Double vision occurs when a person sees a double image where there should only be one. The two images can be side by side, on top of one another, or both.
The condition can affect balance, movement, and reading ability.
If double vision affects just one eye, it is monocular. If it affects both eyes, it is binocular. Treatments depend on the cause and type, but they include eye exercises, specially designed glasses, and surgery.
This article will look at the causes, diagnosis, and treatment of double vision.
Fast facts on double vision
Here are some key points about double vision. More detail is in the main article.
- Double vision, or diplopia, can result from a range of underlying conditions.
- Diplopia can affect just one eye or both.
- A childhood squint, or eye turn, can sometimes recur and cause double vision.
- Temporary double vision can be caused by alcohol or other recreational drugs.
- Treatments can include surgery, eye exercises, or corrective lenses.
Nerve or muscle damage in the eye might cause double vision.
Each eye creates its own image of the environment. The brain combines the representations from each eye and perceives them as one clear picture.
Damage to the muscles that move the eyes or the nerves that control eye movement can create a double image.
The eyes must work together to create depth of field.
Certain illnesses can weaken the muscles moving the eyes and produce double vision.
Causes of binocular double vision
A common cause of binocular double vision is a squint or strabismus.
This occurs when the eyes are not properly 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 might be because the affected eye muscles have the following difficulties:
- They are paralyzed or weak.
- They have restricted movement.
- They are too strong or overactive.
- The nerves controlling the eyes muscles have abnormalities.
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 actually cause double vision, despite the individual's vision being normal before the squint was treated.
This is because the brain had been suppressing signals from one of the eyes in an attempt to maintain normal vision.
Other conditions 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 Grave's ophthalmopathy, in which the eyes can appear to protrude because fat and tissue build up behind the eye.
- Stroke or transient ischemic attack (TIA): 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: An aneurysm is a bulge in a blood vessel. This 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 it is thought to be due to the muscles that control the eye not lining up correctly.
- 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 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 is a disease that 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.
Causes of monocular double vision
If double vision is noted when one eye is covered but not the other, this is referred to as monocular double vision.
Monocular double vision is less common than binocular double vision. The following conditions can cause monocular double vision and can be caused by the following conditions:
- 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 it dries out too quickly.
- Keratoconus: This is a degenerative condition of the eye 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 United States over the age of 80 years and can sometimes cause double vision in one eye.
Temporary double vision
Double vision can sometimes be temporary. Alcohol intoxication, benzodiazepines, opioids, or certain medications for seizures and epilepsy 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 normal vision does not return quickly, seek medical attention as soon as possible.
Diagnosing double vision can be challenging for an eye specialist because there are so many possible causes.
An article from the American Academy of Ophthalmology website states:
"The patient who complains of double vision can have something as benign as dry eye or as life-threatening as an intracranial tumor. The cause may be as rare as Wernicke encephalopathy or as common as convergence insufficiency."
The specialist will start by asking whether the double vision is monocular or binocular.
If the double vision is monocular, it means that the problem is more likely to be within the eye, rather than in the nerves. It is likely to be less serious.
Diagnosis in children
Children cannot always express what they see, and this can make diagnosis difficult.
Physical signs of double vision include:
- squinting or narrowing the eyes to see
- covering one eye with their hand
- turning their head in an unusual way
- looking at objects from the side rather than facing forward
- flicking eyes side to side, between images
This will depend on the underlying cause.
Treatment for monocular double vision
Treatment will depend on the cause.
Astigmatism: This refers to an abnormally curved cornea. Corrective glasses or contact lenses often can counteract the curvature and correct the passage of incoming light into the eye.
Laser surgery is another option. This treatment involves reshaping the cornea with a laser.
Cataracts: Surgery is usually the best option. The surgical procedure removes the clouding and the cause of the double vision. Complications include infection, pain, and possibly continued blurry or double vision, but prompt treatment can usually resolve these.
Dry eye: If the eyes do not produce enough tears or dry out too quickly, they can become inflamed and sore. This can result in double vision. Often, a prescription for tear substitute eye drops will relieve symptoms.
Treatments for binocular double vision
Depending on the cause, the treatments for binocular vision vary, but they include:
- wearing glasses
- eye exercises
- wearing an opaque contact lens
- botulinum toxin (Botox) injections into the eye muscles, causing them to remain relaxed
- wearing an eye patch
- surgery on the muscles of the eye to correct their positioning
A stick-on prism, placed between the eyes at the center of the spectacle frame, can also help to realign the images from each eye.
Exercises cannot treat many of the conditions that cause double vision. However, some exercises can help with convergence insufficiency.
- Focus on a detailed target, perhaps a thin stick or small text in a magazine.
- Hold this at eye-level, an arm's length away from you.
- Aim for the image to remain as a single image for as long as possible.
- Move the target towards the nose in a slow, steady fashion.
- When the single image becomes two images, your eyes have stopped collaborating. Focus intensely on bringing these images back together. Once they join, bring the target closer to the nose.
- Once you become unable to rejoin the images, move your hand back to its original position and start the exercise again.
- The normal convergence range is 10 centimeters (cm) away from the nose. Aim to keep the image as a single image up to the 10cm mark.
- An orthoptist may provide a tool known as a Dot Card to assist these steps.
- Choose a similar target to that in the smooth convergence exercise.
- Start the target at a 20 cm distance from the nose.
- Fix your gaze on the target for between 5 and 6 seconds.
- Switch to looking at a fixed object around 3 meters (m) away for around 2 to 3 seconds.
- Switch your vision back at the nearer target.
- Repeat this, gradually moving the target closer, until you can focus on the object when it is 10 cm away without double vision.
The effectiveness of these exercises is largely restricted to treating convergence insufficiency.
If symptoms do not improve, visit a doctor for further testing.