When an individual sees a double image where there should only be one, it is referred to as double vision or diplopia. The two images can be side by side, one on top of the other, or a combination of both.
Double vision can affect just one eye (monocular) or both eyes (binocular). Treatments vary depending on the cause and type but include eye exercises, specially designed glasses, and surgery.

Other than the obvious difficulties of navigating the world while viewing a double image, double vision can also cause disruptions in balance, movement and reading ability.

There are a number of potential causes for double vision. As a result, the treatment depends on the particular patient. Some of the underlying causes are minor, others are serious. For this reason, any instances of double vision should be checked by a physician sooner rather than later.
This article will take a look at the potential causes, diagnosis, and treatment of double vision.
Fast facts on double vision (diplopia)
Here are some key points about double vision (diplopia).

• Double vision can be caused by a number of underlying conditions
•  Diplopia can affect just one eye or both
•  A childhood squint 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

Causes of binocular double vision

Double vision can be caused by a number of conditions.

Each eye creates its own image of the environment. These two representations are combined by the brain so that they are be perceived as one clear picture.
If damage occurs to the muscles that move the eyes or the nerves that control that movement, a double image can be created.
It is necessary for both eyes to be functioning together to create depth of field.
Alternatively, the muscles moving the eyes can become weakened by certain illnesses and produce double vision.
Squint
In some cases, the eyes may not be properly aligned. This condition is referred to as a squint or strabismus. Squints are relatively common in children but do not necessarily result in double vision.
Squints cause the eyes to look in slightly different directions. This might be because the affected eye muscle(s):

• Are paralysed or weak
• Have restricted movement
• Are too strong
• Have abnormal nerves controlling them

Sometimes, a treated squint in childhood can return in adult life and 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.
Temporary double vision
In some cases, double vision can be temporary. Temporary double vision is often caused by alcohol intoxication, benzodiazepines, opioids, or certain types of drugs for seizures and epilepsy. Head injuries, like 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 come back quickly, medical attention should be sought as soon as possible.
Other causes of double vision
Other than a squint, there are a number of potential conditions that can cause double vision. These include:

Certain thyroid problems can cause double vision.

•  Thyroid: the thyroid gland is situated 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 disease.
• Stroke or transient ischemic attack: blood vessels supplying the brain or nerves controlling the eye muscles can be affected.
•  Aneurysm: an aneurysm is a bulge in a blood vessel. This can press on the nerve of the eye muscle.
• Convergence insufficiency: a condition where 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 properly.
•  Keratoconus: a degenerative condition of the eye that causes the cornea to thin and become cone-shaped.
• Diabetes: the blood vessels that supply blood to the retina at the back of the eye can be affected by diabetes.
• Myasthenia gravis: this condition can cause weakness in the body’s muscles, including those controlling the eyes.
•  Brain tumour/cancer: a tumour or growth behind the eye can interfere with free movement or damage the eye’s nerves.
• Multiple sclerosis: a disease that affects the central nervous system, including nerves that supply the eyes.
• Head injury: physically damaging 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 and not when the other eye is covered, this is referred to as monocular double vision.
Monocular double vision is less common than binocular double vision and can be caused by the following conditions:

• Astigmatism: the cornea is irregularly shaped (the transparent layer at the front of the eye). This causes a refractive error; in other words, the light is bent more, or less, than it should be.
•  Dry eye: the eye does not produce enough tears, or it dries out too quickly
•  Black eye: an injury can cause blood and fluid to collect around the eye. This can press on the eye itself or the muscles and nerves around it.
•  Abnormality of the lens: for instance, cataracts, which are cloudy patches over the lens
• Abnormality of the retina: for example, macular degeneration, where the centre of an individual’s field of vision slowly disappears.

Diagnosing double vision
Diagnosing double vision can be challenging for an ophthalmologist (eye specialist) because of the vast array of potential causes. These can range from minor causes to some that are potentially more serious.
One of the first questions a specialist will ask is whether the diplopia 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 with the nerves that control the muscles of the eyes or with the muscles themselves.

Diagnosing double vision in children
As children cannot always express what they are perceiving, diagnosing double vision in children can be more difficult. Things to watch out for 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

Treatments for double vision
The course of treatment for double vision depends on the underlying cause:
Astigmatism
Astigmatism refers to an abnormally curved cornea. In many cases, corrective lenses (glasses or contact lenses) can counteract the curvature and correct the passage of incoming light into the eye. In certain cases, surgery may also be an option.

Cataracts
Cataracts commonly occur as we age and can sometimes cause double vision. In most cases, surgery is the best option. During the surgical procedure, the ‘cloudy’ lens which is the cause of the double vision is removed.

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
• Fixing thin, plastic, see-through prisms to glasses
• Botulinum toxin (Botox) injections into the eye muscles cause them to remain relaxed
• Wearing an eye patch
• Surgery on the muscles of the eye to correct their positioning