Diplopia is the medical term for double vision. This is when a person sees two images of the same object. The images can be side by side (horizontally), one above the other (vertically), or both, making the images diagonally across from each other. Diplopia occurs when the six extraocular muscles that surround each eye begin to weaken and have difficulty in converging the eyes. These muscles are controlled by three cranial nerves: oculomotor (cranial nerve III), trochlear (cranial nerve IV), and abducens (cranial nerve VI).
In general, diplopia is the result of an underlying systemic disease and can cause the person to have difficulty with balance, reading, and walking. Some diseases resulting in diplopia include but are not limited to brain tumors; damage to cranial nerves III, IV, or VI; diabetes; Lyme disease; migraine; multiple sclerosis; and stroke.
There are four classifications of diplopia: binocular, monocular, temporary, and voluntary. Binocular diplopia is generally associated with strabismus where the six extraocular muscles do not align the eyes properly so that convergence cannot occur. For instance, one eye focuses on an object and the other eye turns inward or outward and focuses on a completely different object. Thus, two different images are sent to the brain, which can be confusing for visual perception. In young children, their brains may learn to ignore the image from the weaker eye—usually the turned eye. If the turned eye is only slightly askew, then both eyes may focus on the same object but the images are focused on different regions of the two retinas. The brain will try to determine where the object is in space based on the two images sent to the visual cortex, which can also be confusing.
Monocular diplopia or monocular polyopia is when one eye perceives more than one image. This can occur if the surface of the eye—the cornea—thins and changes shape to be more cone-like (keratoconus). Keratoconus is a degenerative disorder of the eye and is usually diagnosed in a person’s teenage years. If the keratoconus is severe, surgery may be necessary to correct the shape of the cornea. However, in some cases the cornea may need to be replaced (corneal transplant). Other causes of monocular diplopia include structural defects of the eye, such as a displaced or misaligned lens, and astigmatism. Astigmatism is a common vision condition and is caused by a misshaped cornea, resulting in light not focusing properly on the retina.
Being tired, trauma to the head, or overindulgence in alcohol generally causes temporary diplopia. Temporary diplopia should resolve on its own with rest. However, if a person has trauma to the head, he or she should see a health care provider immediately for evaluation.
Finally, voluntary diplopia occurs when a person purposely crosses the eyes (such as focusing on the tip of the nose), unfocuses the eyes (which can help in viewing stereo images), or focuses on an object that is behind another object. The object closest to the person will be doubled. It is an urban legend that if you cross your eyes, it will become permanent. In reality, voluntary diplopia is not dangerous but if it is prolonged, it may cause headaches.
Treatment for diplopia depends on the cause of the double vision. The underlying cause must be treated first for best results. If diplopia is acute, it may be resolved by restful sleep; if it is chronic diplopia, an optometrist may prescribe eye exercises, wearing an eye patch, or wearing eyeglasses with a prism correction. For severe cases and where other treatments have not been effective, eye surgery may be necessary to correct the diplopia.
Jennifer L. Hellier
See also: Astigmatism; Cranial Nerves; Myopia; Presbyopia; Strabismus; Visual System
American Optometric Association. (2015). Astigmatism. Retrieved from http://www.aoa.org/patients-and-public/eye-and-vision-problems/glossary-of-eye-and-vision-conditions/astigmatism?sso=y
Blumenfeld, Hal. (2010). Neuroanatomy through clinical cases. Sunderland, MA: Sinauer.
Graf, M., & B. Lorenz. (2012). How to deal with diplopia. Revue Neurologique (Paris), 168(10), 720—728. http://dx.doi.org/10.1016/j.neurol.2012.08.001