Anisocoria in Cats: Why Your Cat’s Eyes Are Different Pupil Sizes

If you’re spending time with your furry feline and you notice the pupils of their eyes are not the same size, it can be alarming. This could be a condition known as anisocoria.  

If you have just noticed this for the first time, visit a veterinarian immediately so the underlying cause can be identified and treated. Delaying treatment can put your cat’s vision at risk.  

What Is Anisocoria in Cats?

Anisocoria in cats refers to an eye condition in which the pupils are different sizes. There are many potential causes, ranging in severity.  

A cat’s pupil size is controlled by the nervous system. When these nerve pathways are disrupted, the pupils don’t react to light as they should—so one may dilate while the other constricts, resulting in two different pupil sizes.  

Anisocoria should be treated as a medical emergency because it often develops from serious underlying health issues. If you notice one pupil suddenly becoming larger than the other, call your veterinarian immediately. Prompt diagnosis and treatment are crucial for a positive outcome.   

Causes of Anisocoria in Cats

Several health issues can cause a cat to have pupils of different sizes, including:  

• Trauma or injury  

• Horner’s syndrome 

• Glaucoma 

• Uveitis  

• Corneal ulcers 

• Aging  

• Bacterial or viral infections (such as FeLV) 

• Toxin ingestion (such as permethrins) 

• Medication overdose (such as ivermectin) 

• Eye or brain tumors 

• Optic neuritis  

Symptoms of Anisocoria in Cats

In addition to different pupil size, accompanying symptoms of anisocoria may include:  

• Eye redness 

• Eye discharge 

• Pawing at the eye 

• Squinting 

• Cloudy cornea (outer part of the eye) 

• Droopy eyelids 

• Prominent third eyelid 

• Lethargy 

• Decreased appetite 

How Veterinarians Diagnose Anisocoria in Cats

If a cat has pupils of different sizes, they should be seen by a veterinarian promptly.  

At the clinic, the veterinarian will perform a complete physical exam to look for abnormalities in the cat’s condition and will ask when the signs began, whether the cat may have ingested anything unusual, and if there was any recent trauma.  

Additional tests that may help determine the cause of anisocoria include:  

• Complete ophthalmic exam: Using special tools such as an ophthalmoscope and tonometer to check eye pressure, internal eye structures, and nerve function  

• Neurological evaluation: To check eye nerve reflexes and function  

• Bloodwork: to assess overall health and organ function 

• Urinalysis: To rule out systemic (whole-body) illnesses that may be contributing to anisocoria 

• Blood pressure test: High blood pressure can affect the retina and the nerves within the eye 

• Imaging: X-rays, ultrasound, or CT scans to identify trauma or tumors  

Treatment of Anisocoria in Cats

Treatment of anisocoria in cats is directed at the underlying cause and may last from a couple of days to a few weeks. A veterinarian may prescribe:  

• Eye ointment (such as Terramycin) 

• Eye drops (such as dorzolamide) 

• Antibiotics (such as doxycycline) 

• Pain medications (such as Onsior) 

• Anti-inflammatories (such as prednisolone) 

• Activated charcoal (to be administered for toxin ingestion) 

• Surgery (to correct physical trauma or remove tumors) 

Additional medications may be recommended based on your cat’s specific condition. Some cats may need hospitalization and supportive care to treat their underlying health issues before continuing their recovery at home.  

Recovery and Management of Anisocoria in Cats

In many cases, anisocoria can be resolved when the underlying cause is treated successfully. The prognosis is better when treatment is started promptly. If left untreated, serious complications such as blindness can occur.  

Sometimes anisocoria becomes permanent, especially if the nerves that control pupil function are damaged due to trauma.