Double vision

Double vision

 Double vision occurs when a person sees two images of a single object either some or all of the time. The medical name for double vision is diplopia.

In double vision, the two images may be vertically separated (one on top of the other), horizontally separated (side by side), or both vertically and horizontally separated (oblique).

Single binocular vision

Each eye works independently of the other, creating its own, slightly different image. However, you usually only see one image because the brain is able to control the eye muscles so that both eyes are pointing accurately at the object you are looking at. The brain joins the images produced by each eye together into a single image. This is known as ‘single binocular vision’.

However, if your eye muscles, or nerves, are damaged, or if they are weakened as a result of a health condition, such as myasthenia gravis, the muscles may not be able to control your eyes properly and you may develop double vision.

Types of double vision

There are three types of double vision:

  • binocular double vision,
  • monocular double vision, and
  • physiological double vision.

Binocular double vision

Binocular double vision is double vision that occurs 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.

Monocular double vision

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

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

Physiological double vision

Physiological double vision sometimes occurs ‘naturally’ when background objects (objects in a field of vision that someone is not specifically focusing on) appear double.

The 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 double vision, or other eye-related conditions, it is likely to have implications for driving. Seek advice from your GP or opthalmologist (eye care specialist) if you are unsure whether your eye condition affects your ability to drive safely.

Binocular double vision

Most cases of double vision are binocular. If you have binocular double vision, your eyes will point at slightly different angles, causing them to send a different image 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.

A squint (strabismus) is the most common reason for your eyes to point 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 weak or paralysed. Squints are particularly common in young children. Not all squints cause double vision.

Conditions that can cause binocular double vision

In adults, if binocular double vision develops suddenly, there is a high likelihood that it is a sign of disease. Conditions that can cause binocular double vision are listed below.

  • Thyroid disease that affects the external eye muscles.
  • Disease of the arteries that supply blood to the brain.
  • Diabetes. This can cause double vision by damaging the nerves that control eye movement. The damaged nerves often re-grow after several months and, as they do, your double vision will gradually disappear.
  • Myasthenia gravis, a condition that causes the body’s muscles to become weak and to tire easily.
  • Multiple sclerosis, a neurological condition that affects the central nervous system.
  • Aneurysm, bulging of the arteries in the brain.
  • A blood clot behind the eye that prevents normal eye movement.
  • Stroke.
  • Brain tumour, or cancer in or behind the eye that distorts the image produced by the eye.

A head injury that damages the muscles of the eye sockets, or the nerves that control these muscles, can also sometimes result in binocular double vision.

Monocular double vision

Monocular double vision is more rare than binocular double vision. It is usually caused by abnormalities of the:

  • cornea, the clear part of the eye that covers the iris and pupil,
  • iris, the coloured part of the eye,
  • lens, the transparent layer behind the pupil that focuses light on to the retina at the back of the eye,
  • vitreous humour, the jelly-like substance that fills the body of the eye, or
  • aqueous humour, the thin, watery fluid that fills the space behind the cornea, at the front of the eye.

The two most common causes of monocular double vision are:

  • Refractive errors, such as astigmatism (a condition where the cornea is abnormally curved, so that light coming into the eye is not focused properly on the retina).
  • A rare type of cataract (clouding of the lens) that splits the affected eye’s image.

Other possible causes of monocular double vision can include:

  • dry eye, when your eye does not produce enough tears,
  • a mass, or swelling, in your eyelid that presses on the front of your eye, and
  • an eye abnormality, such as a dislocated lens.

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

Initial tests

The first step in diagnosing double vision is to establish whether you have monocular or binocular double vision. To do this, the ophthalmologist will ask you to cover one eye and then the other in order to determine whether your double vision disappears when one of your eyes is covered.

Binocular double vision

If you have binocular double vision, the ophthalmologist will carry out a test to find out which eye muscles are being affected. They will ask you to look at their finger as they move it up, down, left and right. This will help them to determine how far your eye can move in each direction.

The ophthalmologist will also cover one of your eyes, followed by the other, while you focus on a target. If your eyes shift as the cover is removed, it means that your eyes are not properly aligned.

The test will be repeated, this time with a prism (a piece of glass that bends light) placed in front of each eye in turn to shift the image that you are seeing. The prism will enable the ophthalmologist to measure the degree of your double vision when you are looking in different directions.

Monocular double vision

If you have monocular double vision, the ophthalmologist will examine your eyes for eye conditions such as cataracts that could be causing the problem.

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

 

 

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

When it happens 

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

If you only get double vision when you are looking in a particular direction (to the left or right), it may be caused by paralysis (muscle weakness) of either the lateral rectus muscle or the medial rectus muscle. These muscles are located on either side of the eyeball and are responsible for turning the eye to the right and to the left.

If you have muscle paralysis, your eyes may appear ‘crossed’, misaligned, 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 clearly express what is wrong.

If your child has double vision, they may squint (narrow 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.

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

If you have double vision, your treatment will depend on whether you have binocular or monocular double vision, and the underlying cause.

Binocular double vision

The treatment of binocular double vision will depend on the underlying cause and the prognosis (outlook).

Treatment for binocular double vision may include:

  • wearing an eye patch over one eye,
  • using filters on glasses,
  • wearing an opaque (not transparent) contact lens in one eye,
  • having an eye muscle botulinum toxin injection, or
  • having eye muscle surgery.

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

Squint

Double vision that is caused by a squint (strabismus) can be corrected by wearing prescription glasses, doing eye exercises, having surgery, or by a combination of all three.

For most children with double vision that is caused by a squint, the outlook is very good provided that the condition is detected and treated early. It is much more difficult to correct double vision that is caused by a squint if it continues into adult life.

Botulinum toxin injections are sometimes used to treat eye movement disorders, such as squints. Botulinum toxin is a protein that is made from the same bacterium that causes food poisoning.

In very small doses, botulinum toxin injections can be used to stop muscle spasms. Chemical messages that are sent from the nerves cause the muscles to contract (tighten). The toxin helps to prevent the spasms by blocking the chemical messages.

The botulinum toxin is mixed with a saline solution (salt water) and injected into the muscle using a very fine needle. After the injection, your eye muscle may be slightly sore, but this should improve quickly.

Opaque intraocular lens (IOL)

For some cases of intractable double vision (double vision that persists), a new surgical procedure known as opaque intraocular lens (IOL) is now available. However, this procedure is only recommended when other treatment methods have been unsuccessful.

IOL can only be used to treat patients who have the binocular form of double vision (not monocular). The procedure involves removing the lens of the eye, either under local or general anaesthetic and replacing it with an implant.

There are two different techniques for removing the lens. Phacoemulsification is where the lens is removed in fragments, and extracapsular surgery involves removing the lens in one piece (similar to cataract surgery). An incision (cut) is made in the cornea, so that an opaque IOL can be inserted.

Alternatively, a specially manufactured tinted IOL, known as an iris claw lens, is surgically attached to the iris (the coloured part of the eye). The IOL is attached to the tissue of the iris by using a claw-like mechanism. This technique allows the eye’s natural lens to be kept in place.

Generally speaking, significant improvements can be expected in the symptoms of patients who have had the IOL procedure. For example, in a case study of 12 patients, an average satisfaction score of 3.4 was reported (using a scale where 0 = unhappy, and 4 = very satisfied).

However, there are also some risks that are associated with the IOL procedure, such as the possibility of the intraocular lens breaking after it has been inserted, or causing damage to the natural lens of the eye (where it is not removed).

Therefore, before having an IOL implant, you should fully discuss the implications of the procedure with your ophthalmologist. The IOL procedure is irreversible, and removing the implant would involve significant risk of damaging your eye and impairing your vision.

Monocular double vision

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 (abnormal curvature of the 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 to the retina.

Laser surgery may be another option for treating astigmatism.

Cataracts

If you have monocular double vision that is caused by cataracts (clouding of the lens), you may have cataract surgery in order to remove them. If you have cataracts in both eyes, you will have them removed on separate occasions in order 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.

See Useful links for more information about cataract surgery.

Dry eye syndrome

Dry eye syndrome (also known as keratoconjunctivitis sicca) is a condition in which the eyes do not make enough tears, or the tears evaporate too quickly. This can cause your eyes to dry out, and become sore and inflamed. Sometimes, it can also lead to vision problems such as double vision.

If you have monocular double vision that is caused by dry eyes, you may be given eye drops that contain ‘tear substitutes’, a liquid that is designed to mimic the properties of tears. Rarely, in severe cases, surgery may be required.