Whole or partial failure of one or more of the muscles responsible for moving the eye in its socket, caused either by a disorder in the associated nerve or the muscle itself; the former is more common. The condition usually affects one eye. The eye is normally moved in its socket by six muscles, provided with impulses by three nerves. If a muscle is paralysed the eye cannot move in certain directions, and is dragged adrift by the unaffected muscles. This causes double vision (diplopia), because the images presented to the brain by the two eyes no longer correspond. The