Encyclopaedia


Double vision

Introduction

Double vision (medically known as diplopia) is where you see two images of a single object either some or all of the time. The two images may be one on top of the other, side by side or a mix of both.

The underlying cause of your double vision depends on whether your double vision is coming from one eye or both eyes. This will also affect any treatment you receive.

Treatment options range from special glasses and eye exercises, to surgery to remove a cataract. Read more about treating double vision.

When to see your GP

You should visit your GP as soon as possible if you have developed double vision. If you have not had double vision before, it is very important that you have it checked as it could be a symptom of a serious medical condition.

Read more about the symptoms of double vision.

What causes double vision?

Each eye creates its own, slightly different image because the two eyes are physically located in different positions. However, you usually only see one image. This is because your brain is able to control your eye muscles so that both eyes are pointing accurately at the object you are looking at. Your brain joins the images produced by each eye together into a single image in a process known as fusion.
 
If your eye muscles or nerves are damaged, the muscles may not be able to control your eyes properly and you may develop double vision. Your eye muscles can also be weakened as a result of a health condition, or your eyes may not be aligned properly. This is sometimes known as a squint.

Read more about the causes of double vision.

Double vision in one eye

Double vision in one eye is known as monocular double vision. You should be able to see normally if the affected eye is covered, but double vision continues when the unaffected eye is covered.

In cases of monocular double vision, the two images are often only slightly separated. This is sometimes referred to as ‘ghosting’.

Double vision in both eyes

Double vision in both eyes is known as binocular double vision. It happens when both eyes fail to work together properly. If you have binocular double vision, your vision will usually return to normal if either eye is covered.

Physiological double vision

You may experience physiological double vision when objects in your background field of vision, which you are not specifically focusing on, appear double.

Your brain usually compensates for this type of double vision and it often goes unnoticed. However, children who complain of having double vision sometimes have physiological double vision.

Driving

If you have new double vision, or other eye-related conditions, your driving ability is likely to be affected. You should seek advice from your GP or ophthalmologist (eye care specialist) if you have an eye condition and you are unsure whether it affects your ability to drive safely.

If you have a problem with your vision that affects your ability to drive and you hold a current driving licence, it is your legal obligation to inform the Driver and Vehicle Licensing Agency (DVLA). Visit the Directgov website to find out how to tell the DVLA about a medical condition.

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Symptoms

Symptoms of double vision 

Double vision is where you see two images of a single object. It is sometimes confused with blurred vision. If you have blurred vision, a single image will appear, but will be unclear, whereas if you have double vision you will see two images at the same time.

Double vision may be constant, it may come and go, or it may only occur when you are looking in a particular direction.

A weakness in the muscles on either side of your eyeball can cause horizontal double vision, where you see two images side by side, when you are looking in a particular direction (to the left or right). It can also cause your eyes to appear ‘crossed’, or as if they are wandering. 

You should visit your GP as soon as possible if you have developed double vision. If you have not had double vision before, it is very important that you contact your GP immediately. It could be a symptom of a serious medical condition.

Recognising double vision in children

Whereas adults are able to describe what is happening with their sight, young children may not be able to explain clearly what is wrong.

If your child has double vision, they may narrow or squeeze their eyes in an effort to see, or they may frequently cover one eye with their hand. They may also turn their head in unusual ways or look sideways instead of facing forward.

Sometimes, children may also look between the two images. However, they usually quickly learn to ignore one image. This process is called suppression and can lead to permanent loss of vision in that eye (amblyopia).

If you suspect that your child has double vision or a squint (ocular misalignment), take them to see your GP as soon as possible. It is very important that the condition is diagnosed and treated quickly.

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Causes

Double vision in one eye is known as monocular double vision. Double vision continues when the unaffected eye is covered, but you should be able to see normally when the affected eye is covered.

It can be caused by:

  • an irregularly shaped cornea (astigmatism), which is a refractive error
  • dry eye, which is when your eye does not produce enough tears
  • abnormalities of the cornea  
  • abnormalities of the lens, such as cataracts (cloudy patches over the lens) 
  • abnormalities of the retina, such as macular degeneration (a painless eye condition that causes vision loss)

Monocular double vision is rare. Astigmatism and cataracts are the most common causes.

Double vision in both eyes

Double vision that occurs when both eyes fail to work together properly is known as binocular double vision. If you have binocular double vision, your vision will usually be normal if either eye is covered.

Most cases of double vision are in both eyes. If you have binocular double vision, your eyes will point at slightly different angles, causing them to send different images to your brain. The images from each eye are too different for your brain to create a clear, single image. This results in you seeing double images.

Squints

A squint (strabismus) means your eyes are pointing at different angles. If you have a squint, your eyes do not look in the same direction because some of the muscles that control your eyes are either:

  • weak or paralysed (unable to move)
  • too strong 
  • restricted (prevented from moving) 

Squints are particularly common in young children. Not all squints cause double vision.

A squint can reoccur in someone who had a squint as a child. In some cases, treating the squint and realigning your eyes can result in double vision, as previously your brain may have been ignoring the signals from one of your eyes. When your brain ignores the signals from one eye, this is called suppression.

A reoccurrence of a childhood squint is a common cause of double vision in adults.

Conditions that can cause double vision

In adults, if double vision develops suddenly and is not due to the reoccurrence of a childhood squint, it may be a sign of another condition. Conditions that can cause binocular double vision include:

  • A thyroid condition that affects the external eye muscles. Your thyroid gland is found in your neck and produces hormones. 
  • A condition that affects the blood vessels that supply blood to the brain, or to the nerves that control the eye muscles (for example stroke or transient ischaemic attack).
  • Diabetes, which can cause double vision by damaging the blood vessels that supply blood to the eye. 
  • Myasthenia gravis, which is a condition that causes the body’s muscles to become weak. 
  • Multiple sclerosis, which is a condition that affects the central nervous system.
  • An aneurysm, which is a bulge in a blood vessel that is caused by a weakness in the blood vessel wall. 
  • A brain tumour or cancer behind the eye that prevents free movement or damages the nerves to the eye muscles.
  • A head injury that damages the brain or the nerves that move the eye muscles, or damages the eye socket and restricts the movement of the eye muscles.

How your eyes work

Your eyes are made up of: 

  • the cornea and lens at the front of the eye, which act like a camera lens, helping to focus the light coming into the eye 
  • the retina at the back of the eye, which is a light-sensitive layer of tissue that senses light and colour and converts the light that enters the eye into electrical signals 
  • the optic nerve at the back of the eye, which transmits the electrical signals from the retina to the brain, where they are converted into an image
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Diagnosis

If you have double vision, your GP will probably refer you to an ophthalmologist (eye specialist) at the eye department of your local hospital.

Ophthalmologists commonly work in a team with:

  • orthoptists, who are specialists in problems that relate to eye movements and how the eyes work together 
  • optometrists, who carry out eye examinations and assess your need for glasses

Initial tests

The first step in diagnosing double vision is to establish whether your double vision affects:

  • both eyes (binocular double vision)
  • one eye (monocular double vision)

Read more about the causes of double vision.

Sight tests

Your eye care team will usually start by assessing your vision. For example, they may ask you to read letters off a chart, and will also look at the position of your eyes and how well you can move them. They may also assess how well your eyes are working together (your binocular vision). Sometimes your optometrist will try out different lenses in front of your eyes to see if you need glasses. This is called a refraction test.

Your ophthalmologist will also use a microscope with a very bright light, called a slit lamp, to examine the structures at the front and back of your eyes.

Your ophthalmologist will use the results of this examination, together with your medical history and any other symptoms you have, to determine what could be causing your double vision. They may then arrange for you to have any appropriate tests and treatment.

Read more about treating double vision.

Further tests

Further tests may include blood tests, and a picture of your brain or eye muscles may be taken using: 

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Treatment

If you have double vision, your treatment will depend on whether you have double vision in one or both eyes and the underlying cause. Monocular double vision is double vision in one eye. Binocular double vision is double vision that occurs when both eyes fail to work together properly.

An underlying condition that is causing your double vision may be treated separately. Read more about the causes of double vision.

Double vision in one eye

If you are diagnosed with monocular double vision, the type of treatment you receive will depend on the underlying condition that is causing the problem.

Astigmatism

If you have monocular double vision that is caused by astigmatism (an abnormally curved cornea), you may be prescribed corrective lenses, such as glasses or contact lenses. Most children who are over 12 years of age are able to wear contact lenses.

Corrective lenses work by compensating for the uneven curve of the cornea so that the incoming light that passes through the corrective lens is properly focused on the retina.

Laser surgery is another possible option for treating astigmatism, but it is not usually available free of charge on the NHS.

Read more about treating astigmatism.

Cataracts

If you have monocular double vision that is caused by cataracts (clouding of the lens), you may need to have cataract surgery to remove them. If you have cataracts in both eyes, you will have them removed on separate occasions to give the first eye time to heal and to allow your vision to return.

Cataract operations are common procedures and keyhole surgery is often used. The procedure is usually performed as day surgery, which means that you should be able to go home shortly afterwards. After the operation, you will need someone to look after you for the first 24 hours.

Read more about cataract surgery, including how it is carried out and your recovery.

Double vision in both eyes

Treatment for binocular double vision, as with monocular double vision, depends on the underlying cause. Possible treatment options include:

  • wearing glasses
  • doing special eye exercises 
  • fixing prisms on your glasses
  • wearing an eye patch over one eye
  • wearing an opaque (not see through) contact lens in one eye
  • having botulinum toxin injections into an eye muscle
  • having surgery on your eye muscles

Sometimes, children are able to ignore double vision because their brain is able to deal with the problem by ignoring, or ‘suppressing’, one of the two images.

For most children with a squint (when the eyes are pointing at different angles), the outlook is very good provided the condition is detected and treated early. Read more about treating squints.

Prisms

A prism is a wedge-shaped piece of glass or plastic that bends the light that shines though it. Special prisms called Fresnel (pronounced frennel) prisms can be attached to your glasses.

Fresnel prisms are thin, see-through sheets of plastic. One side sticks to the lens of your glasses and the other side has special grooves in it that change the way the light enters your eye. Fresnel prisms can be very effective at treating double vision.

You may need to wear the Fresnel prisms for several months. The strength of the Fresnel prisms can be adjusted to suit your eyes. If the Fresnel prisms are successful, you can have glasses made with prisms built in.

Botulinum toxin

Botulinum toxin injections are sometimes used to treat eye movement disorders, such as squints. Botulinum toxin is a poison, but in very small doses it is used to treat a range of different health conditions.

The botulinum toxin is injected into one of the muscles that controls the movement of your eye. It blocks the chemical messages that are sent from the nerves and causes the muscle to relax. This means the muscle can no longer move your eye and the other muscles take over and straighten the eye.

After the injection, you may experience some temporary side effects such as:

  • a droopy eyelid (ptosis)
  • overcorrecting the position of your eye so that your eyes are not aligned properly, which can mean your eye ‘drifts’ slightly and appears as if one eye is looking up or in the opposite direction

Surgery on your eye muscles

If you have a squint that is causing double vision, you can have surgery on your eye muscles to correct the position of your eyes. Any decision to have surgery is made on the basis that the benefit offered by the surgery (usually to align your eyes) is bigger than any risks of the surgery.

There are six muscles controlling the position of each eye. Each muscle can be weakened, strengthened or moved. Depending on your particular squint, surgery can be carried out on the eye muscles to change the position of your eyes and realign them.

The risks of squint surgery include:

  • making any double vision worse
  • damaging the eye or your vision
  • having a poor result from the operation that could mean you need further squint surgery

Your eye care team will discuss the risks and benefits with you.

Opaque intraocular lens

For some cases of double vision, a surgical procedure known as opaque intraocular lens (IOL) may be an option. However, this procedure is only recommended when other treatment methods have been unsuccessful.

An opaque intraocular lens can only be used to treat people who have double vision in both eyes, not in one eye. The procedure usually involves removing the lens of the eye, either under local anaesthetic or general anaesthetic, and replacing it with an implant.

Although it is considered to be a safe procedure, there are also some risks that are associated with having an opaque intraocular lens fitted. These include:

  • the intraocular lens breaking after it has been inserted
  • damage to the natural lens of the eye (if it is not removed)

Therefore, before having an opaque intraocular lens implant, you should discuss the implications of the procedure with your ophthalmologist (eye specialist). The opaque intraocular lens procedure is irreversible and removing the implant would involve significant risk of damaging your eye and impairing your vision.

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Prevention

Most forms of double vision, such as double vision that is the result of a squint or astigmatism, cannot be prevented. However, there are some things that you can do to help lower your risk of getting double vision.

Diabetes

If you have diabetes, you can lower your risk of developing double vision by keeping your blood sugar level under control. Read more about diabetes.

Cataracts

Although it is not possible to prevent age-related cataracts, you may be able to lower your risk of developing cataracts by: 

  • stopping smoking (if you smoke)
  • protecting your eyes from sunlight by wearing good quality sunglasses and a wide-brimmed hat
  • ensuring that you eat a healthy, balanced diet

Childhood cataracts often run in families, but they can also be caused by infections caught by the mother during pregnancy.

If you are trying for a baby, it is a good idea to ensure that you are immune to common infections, such as rubella (German measles). Your GP can check your immunity and, if necessary, you can have the MMR vaccine. You should take care to avoid becoming pregnant for one month after having the vaccine.

Head injury

To help reduce the risk of double vision caused by a head injury, you should wear a seatbelt while driving.

Wear protective headgear, such as a helmet and goggles, when using machinery such as drills or angle grinders, and during sporting activities such as cycling, rollerblading and skiing.

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The information on this page has been adapted by NHS Wales from original content supplied by NHS Choices.

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