Diplopia is a very strange sensation! Commonly referred to as double vision, diplopia can actually have many causes. It can be due to a diverse range of infectious, neurological, autoimmune, ophthalmologic and neoplastic causes. Most of these include damage to the third, fourth or sixth cranial nerves (those which control eye movements), cancer, trauma, MS, botulism, Guillain-Barre syndrome, drunkenness, sinusitis, brain tumor abscess, orbital myositis, anisometropia, myasthenia gravis, Grave’s disease, Wernicke’s syndrome, strabismus, salicylism, Lyme disease, corneal scars, cataracts, ischemic vascular disease, following eye surgery or something as simple as the need for glasses.
Diplopia is the simultaneous perception of two images of a single object that may be displaced vertically, horizontally or diagonally in relation to each other. Normally it is the result of impaired function of the EOM’s (extraocular muscles) whereby both eyes are still functional but they cannot converge to target the desired object. Problems with EOM’s can be mechanical, ingestion of toxins or disorders of the cranial nerves. It is often the first sign of a systemic disease, particularly to a muscular or neurologic process. Further, it can disrupt a person’s reading, movement or balance.
The brain naturally guards against double vision. It can sometimes ignore the image from one eye (referred to as suppression). The ability to suppress is usually found in childhood, when the brain is still in the stages of development. Thus, those with childhood strabismus almost never complain of diplopia, as opposed to adults with strabismus almost always do.
Finally, there is a distinction between monocular and binocular diplopia. Monocular is more rare and diplopia can occur with viewing with only one eye, or where the patient perceives more than two images, monocular polyopia. Binocular diplopia is double vision arising as a result of the misalignment of the two eyes relative to each other. This particular phenomenon occurs with esotropia or exotropia. In such a case while the fovea of one eye is directed at the object of regard, the fovea of the other is directed elsewhere, and the image of the object of regard falls on an extra-foveal area of the retina. Images falling on the fovea are seen as being directly ahead, while those falling on retina outside the fovea may be seen as above, below, right or left or straight ahead depending upon the area of retina stimulated. Therefore, when the eyes are misaligned, the brain will perceive two images of one target object, as the target object simultaneously stimulates different, non-corresponding, retinal areas in either eye, thus producing double vision.