You notice that the pupils—the black circles in the center of your dog’s eyes—are no longer the same size. One has become significantly larger than the other, blocking out much of the iris (the colored part). Or one has become significantly smaller. Either way, it’s not normal. Both pupils are supposed to enlarge to the same degree to allow more light in when the ambient light has dimmed, and they’re supposed to shrink to the same degree to let in less light when the surrounding light has brightened. The condition of one pupil remaining a different size from the other no matter what is called anisocoria, from the Greek aniso, meaning “unequal,” kore, meaning “pupil,” and the Latin suffix ia, signifying “abnormal.”
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