Diplopia is the perception of two images when only one is present. The double images can be side by side (horizontal), on top of each other (vertical), diagonal (horizontal and vertical), or tilted (torsional). Additionally, there is constant diplopia, where double images are seen all the time, and intermittent diplopia, where this occurs only from time to time.
Monocular diplopia occurs when the double images are seen with only one eye and are present even when the other (unaffected) eye is closed.
Monocular double vision is usually caused by a refractive error (e.g., astigmatism), dry eyes, or cataracts. Rarely, issues at the back of the eye (retina) can also cause monocular double vision.
Binocular diplopia occurs when double images are seen with both eyes open, but the second image disappears when one eye is closed. Binocular diplopia is usually caused by a misalignment of the eyes (strabismus).
Some patients may experience both monocular and binocular double vision simultaneously.
The most common cause of binocular double vision is a misalignment of the eyes (strabismus). Strabismus can be constant (present all the time) or intermittent (occurring occasionally). When strabismus begins in early childhood, double vision typically does not occur. These children usually learn to “ignore” the double image, a process known as suppression.
Older children and adults who develop strabismus later in life have not learned to suppress the double image and therefore will see double vision.
A sudden onset of such an eye misalignment (strabismus) can be caused by a nerve palsy. For this reason, any type of double vision should be examined by an ophthalmologist.