Anisocoria is the medical term used to describe pupils of unequal size. Anisocoria is not a disease in and of itself. Instead, it is a symptom of disease, much as pain and swelling are symptoms of a broken bone.
The pupil is the black part in the center of the iris (the colored part of the eye). It is where light enters the eye, and it can dilate or constrict depending on how much light is present in a given situation. If you’ve ever been to the ophthalmologist or optometrist and had your eyes dilated, you know how sensitive you become to light as your pupils become wider than they normally would be.
When a pet has anisocoria, your vet will first try to answer two questions.
Which eye is abnormal?
Is the anisocoria a neurologic problem or an ophthalmic problem?
Because the abnormal pupil can be either larger or smaller than the normal pupil, it is important to determine which eye is normal. Knowing that the abnormal pupil is larger or smaller than it should be can give some clues as to the underlying reason for the anisocoria.
The muscles that dilate and constrict the pupil are controlled the same way as other muscles in the body—they receive impulses from the brain through nerves. When any hitch it the neural pathway occurs, be it from the brain or along the nerve itself, the message to the pupil to constrict or dilate can get lost, causing the difference in pupil size. A common neurologic cause of anisocoria is head trauma or concussion.
Several ophthalmic conditions can cause anisocoria, including:
- ocular lymphoma (or any ocular cancer)
- degenerative iris atrophy
- ocular pain
And then there are just some medical conditions that are not strictly neurologic or ophthalmic conditions that can lead to anisocoria. The most common one of these (in cats, at least) is feline leukemia. Feline leukemia can cause feline spastic pupil syndrome, leading to anisocoria that comes and goes, and often moves from one eye to the other.
Horner’s syndrome is another common cause of anisocoria in dogs and cats. This syndrome encompasses four symptoms: anisocoria, elevated third eyelid, recessed eyeball, and droopy upper eyelid. Horner’s syndrome occurs due to disrupted enervation to the eye for a myriad of reasons. In dogs, about half of the cases of Horner’s are called idiopathic, meaning that an underlying cause cannot be found. The cause of Horner’s in cats is found much more often.
Anisocoria is not generally uncomfortable, as long as the underlying cause is not uncomfortable. Like the pain associated with a broken bone, anisocoria will not resolve until the underlying problem causing it is resolved. The presence of anisocoria can mean that something very serious is going on with your pet’s health. If you notice your pet’s pupils are consistently two different sizes regardless of changing light, it’s important to schedule an appointment with your vet to have it checked out.