Understanding Double Vision

Double vision (also known as diplopia) is a condition where someone sees two images of a single object, the images may overlap, intermingle, or sit side by side. Because your eyes are a few centimetres apart each eye perceives a slightly different image. What happens normally is that the brain fuses these two images so we see one consistent image in a process called fusion. With diplopia, this process is compromised.

There are different kinds of double vision – monocular, binocular, and temporary. Temporary double vision may be caused by something external like a head injury or drunkenness. It should pass when the effects have worn off or if the underlying condition is successfully treated. Monocular double vision is when only one of your eyes is affected if this eye is covered up images should appear correctly.

Binocular double vision is when both eyes are failing to co-ordinate properly – if either eye is covered then vision should be normal.

Signs and symptoms of double vision

Double vision has quite clear symptoms – it may sometimes be confused with other eye conditions, but essentially it is when you see two images of a single object. If your vision is blurred or you have interference, such as lights or floaters, these will likely be another condition.

Sometimes diagnosing double vision in children may be difficult as they cannot articulate exactly what they are seeing. But if they exhibit behaviour such as squinting or covering their eyes or tilting their head in an unusual fashion, they may be experiencing some issues. If left untreated this kind of vision problem in children can develop into amblyopia (lazy eye) as the brain tries to compensate for the error. 

Regular eye tests for children and adults are essential for the early detection of such problems.

Causes of double vision

There are several different conditions that can cause double vision, and depending on whether you are talking about monocular or binocular double vision, they may be different. 

 Monocular double vision

There are several different causes, including:

• Astigmatism is when the cornea has an irregular curvature and leads to short- or long-sightedness. 
• Abnormalities of the lens or cornea, such as cataracts.
• Conditions that affect the retina, e.g. macular degeneration, where the cells of the macula – the part of the eye responsible for central vision – are damaged by excess deposits or abnormal blood vessels.
• Dry Eye Syndrome, where the eyes do not produce enough tears to sufficiently lubricate the eyes, causing them to become red and irritated, potentially leading to intermittent blurred vision.

Binocular double vision

Binocular Double Vision, where double vision is caused by both eyes not working properly, can be caused by a ‘squint’ (strabismus) – which is where the eyes point in different directions. This is caused when the extraocular muscles that control the movement of the eye are too weak, too strong, or not working in coordination. 

Treatments for fixing a squint may help with double vision, although sometimes can also bring on temporary double vision as the brain over-compensates for previous deficiencies. Often the cause of double vision in adulthood is the reoccurrence of a childhood squint. 

Double vision in adulthood

If double vision is persistent in someone with no previous history of a squint, there may be another factor affecting the way the eyes work. These include:

• Head injuries. If there is damage to the nerves, the eye muscles, or the socket.
• Diabetes. This condition can affect the blood supply to the eye as high glucose levels damage the blood vessels.
• Pressure on the optic nerve. This could be caused by a tumour, an aneurysm, or an abscess that interferes with the nerve linking the eye to the brain.
• Multiple Sclerosis. MS disrupts the ability of nerves to communicate often leading to problems with vision, such as diplopia. 

How the eyes work – Extraocular muscles

Movement of the eye is controlled by a group of muscles called the extraocular muscles. These six muscles pull the eye in different directions allowing it to focus on particular objects.

The fovea centralis is a tiny part of the eye that allows us to process incredibly sharp and accurate vision. The extraocular muscles are crucial for eye movement that is both fast and accurate, allowing the fovea to focus on specific objects.

When it comes to eye conditions like a squint or double vision, these muscles that typically work involuntarily and incoordination, do not work together. As such, one of the eyes may move and focus on objects perfectly well, while the other works independently. This may be a result of congenital problems with the nerves which send and receive messages to the brain, or as was outlined above, a variety of other illnesses.

How is double vision diagnosed? 

There are various types of doctor that may be involved in the diagnosis of double vision. Your GP may refer you to an ophthalmologist, but they will work together with an optometrist and an orthoptist who deals with eye examinations and eye movements respectively.

Determining whether the double vision is in both eyes or just in one is the most important initial step, as this could lead to very different diagnoses. They will start with basic eye tests – using an eye chart with lines of increasingly small letters to test your visual acuity (how clear your vision is). They will also conduct tests to check eye movement, such as covering one eye and asking you to focus on an object or asking you to track an object like a pen as they move it from side to side. This will allow them to see how well co-ordinated your eye movement is and how responsive your eyes are.

They will examine the back of the eyes using a slit lamp test, which shines a slit of very bright light into your eye while the specialist examines the front and back of the eye with a special microscope. They may also use eye drops that contain a yellow dye - when a light is shone on the dye it illuminates any damage to the cornea making it easy to see. The dye is harmless, but will dilate/blur your vision for a few hours as well as leaving temporary stains.

Usually, these tests will be enough to determine the cause of your double vision, especially if it is a condition you also suffered as a child. If further tests are needed you may be referred for an MRI or CT scan where doctors can access detailed images of your eye muscles and surrounding areas using magnetic fields or X-rays.

How do you treat double vision? 

The treatments depend on which form of double vision you have and what the specific cause is. Typically, an underlying condition is causing double vision and the treatment will involve rectifying that problem.

Monocular double vision

If the problem is caused by astigmatism, then corrective lenses, such as eyeglasses or contact lenses, can be used to compensate for the cornea’s irregular curve. Laser eye surgery can be used to permanently correct astigmatism; however, it is not available on the NHS – you would have to visit a specialist clinic. 

If cataracts are the problem, then surgery is necessary. Cataract surgery is quite common so any procedure should be quick and complication-free. The cloudy lens will be removed and replaced with a man-made lens. 

Binocular double vision

There are multiple treatments for Double Vision depending on the cause.

• Prescription glasses may be used to correct refractive errors like short- or long-sightedness. If the condition has developed into a lazy eye, then an eye patch will be used, which forces the patient to see only through the weaker eye, strengthening it and correcting the alignment.
• Eye exercises may be beneficial, they are designed to strengthen and re-align eyesight, in the same way, using a patch forces an eye to improve in strength. Most exercises involve repetitively focusing on different objects, such as a pencil until the eye muscles become tired. Much like with working out any other muscles, repetition and muscle memory are key to improvement.
• Fresnel lenses are sheets of plastic that can be attached to a pair of glasses in order to bend the light that shines through them. They act like prisms and are worn for several months, they can also be manufactured directly into a pair of glasses. 
• A botulinum toxin injection may sometimes be given. The toxin relaxes the muscles around the eye which can then gradually realign in the right direction. Unfortunately, there are sometimes side effects like a worsening of double vision, droopy eyelids, or muscle weakness in the area of the injection.
• If the other treatments fail, then surgery may be necessary. The surgery will be performed on the extraocular muscles, changing their tightness to balance the different muscles. Under a general anaesthetic a surgeon will operate on the muscles, adjusting their position and sewing them into a new position, held in place with dissolvable sutures. There are potential complications with this surgery, such as a deterioration of the condition, or in very rare cases, damage to the eye.
• The last resort may be the use of an IOL (opaque intraocular lens), the same man-made lens that is used to treat cataracts. This carries with it several risks, such as potential damage to the existing lens and reduced vision.