An owl with unequal pupils

This barn owl has unequal pupils – anisocoria if you want a bit of ancient Greek thrown in for good measure! But how are we to know if its the eye with the small miotic pupil which is abnormal or the one with the large mydriatic pupil?

Well truth be told bird eyes are a bit different from mammalian ones – their irises consist of striated not smooth muscle and their optic nerves decussate (cross over) completely at the optic chiasm. But those differences don’t change this simple test in cases of anisocoria. What happens when the animal is in the dark?

Sorry the photo is a bit blurred – the camera has difficulty focussing in the dark! But you can see that the anisocoria is less in the dark than it is in the light. So the left pupil which is miotic in the light dilates in the dark showing that this iris and its associated nerve supply is working correctly. It is the right eye where the pupil is not constricting in the light. This may be a defect of iris atrophy in this old bird or an oculomotor nerve defect. As the iris looks normal I’m leaing towards a neurological explanation. You might expect a lateral strabismus in an oculomotor neuropathy, but the owl eye is so tightly packed into the orbit that such a change in globe position will not happen.