In normal vision, the two separate images from each of your eyes are combined by the brain into one single image. Double vision occurs when the brain cannot combine the images from each of the eyes into a single image. This is usually because the images are not similar enough because of problems with the muscles or nerves that control precisely where each eye is looking.
There are two main types of double vision: binocular and monocular.
Binocular diplopia is the most common type of double vision. It occurs when your eyes don’t align with each other as they normally would. Those with binocular double vision will find that covering either eye gets rid of the double image. A squint usually causes binocular double vision, but it could be a symptom of a much more serious medical condition if it occurs suddenly.
Diseases of the thyroid or arteries, diabetes, multiple sclerosis, stroke, and other serious conditions can cause double vision. It’s therefore essential to make an appointment with an eye specialist as soon as possible if you have double vision.
Monocular diplopia is double vision in one eye only. It is much less common than binocular diplopia. If you have monocular double vision, only covering the affected eye will make the double image disappear. An abnormality usually causes monocular double vision within the eye. This could be a refractive error like astigmatism or a rare type of cataract. It can also be caused by abnormalities of the iris, lens, or fluid within the eye, and even dry eye.