Double vision (medically known as 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. Read more about treating double vision.
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.
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).
Tests for double vision in hospital
Your eyecare team will usually start by assessing your vision. They may ask you to read letters off a chart, measure any current glasses, 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 depend on whether the double vision is monocular or binocular, but may include taking images of your eye. A picture of your brain or eye muscles may also be taken using:
Blood tests may also be used to check for an underlying condition.
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.
Read more about the causes of double vision.
What causes monocular double vision?
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".
Causes of monocular double vision can include:
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're 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.
If you have just developed double vision, or you have other eye-related conditions, your driving ability is likely to be affected. Check with your GP or ophthalmologist (eye care specialist) if you have an eye condition and you're unsure whether it affects your ability to drive safely.
If problems with your vision affect your ability to drive and you hold a current driving licence, it's your legal obligation to inform the Driver and Vehicle Licensing Agency (DVLA). Visit the GOV.UK website to find out how to tell the DVLA about a medical condition.