopia from developing. Otherwise, it may become permanent. A young child with strabismus should be seen by a physician to find the cause and to begin treatment.
The onset of double vision in adult life needs immediate investigation to exclude the possibility of a tumor, aneurysm, and/or neurologic abnormality. The double vision could be a symptom of a very serious underlying disorder that requires prompt attention and treatment, including surgery.
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