Double vision refers to a symptom where two objects are seen when only one exists in reality. The objects may be separated horizontally, vertically or obliquely. The most important distinction to make in patients with double vision is whether it persists after either eye is covered. Monocular diplopia is the term used to describe double vision that persists when one eye is covered and binocular diplopia refers to double vision that resolves when one eye is covered.
Monocular diplopia is a result of problems with the eye itself such as an abnormal shape of the cornea (eg. Astigmatism) or lens. These problems can be diagnosed by an ophthalmologist. Binocular diplopia is a result of misalignment of the eyes. Normally, our eyes move together and are directed towards the same object. When there is weakness or restriction of an eye muscle, the eyes may be directed towards different objects resulting in double vision.
Binocular diplopia may be caused by a number of conditions. It may be secondary to a problem with a cranial nerve, extraocular muscle, or the communication between a cranial nerve and muscle (neuromuscular junction). It may also be the result of congenital eye misalignments that decompensate later in life. To elucidate the underling cause of binocular diplopia, investigations such as MRIs or blood tests may be required. Some causes may resolve on their own whereas others may require treatment.
Regardless of the cause, double vision can be symptomatically treated in one of three ways:
- Patching one eye or placing a piece of tape over an eyeglass lens. In adults, patching one eye will not harm the eye, weaken the eye, or strain the other eye. It does not matter which eye is covered.
- Prism glasses. These glasses are used in patients with stable double vision with a deviation between the eyes that is not very large. Prisms shift the image to help align it with the other eye. They are useful for eliminating double vision when looking straight ahead or when reading, but may not eliminate double vision when looking in the other directions.
- Strabismus surgery. This surgery involves altering the position of the eye muscles on the eye and can be used in patients with a stable deviation between the eyes. An ophthalmologist specializing in strabismus surgery typically performs this procedure.