Diplopia is the medical term for double vision or seeing double. Diplopia is defined as seeing two images of a single object when you’re looking at it.
Double vision is usually a temporary issue, but it can also be a sign of more serious health conditions. Even if it’s caused by something as simple as needing new glasses, it’s important to get your eyes examined by your healthcare provider right away if you start seeing double.
Your healthcare provider will classify your double vision as either monocular (one eye) or binocular (both eyes). Monocular diplopia is present when you use only one eye at a time. It may appear as a shadow. Binocular diplopia is present when both eyes are open at the same time. It goes away if you cover one of your eyes.
Monocular diplopia is more common and usually less serious. Binocular diplopia is usually caused by your eyes being out of alignment or other, more serious underlying conditions.
If you have binocular diplopia, your double vision can appear either vertically (top to bottom) or horizontally (side to side). Which one you have depends on what’s causing your diplopia and how (or why) your eyes are out of alignment.
Each eye creates its own image of the environment. The brain combines the representations from each eye and perceives them as one clear picture. The eyes must work together to create depth perception. Anything that disrupts this process can cause diplopia. It can be nerve or muscle damage.
Certain illnesses can weaken the muscles moving the eyes and produce double vision. Damage to the muscles that move the eyes or the nerves that control eye movement can create a double image.
Damage in specific parts of the eye, like the lens or cornea, can also cause diplopia.
A common cause of binocular double vision is a squint or strabismus. This condition occurs when the eyes are not correctly aligned. Strabismus is relatively common in children. However, the condition does not always result in double vision. Strabismus causes the eyes to look in slightly different directions.
This condition might be because the affected eye has the following difficulties:
Sometimes, a squint can return later in life for people who had a squint as a child. In some cases, the treatment of a squint can also cause double vision. This occurs because the brain had been suppressing signals from one of the eyes in an attempt to avoid double vision.
Other conditions that can cause double vision include:
Thyroid dysfunction: The thyroid gland is in the neck and produces a hormone called thyroxine. Changes in thyroid function can affect the external muscles that control the eye. This includes Graves’ ophthalmopathy, in which the eyes can appear to protrude because fat and tissue build up behind the eye
Stroke or transient ischemic attack: In a stroke, blood fails to reach the brain due to an obstruction in the blood vessels. This can affect the blood vessels supplying the brain or nerves controlling the eye muscles and cause double vision.
Aneurysm: This is a bulge in a blood vessel, and it can press on the nerve of the eye muscle.
Convergence insufficiency: In this condition, the eyes do not work together correctly. The cause is unknown but thought to be due to the neuromuscular ability (the nerves’ control of muscle function) being abnormal
Diabetes: This can affect the blood vessels that supply the retina at the back of the eye. It can also affect the nerves that control eye muscle movements.
Myasthenia gravis: This condition can cause weakness in the muscles, including those that control the eyes.
Brain tumors and cancers: A tumor or growth behind the eye can interfere with free movement or damage the optic nerve.
Multiple sclerosis (MS): MS affects the central nervous system, including the nerves in the eyes.
Black eye: An injury can cause blood and fluid to collect around the eye. This can put pressure on the eye itself or the muscles and nerves around it.
Head injury: Physical damage to the brain, nerves, muscles, or eye socket can restrict the movement of the eye and its muscles.
If double vision is noted when one eye is covered but not the other, this is referred to by eye specialists as monocular double vision. Monocular double vision is less common than binocular double vision.
The following conditions can cause monocular double vision or vice versa:
Astigmatism: The cornea, or the transparent layer at the front of the eye, is irregularly shaped. With astigmatism, the cornea has two curves on the surface similar to a football instead of being perfectly round like a basketball.
Dry eye: The eye does not produce enough tears or dries out too quickly.
Keratoconus: This is a degenerative eye condition that causes the cornea to become thin and cone-shaped.
Retinal abnormalities: In macular degeneration, for example, the center of an individual’s field of vision slowly disappears, and sometimes there is swelling, which can cause double vision in one eye.
Cataracts: Cataracts occur in more than half of all people in the U.S., ages 80 years and over. It can sometimes cause double vision in one eye.
Double vision can sometimes be temporary. Alcohol intoxication and drugs such as benzodiazepines, opioids, or certain anti-seizure medications can sometimes cause this. Head injuries such as concussions can also cause temporary double vision.
Being particularly tired or having strained eyes can bring on temporary double vision. If correct vision does not return quickly, seek medical attention as soon as possible.
If you have double vision, you may also notice:
Double vision that's new or doesn’t have a clear cause can be serious. Your doctor will probably use more than one test to find out what’s causing your diplopia. They might try:
Note: In accordance with 38 CFR 4.31, diplopia that is occasional or that is correctable with spectacles is evaluated at 0 percent.
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