What does the size and shape of a cat’s pupils mean?

Disclaimer: This article, its content, and its related references do not constitute veterinary advice and should not be considered a substitute for veterinary care. If your pet is showing any symptoms that could indicate a medical emergency, please seek immediate emergency care.

A cat’s pupil size naturally changes in the dark and in bright light. However, constricted or dilated pupils that are not responding to light changes, pupils that appear misshapen, or pupil sizes that do not match each other can be a sign of an underlying health or eye condition. If you have a cat, read on to discover:

• How a cat’s pupils normally work
• Conditions that can affect a cat’s pupils
• How pupil changes in cats are diagnosed and treated

Diagnostic testing may be necessary to determine what’s causing a cat’s pupils to be too large, too small, misshapen, or unequal. There’s a wide range of conditions that can affect a cat’s pupils, each with different treatment options. Treatment can range from home supportive care to surgery and hospitalization, depending on the underlying cause.

The pupil is a black hole in the center of a cat’s eye that light passes through. The edges of the iris (the colored part of the eye) are what determine the size and shape of a cat’s pupil. The size of the opening of the iris is controlled by tiny muscles in the eye that adjust how much light enters the eye. Pupils become small or constricted in bright environments and larger or dilated in darker environments. Cats have vertical, elongated pupils rather than round pupils, such as those found in dogs and humans. Vertical-slit pupils are common in predatory animals that hunt in low light and may help a cat gauge distance while hunting.

Normal vs. abnormal pupils in cats

A healthy cat’s pupils should be black, slit-shaped, and of equal size. Pupil size will naturally fluctuate when the light in their environment changes. They should be large and dilated in dark environments and narrow or constricted in bright light.

Types of abnormal pupils in cats include:

• Pupils that are unresponsive to light
• Unequal pupils (anisocoria)
• Misshapen pupils

Unresponsive pupils don’t change as expected in different light conditions. They may appear as dilated in bright environments (mydriasis) or constricted when they shouldn’t be (miosis). Only one or both pupils may be affected. A misshapen pupil may be due to something within the eye partially or fully obstructing the pupil, such as a luxated lens or a tumor.

“In an unhealthy eye, the cornea is often cloudy and this makes it hard to see the pupil and iris,” explains Dr. Jo Myers, a veterinarian on Vetster. “The clear bubble over the eye (cornea) and fluid underneath (anterior chamber) need to be clear in order to clearly see the pupil and iris.”

Various health issues can cause pupils to become abnormally dilated, misshapen, or unequally sized. A veterinarian should be notified right away if abnormal changes to the eyes are observed at home.

Eye disorders

There are various eye issues that can lead to abnormal pupils in cats, such as:

• Glaucoma
• Progressive retinal atrophy
• Atrophy of the iris
• Retinal detachment
• Luxating eye lens
• Uveitis
• Eye tumors and cancer

Depending on the condition, a cat may have unequal pupils, abnormally constricted or dilated pupils, or misshapen pupils.

Horner’s syndrome, while not a disease itself, is a collection of symptoms that can help point a vet toward an accurate diagnosis. Symptoms include:

• Constricted pupil
• Elevated third eyelid
• Retraction of the eyeball into the head
• Slight drooping of the eyelid
• Increased pink color and warmth on the face

Horner’s syndrome typically affects only one side of the face, resulting in only one pupil becoming abnormally constricted.

Eye injury or toxicosis

Injuries to the eye can damage the iris or other parts of the eye, changing the pupil’s appearance. Common eye injuries in cats include:

• Corneal ulceration
• Facial burns
• Traumatic injury to the eye, head, or neck

Exposure to or ingestion of some toxins can also affect the appearance of a cat’s pupils. These toxins include:

• Flea and tick medication
• Venomous snake bite
• Rat bait poison (bromethalin toxicosis)
• Snail or slug bait
• Pesticides and herbicides (organophosphate toxicosis)

Toxicities often cause other symptoms, such as vomiting, diarrhea, lethargy, and ataxia, in addition to changes to the pupils. Some toxins, such as flea and tick medication or pesticides, do not need to be ingested to become toxic, as they are absorbed through the skin.

Infectious disease

Various systemic diseases in cats can alter pupil size and shape, or result in unequal pupil sizes. Infectious diseases that can affect a cat’s eyes include:

• Feline infectious peritonitis (FIP)
• Fungal or bacterial infections
• Rabies

These diseases often cause additional symptoms, such as appetite loss, changes in behavior, vomiting, diarrhea, and lethargy.

When a cat presents with unequal, abnormally shaped, or abnormally sized pupils, a veterinarian typically recommends a physical exam, eye (ocular) exam, and diagnostic tests to determine the underlying cause. Diagnostics may include:

• Tonometry, Schirmer tear test, or fluorescein stain test
• Blood tests and infectious-disease testing
• Neurologic exam
• Diagnostic imaging, such as X-rays, ultrasounds, CT scans, or MRI scans
• Biopsy

A definitive diagnosis is necessary to determine an effective treatment plan because there are many potential causes for abnormal pupils with varying treatment options. Once a diagnosis is confirmed, treatment may include:

• Eye or oral medication
• Toxicosis treatment
• Supportive care
• Surgery

Many eye problems can be managed at home with oral or topical medication, along with supportive care. In severe cases, hospitalization or surgery may be required to treat the underlying problem.

Do not attempt to treat your cat’s eye issue at home without consulting a veterinarian. A sudden change in a cat’s pupils can be a sign of a serious problem that requires veterinary care. Ineffective home treatment can further damage the eye and delay proper care.

Are large, small, or different-sized pupils an emergency?

A cat who has abnormal pupil sizes, unequal pupils, or different-looking pupils but is otherwise acting fine is probably not having a medical emergency. Most emergencies that involve pupil changes will have additional severe symptoms such as:

• Excessive drooling
• Vomiting or diarrhea
• Difficulty walking
• Loss of appetite
• Lethargy

It’s recommended that you promptly talk to a veterinarian if pupil changes are seen, just in case there’s a serious underlying issue. Earlier detection and treatment may lead to a better prognosis.

Talk to a veterinarian right away any time you notice changes in your cat’s eyes. Sudden changes in the pupils may be a sign of a serious problem that needs veterinary attention. Various health and eye disorders can threaten a cat’s vision. If you’re unsure how quickly your cat needs treatment, an online vet through the Vetster platform can help triage and offer advice based on your cat’s symptoms.

Why are my cat’s pupils so big?

A cat’s pupils will naturally become larger or dilated in a dark environment. It’s normal to observe changes in pupil size when the light changes. If a cat’s eyes remain dilated in a bright room and do not adjust, seek veterinary care, as this can point to an underlying health or eye issue.

Why is my cat’s pupil misshapen?

A healthy cat has vertical-slit pupils that grow larger and smaller based on the amount of light in their environment. Changes to the pupil’s elongated shape are usually due to a condition affecting the iris or something within the eye obstructing the pupil. Seek veterinary care if your cat has a misshapen pupil in one or both eyes.

What does it mean when a cat has one pupil bigger than the other?

Cats can have conditions and injuries that affect only one eye. One pupil may be bigger than the other because it’s abnormally dilated, or it may be normal and appear larger because the other pupil is abnormally constricted. Other conditions, such as iris or retinal atrophy, can change the shape of the iris or retina, resulting in a pupil that’s abnormally large. It’s important to seek veterinary advice if your cat’s pupil sizes are unequal.