Crossed diplopia is seen in exotropia (divergent squint) and uncrossed diplopia is seen in esotropia (convergent squint). I shall first let you experience this before explaining how it happens.
First, keep your index finger about 20-30cm in front of your nose. Now look at the tip of your nose. This simulates convergent squint. You will now be able to see the finger as 2 separate images – one on the left and another on the right. Keeping your eyes fixated at the tip of your nose, close your left eye. The image on the left side disappears. Open your left eye and close your right eye. Now the image on the right side disappears.
Explanation: In convergent squint, the eyeball is rotated inward. Instead of falling on the fovea, the light from the object falls on the nasal side of the retina. Nasal side of the retina is is supposed to receive light from the temporal side of the visual field. So the image formed on the nasal side of the retina will be perceived by the brain as being on the temporal side of the visual field. So in convergent squint, instead of seeing a single image, 2 images will be seen in the temporal fields of each eye. The left eye sees the image on the left and the right eye sees the image on the right – This is UNCROSSED diplopia.
Now we can experience crossed diplopia. Keep your eyes fixated on the computer monitor. Keep your index finger about 15 cm in from of your eye. You can see 2 images of your finger. This is a simulation of divergent squint. Close your left eye. The image on the right side disappears! Open your left eye and now close your right eye. The image on the left will disappear. This is crossed diplopia! 🙂
Explanation: The mechanism acts in reverse in case of divergent squint. The eyeball being rotated outward, the image falls on the temporal side of the retina. Images formed on the temporal side of the retina is perceived as being from the nasal field by the brain. So the image seen by the left eye will be perceived as being on the right side and that seen by the right eye will be perceived as being on the left side – CROSSED diplopia.
It took me some time to understand the concept of crossed and uncrossed diplopia. So I decided to write this article to help other learn about them. I have tried my best to explain the concept. If you have any doubts regarding, please leave a comment below.