DOUBLE VISION

also called Diplopia, perception of two images of a single object, most commonly caused by temporary or permanent paralysis of eye muscles. Normal binocular vision results from the brain's fusion of slightly different images from each eye. Points on the retina of each eye correspond to points on the opposite retina. When one of the extraocular muscles of one eye is paralyzed, the image of an object falls on the retinal fovea (small area at the centre of the retina that affords acute vision) at a point different from that where the same image is projected on the retinal fovea of the opposite eye, and the images do not correspond. The particular muscle that is paralyzed determines the orientation of the double image. Objects that are near at hand are frequently seen in crossed images; that is, the image received by the left eye appears to the right of the image from the right eye. Farther objects are also doubled, but the images bear the appropriate relationship to each other. Generally, the image from the paralyzed eye is less distinct than that from the normal eye because the paralysis makes proper focusing difficult. The two images can also be distinguished by placing a coloured filter over one eye. Double vision may be among the first symptoms of botulism and also occurs in other infections, head injuries, and nerve palsies or muscle disorders. The most common ocular muscle disease causing double vision is myasthenia gravis, in which diplopia may be the first symptom. The disease later affects other muscles of the body as well. Double vision also occurs in benign ocular myasthenia, which resembles myasthenia gravis but does not spread to other muscles.