Double vision (its medical term is diplopia) is seeing two images of a single object. The two images may be one on top of the other, side by side, or a mix of both.
Double vision may be constant, it may come and go, or it may only occur when you're looking in a particular direction.
The cause of your double vision depends on whether your double vision is coming from one eye (monocular) or both eyes (binocular). This also affects which treatment you receive.
Treatment ranges from special glasses and eye exercises, to surgery to remove a cataract.
Recognising double vision in children
Adults are able to describe what is happening with their sight, but young children may not be able to explain what is wrong clearly.
If your child has double vision, they may narrow or squint their eyes closed in an effort to see better, or they may frequently cover one eye with their hand.
They may also turn their head in unusual ways (for example, tilting their head) or look sideways instead of facing forward to try to avoid double vision.
What causes binocular 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 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.
Adults who had a squint as a child may sometimes develop double vision over time.
When to see your GP
Visit your GP as soon as possible if you develop double vision or you suspect your child has double vision or a squint.
If you've never had double vision before, it's important to have it checked as it could be a symptom of a serious medical condition.
Your GP will probably refer you to an ophthalmologist (eye specialist) at your local hospital, or an orthoptist (eye movement specialist).
Causes of double vision
Double vision can occur in one or both eyes, and can be caused by several different conditions.
Double vision in one eye (monocular)
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
The cornea is located at the front of the eye and acts like a camera lens, helping to focus the light coming into the eye.
The retina is a light-sensitive layer of tissue at the back of the eye that converts light entering the eye into electrical signals.
Monocular double vision is rare. Astigmatism and cataracts are the most common causes.
Double vision in both eyes
Double vision caused by both eyes failing 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.
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.
A squint (strabismus) means your eyes are pointing at different angles. If you have a squint, your eyes don't look in the same direction because some of the muscles controlling 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 recur 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 childhood squint recurring is a common cause of double vision in adults.
Conditions that can cause double vision
In adults, if double vision develops suddenly and isn't caused by a childhood squint, it may be a sign of another condition affecting the free movement of the eye, or the muscles, nerves, or brain. Conditions may include:
- a thyroid condition affecting the external eye muscles – your thyroid gland is found in your neck and produces the hormone thyroxine
- a condition affecting the blood vessels supplying blood to the brain, or to the nerves controlling the eye muscles (for example, a stroke or a transient ischaemic attack)
- diabetes – which can damage the blood vessels supplying blood to the nerves of the eye muscles
- myasthenia gravis – which causes the body's muscles to become weak, including the eye muscles
- multiple sclerosis – which affects the central nervous system, including the nerves of the eye muscles
- an aneurysm – a bulge in a blood vessel caused by a weakness in the blood vessel wall, which can press on a nerve of the eye muscle
- a brain tumour or cancer behind the eye that prevents free movement or damages the nerves to the eye muscles
- a head injury damaging the brain or the nerves that move the eye muscles, or damages the eye socket and restricts the movement of the eye muscles
Diagnosing double vision
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 – specialists in problems relating to eye movements and how the eyes work together
- optometrists – who carry out eye examinations and assess your need for glasses
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)
Your eyecare team will usually start by assessing your vision. They may ask you to read letters off a chart, look at the position of your eyes, and assess how well you can move them. They may also assess how well your eyes are working together (your binocular vision).
Your ophthalmologist will also use a microscope with a very bright light, called a slit lamp, to examine the front and back of your eyes.
These results, together with your medical history and any other symptoms you have, should determine what could be causing your double vision.
Further tests may include blood tests. A picture of your brain or eye muscles may also be taken using:
- a computerised tomography (CT) scan – this takes a series of X-rays at slightly different angles and uses a computer to put the images together
- a magnetic resonance imaging (MRI) scan – this uses a strong magnetic field and radio waves to produce detailed images
Treating double vision
If you have double vision, your treatment will depend on whether you have double vision in one eye (monocular) or both eyes (binocular), and the underlying cause.
Underlying conditions may be treated separately.
Double vision in one eye
If you're diagnosed with monocular double vision, treatment will depend on the underlying condition causing the problem.
If you have monocular double vision caused by astigmatism (an abnormally curved cornea), you may be prescribed corrective lenses, such as glasses or contact lenses. Most children over the age of 12 are able to wear contact lenses.
Corrective lenses compensate for the uneven curve of the cornea so 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's not usually available free of charge on the NHS.
If you have monocular double vision caused by cataracts (clouding of the lens), you may need cataract surgery to remove them.
If you have cataracts in both eyes, you'll have them removed on separate occasions to give the first eye time to heal and allow your vision to return.
Cataract operations are common procedures, usually performed as day surgery, which means you should be able to go home shortly afterwards.
Read more about cataract surgery, including how it's carried out and your recovery.
Double vision in both eyes
Treatment for binocular double vision also depends on the underlying cause. Read more about the causes of double vision.
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 (Botox) injections into an eye muscle
- having surgery on your eye muscles
Sometimes children are able to ignore double vision because their brain deals with the problem by ignoring, or "suppressing", one of the two images.
Most squints in children (when the eyes are pointing at different angles) are treated successfully if the condition is detected and treated early. Read more about treating squints.
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, and are an effective way of treating double vision.
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.
You may need to wear prisms for several months. The strength can be adjusted to suit your eyes. If the Fresnel prisms are successful, you can have glasses made with prisms built in.
Botulinum toxin (Botox) injections are sometimes used to treat eye movement disorders, such as squints.
The botulinum toxin is injected into one of the muscles controlling the movement of your eye. It blocks the chemical messages 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 your eyes are not aligned properly – this can mean your eye "drifts" slightly, and appears as if one eye is looking up or in the opposite direction
- worse double vision
As the botulinum toxin wears off, you may be able to control the position of your eyes for a period of time. However, this timeframe can vary and further injections may be necessary.
Surgery on your eye muscles
If your double vision is caused by a squint, surgery on your eye muscles can 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.
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
- having a poor result from the operation, which could mean you need further squint surgery – this is estimated to affect around 1 in 10 people who have squint surgery
- in rare cases, damaging the eye or your vision – this is estimated to affect 1 in 2,400 people who have squint surgery
Your eyecare 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 treatments have been unsuccessful.
An opaque intraocular lens can only be used to treat people with binocular double vision (both eyes). The procedure usually involves removing the lens of the eye and replacing it with an implant.
Although it's considered a safe procedure, there are risks 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)
- inability to examine the back part of the eye (retina)
- reduced visual field
Before having an opaque intraocular lens implant, you should discuss the implications of the procedure with your eye specialist. The procedure is irreversible, and removing the implant would involve significant risk of damaging your eye and impairing your vision.
Source - www.nhs.uk