Double vision, or "seeing double," occurs when two nonmatching images are sent to the part of the brain that is responsible for processing visual input. If this occurs over the long term, the brain will eventually compensate for the two signals by suppressing one signal, so that a single image is perceived. The suppressed eye may eventually become amblyopic ("lazy eye") with resultant vision loss.
Double vision can result from impairment in any part of the vision system, including the cornea, eye muscles, lens, nerves, or the brain. The most common cause of diplopia is misalignment of the two eyes that can arise from several different conditions. Some diseases such as myasthenia gravis and Grave's disease can cause weakness of the eye muscles, leading to misalignment of the eyes and double vision. Poorly controlled diabetes can ultimately result in nerve damage that can manifest as diplopia, and diplopia can arise as a result of stroke, head injury, or other brain damage. While diplopia can be a symptom of the conditions listed below, it is not always present in these conditions.