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.
If you have a problem with your vision that affects your ability to drive, and you hold a current driving licence, you are legally obliged to inform the Driving and Vehicle Licensing Agency (DVLA).