Overview of Double Vision

‘Double vision’ is the common term given to the experience of seeing two images of a single object, rather than seeing a single image.

 The condition’s proper medical name is diplopia, which stems from two Greek words for ‘double’ and ‘of the eye.’  It can affect one eye (when it is known as ‘monocular diplopia’) or both eyes (when it is known as binocular diplopia), depending largely on the cause.  The treatment required will depend on the cause and extent of the diplopia. 

Normally, our brain takes the images seen through both our eyes (which both see a single image from slightly different angles) and combines them to create a single image to be perceived.  That process is known as ‘fusion’ and it breaks down in the case of diplopia.

Below are set out several of the main causes of diplopia and details of the diagnosis process and treatment options.  First, though, is a summary of the main symptoms of double vision and how they can be experienced by a sufferer.

Symptoms of Diplopia

Some sufferers report that they can see two images side by side, whilst others report a blurring of the two images so that they appear almost on top of each other (this is not the same as simple ‘blurred vision’, where a single image can seem blurred, but is rather two images on top of each other).

Someone suffering from diplopia may experience double vision in one or both eyes all the time, or it can come and go from time to time.  Sometimes, looking in a certain direction can induce diplopia, whilst other sufferers experience double vision wherever they look.

Diplopia can be easily articulated by an adult patient and it is easy enough for a GP to make a preliminary diagnosis after careful questioning so that an appropriate referral to an ophthalmologist can be made.  However, children with diplopia may be less able to describe their symptoms and indeed if it is something that they have always experienced they may be unaware that their perception of the world is not ‘normal.’

Children with diplopia can sometimes be seen to squint to narrow their eyes, looking as though they are straining to focus on something.  This may be particularly apparent when they are trying to see detail, such as when reading.  They could also tilt their head to look at objects from another angle.  If they ignore one of the images, they could cause the eye that is ‘seeing’ that image to permanently lose sight through lack of use.  

A squint in one eye is also another sign that double vision may be present and should be investigated by a doctor.

Causes of Diplopia

Diplopia itself can be a symptom of a serious underlying medical condition and as such it is very important that a sufferer should seek the advice of their GP without undue delay. 

Damage to face or head can cause diplopia, either by damaging the orbital socket or the muscles and nerves of the eyes, or by damaging the brain to make it less able to ‘fuse’ the images sent to it by the eyes.

If a head or facial injury is not indicated, there are specific causes that an ophthalmologist would normally consider after identifying  whether a person was experiencing monocular or binocular diplopia.

Common causes of monocular diplopia include:
• A misshapen cornea (often described as being ‘rugby ball shaped’ rather than ‘football shaped’) known as astigmatism, or other problems with the cornea.
• Problems with the lens (the centre of the eye) such as cataracts.
• Sight deterioration due to problems with the retina.
• Dry eye, a lack of sufficient moisture when blinking due to problems with the tear ducts.

Common causes of binocular diplopia include:
• A squint (which in medical terms is called a strabismus).  Your eyes point in slightly different directions, giving very different angles of vision to your brain to fuse.This could be because the muscles in your eyes cannot move properly, or are too weak or too strong.
• Problems with the thyroid gland that affect the muscles of the eye.
• Problems with the nerves that control the muscles of the eye, perhaps due to problems with the blood supply to those muscles or to the brain.
• Multiple Sclerosis.
• Diabetes.
• Myasthenia gravis.
• A brain tumour affecting the eyes by restricting their movement or damaging their nerves.
• An aneurysm (weakness in one of the blood vessels supplying the eyes).

Diagnosis

It is important that a sufferer seeks medical advice and their first point of call is to their GP, who can decide if further treatment is needed.  They will usually refer patients to an ophthalmologist, who may also call upon the skills of an optometrist to test the eyes to see if glasses might help, and an orthoptist to see if there are problems with the muscles of the eyes or with how the eyes are working together.

The ophthalmologist will establish whether a sufferer has monocular or binocular diplopia to narrow down the possible causes and ensure that the proper treatment is provided.  A simple way to test this is to see whether the problem disappears when one eye is covered (if the weaker eye is covered, the problem should disappear in cases of monocular diplopia).  A standard sight test is usually used, just like when a person goes to the optician for a routine eye test. 

Close examination is made of the eyes under a bright light so that the retina, lens and cornea can be seen properly.  Questions will be asked about any history of double vision, or squints in childhood, and a general overview of a patient’s health will be taken.

The ophthalmologist might also take blood tests and may ask for a CT scan or MRI of a patient’s brain if it is thought that the problem could lie in the brain rather than in the physiology of the eye.

Treatment of Diplopia

Treatment will depend on whether the diplopia is monocular or binocular.  The cause will also determine the treatment because treatment of an underlying medical condition might coincidentally resolve the double vision.   If diplopia is the only problem, then how it will be treated will depend on its cause.

Monocular diplopia caused by astigmatism can be resolved quickly and easily by using corrective lenses in the form of glasses or even contact lenses if the sufferer is over 12 years old, or possibly through laser treatment for the astigmatism. 
Monocular diplopia caused by cataracts can be resolved through simple cataract surgery, which removes the clouding from the lens. 

Binocular diplopia can be treated by wearing glasses or by doing eye exercises to strengthen/weaken a problematic muscle.  Sometimes wearing a patch over one eye can help by forcing the ‘weak’ eye’s muscles to work harder and strengthen so that when the eye patch is removed both eyes will be of equal strength. 

If the muscles are the problem, injections of Botox into the muscles can temporarily slacken a too-tight muscle, whilst surgery on the eye muscles is another solution.

Problems with Treatment

There are known side-effects and complications involved in some of the available treatments.  Any surgery carries risks because of the use of local or general anaesthetic (most surgery on eyes is carried out under local anaesthetic). 

It is possible that surgery to correct a problem with an eye muscle could lead to the eyes not being aligned properly and may not resolve the diplopia – it may even make the diplopia worse.

The likelihood of surgery succeeding in curing diplopia can depend (in the case of injury to the orbital socket causing the diplopia) on how quickly treatment is given.