Asymmetrical Pupils in Cats

Unequal pupil size in cats is known as anisocoria.
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Bandit awakens unfazed at what has happened during the night. One of his pupils is larger than the other -- a bizarre medical condition referred to as anisocoria. Bandit’s unusual case of anisocoria may be due to an underlying condition or visual deficit, requiring a visit to the vet.


In all cases of anisocoria, one pupil is visibly larger than the other. This could include one normal pupil with one larger pupil or one normal pupil with one smaller pupil. While anisocoria may not bother Bandit one bit, it could potentially cause eye or head pain, evident as your cat tilts or paws his head. Depending on the underlying condition, your cat’s affected eye may appear red and the cornea could become bluish or cloudy. Bandit may squint and his eye may be droopy. In some cases, discharge may secrete from the eye.


If you suspect your cat has anisocoria, consult your vet immediately. Anisocoria can be caused by several things, but the most common cause of unequal pupil size in cats is anterior uveitis. Uveitis is a type of inflammation of the eye that affects the smaller of the two pupils. Diseases can be found within the iris tissue and scar tissue can build up in the eye, resulting in anisocoria. Bandit may also be suffering from glaucoma, a condition that causes increased pressure in the eye. The affected eye often bulges from the socket and presents with an overly large pupil. Horner’s syndrome causes one pupil to become smaller, caused by a disruption of the innervation to the pupil.


At your vet’s office, Bandit will undergo a physical examination to determine if the cause of his anisocoria is due to head trauma, neurological disorders or any other abnormality of the eye. Your vet may perform tests to measure intraocular pressure and tear production. He may also collect conjunctival biopsies or scrapings that will be sent to the laboratory for testing. Ultrasounds can be performed on the eye to locate possible lesions in the affected eye. Lesions or growths in the brain may also be causing Bandit’s odd symptoms, generally diagnosed using computed tomography or magnetic resonance imaging. In some cases, your vet may decide to take some blood to rule out possible systemic conditions, such as feline leukemia.


Treatment for anisocoria will ultimately depend on your cat’s underlying condition. Depending on the diagnosis, your vet will provide a tailored treatment plan to help your cat recover from his anisocoria. Whether or not Bandit fully recovers will depend on the underlying condition. In some cases, your cat may require medication long-term to control his illness. Surgery may be required to remove any tumors, more commonly seen in older felines compared to younger pets. In severe cases, anisocoria may result in permanent blindness.

Always check with your veterinarian before changing your pet’s diet, medication, or physical activity routines. This information is not a substitute for a vet’s opinion.

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