If you look at your finger and draw it very close to your eyes you’ll be aware that everything in the background becomes two separate images. This is because your eyes have ‘converged’ for the near object and are not now correctly aligned for viewing at a distance. Double vision, known as diplopia, causes a person to see two images of the same object some or all of the time. Diplopia occurs when the eyes no longer work together as a pair and become out of alignment with each other. This produces two images that the brain is unable to fuse together. This may happen suddenly or over a period of time and sometimes you can get double vision in just one eye.
Double vision can also indicate other underlying, sometimes serious visual disorders that affect children and adults, such as difficulty in converging on near objects or visual conditions caused by head injuries.
If you suddenly experience double vision, for example if you wake up seeing double, you should go immediately to your local casualty department.
The Causes of Double VisionThe most common cause of double vision is misalignment of the two eyes due to functional problems in the visual system, when muscles or nerves may have been weakened or damaged. This condition is referred to as binocular diplopia. About 4% of children under six have signs of binocular diplopia, which is often called a ‘squint’ or a ‘turn’.
Double vision can also occur in one eye only (monocular diplopia), due to one or more structural defects in the eye's optical system. The most common cause of monocular diplopia is cataract - a hardening and gradual clouding of the crystalline lens inside the eye which occurs very often in old people. Ageing causes the lens to develop regions where its refractive power varies. These regions may produce separate images on the retina. This form of diplopia may subside with further ageing.
How Double Vision is TreatedThe brain of a young child with diplopia learns to suppress the image seen by the weaker, misaligned eye. This needs to be corrected early, usually with glasses or possibly using surgery. If it is left untreated diplopia can lead to poor vision in the affected eye, a condition known as amblyopia (lazy eye). Occlusion (patching) of the better eye may also improve the vision in the lazy eye by making it work. Without treatment amblyopia may become permanent.
People with permanent double vision can often be treated using a Fresnel prism - a special transparent plastic sheet which can be trimmed to fit over the lens in a pair of glasses. The Fresnel prism has the effect of displacing the image in the misaligned eye so that it falls where it is supposed to on the retina, allowing the brain to fuse the two images together. If the Fresnel prism is not successful it may be necessary to cover one eye in order to block out one of the double images, at least temporarily
Double vision may gradually disappear with time. If it doesn’t, corrective surgery remains an option.
It’s important to remember that suddenly getting double vision, especially if it has not happened to you before, can indicate more serious conditions such as tumours, cancer or dislocation of the crystalline lens. Don’t ignore it in the hope that it will go away!