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What is Unequal Pupil Size?

Normally, the pupils expand and contract in unison. When unequal pupils occur, the abnormal eye can be either the smaller (constricted) pupil or the larger (dilated) pupil. There may or may not be other symptoms present. If unequal pupils occur suddenly, bring your cat to a veterinary clinic or animal hospital immediately as eyesight can be permanently damaged if the problem is not rectified. Certain underlying causes of unequal pupils can be life threatening.

Unequal pupils, or anisocoria as it is called in the medical community, occurs when one of a cat’s pupils (the black openings in the center of the iris) is either too far open or too far closed in relation to the other pupil. This gives the appearance of a small black dot in the center of one eye and a larger black dot in the center of the other. This condition is often the result of a more serious complication within the cat.

Unequal Pupil Size Average Cost

From 370 quotes ranging from $200 - $4,000

Average Cost

$1,000

Symptoms of Unequal Pupil Size in Cats

Only one symptom need be present to merit a veterinary examination. If any of the following arise after head trauma, veterinary attention will be needed immediately. 

  • One pupil larger than the other
  • Eye producing discharge
  • Drooping eyelid
  • Redness of the eye
  • Clouding cornea
  • Bluish cornea
  • Head tilting
  • Eye pain or irritation
  • Squinting
  • Activity decrease
  • Abnormal eye movement
  • Confusion
  • Change of position of eye in socket
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Causes of Unequal Pupil Size in Cats

Unequal pupils can be caused by both neurological (brain or nerves) and ocular (eye) issues. Because the eyes are so closely connected to the brain, often damage and injury of this area affect both parts of the body. All known causes include:

  • Concussion (bleeding and swelling of the brain from head trauma)
  • Injury to optic nerves
  • Neurological disorder (such as Horner’s Syndrome)
  • Oculomotor nerve paralysis (damage to the nerve controlling eye movement)
  • Cerebellum (brain) injury
  • High blood pressure
  • Corneal ulcer or injury
  • Glaucoma (pressure from excess fluid dilates the pupil)
  • Anterior uveitis (inflammation of urea constricts the pupil)
  • Retinal disease
  • Scar tissue
  • Iris atrophy (mainly in older cats)
  • Defective iris from birth
  • Cancer or cancerous tumors
  • Spastic pupil syndrome 
  • Feline leukemia virus
  • Stroke (ruptured blood vessels in the brain)
  • Eye infection
  • Medication (some can cause the pupils to dilate)
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Diagnosis of Unequal Pupil Size in Cats

To start the diagnostic process, the vet will require your cat's full medical history along with all current prescriptions ,including eye drops. Next, the vet will complete a physical examination with detailed focus on the eye region. You will be asked if your cat has undergone any recent trauma that you are aware of. The main objective will be to determine if the unequal pupils stem from a neurological problem or an eye problem. 

Many tests may be administered at this point. Often, the function of the pupils will be evaluated using Visual Pathway Testing. This includes checking for blinking reflexes, object following ability, and proper light sensitivity. Tear production may be measured with a Schirmer Test. Topometry may be applied to test for intraocular pressure. The cornea may be stained with dye to better find ulcers, scratches or other injuries. An ERG (electroretinography) test may be conducted to measure the function of cones and rods within the eye. 

Biopsies or conjunctival scrapings may be collected and sent to labs for further analysis. Blood tests and complete blood counts may be needed in cases which Feline Leukemia Virus or cancer are suspected. An ultrasound, MRI or CT scan may be requested if tumors or lesions are present. Spinal fluids may need to be collected and analyzed in some cases of neurological issues. Urinalysis may be needed in cases of infection. Obvious signs of disease will be noted for diagnostic purposes. If no cause is found, the cat may be referred to an ophthalmologist for further inspection.

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Treatment of Unequal Pupil Size in Cats

As unequal pupils are often the sign of an underlying issue, treatment depends on the issue and not the condition of the pupils themselves. Severity of underlying issues range from harmless to life-threatening. Common issues and their recommended treatments are listed below.

Medication 

In some cases, a medication will be found to cause unequal pupils. If this is the situation, the removal of the medication should stop the pupil issue. 

Glaucoma 

Often medication will be prescribed for this condition, fixing the unequal pupils in the process.

Infection 

If eye infections are found, the responsible bacteria will be identified and the appropriate antibiotics will be prescribed. The average time needed for antibiotics is 2-4 weeks.

Cancer 

If tumors are found, surgery may be an option. The cat would be put under general anesthesia for the procedure. Recovery time will depend on the location of the tumors. Radiation or chemotherapy may also be treatment options to fight against cancer. Both have strong side effects and treatment can be lengthy.

High Blood Pressure 

This can be alleviated with ACE (Angiotensin Converting Enzyme) inhibitors, beta-blockers, calcium channel blockers and a low sodium diet. This treatment may be ongoing. 

Horner’s Syndrome 

This issue will generally resolve on its own, but eye drops may be prescribed to help with symptoms. 

Feline Leukemia Virus 

No cure is currently known, so care designed to ensure the cat’s comfort would be advised.

Hypoplasia & Iris Atrophy 

Both of these conditions require no treatment. 

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Recovery of Unequal Pupil Size in Cats

Depending on the underlying cause of the unequal pupils, recovery can be possible. Often medications will be prescribed over a long period of time. If vision loss or blindness have occurred in the cat, they are often not reversible.

If Feline Leukemia Virus is found to be the cause, focus on all at home adjustments you can make to lengthen your cat's life span. This would include keeping the cat indoors, removing all sources of stress, changing the cat's vaccination schedule, following all antibiotic prescriptions for related infections closely and giving your cat vitamins and anti-viral drugs.

No matter what the underlying cause of unequal pupils is, if the condition does not improve, or if it gets worse, more veterinary care may be needed. 

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Unequal Pupil Size Average Cost

From 370 quotes ranging from $200 - $4,000

Average Cost

$1,000

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Unequal Pupil Size Questions and Advice from Veterinary Professionals

Need pet health advice? Ask a vet

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Ask a Vet

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Unknown

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One Year

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Unknown severity

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6 found helpful

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Unknown severity

Has Symptoms

One Pupil Is Small While The Other Is Large

She isn’t acting any different. Still eating, drinking, sleeping, and playing like normal. Just a few minutes ago I noticed one pupil was small while the other was big. This had to of just happened as it wasn’t like that an hour ago.

July 23, 2020

Owner

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Dr. Michele K. DVM

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6 Recommendations

Thank you for your question. While a change in pupil size is definitely abnormal, if she is acting normal otherwise, it may be fine to keep an eye on her. If the problem continues over the next 12 to 24 hours, it would be best to have her seen by a Veterinarian, as they can assess her neurologic status. If things return to normal, it may have been a short-term problem, and it would be an okay to monitor her for any change in function. I hope that things go well for her.

July 23, 2020

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Domestic long hair cat

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Seven years

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Unknown severity

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2 found helpful

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Unknown severity

Has Symptoms

Pupils Dilated To Different Sizes

My cat is acting slightly more agitated than normal and his pupils are different sizes. It has been quite hot the past few days, that might be behind in irritation. Should I bring him to my vet asap?

July 21, 2020

Owner

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Dr. Michele K. DVM

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2 Recommendations

Thank you for your question. Heat does not cause a change in pupil size, and there may be something going on with your cats brain or neurologic system that needs attention. It would be best to have him seen by your veterinarian as soon as possible, yes. They will be able to assess his eyes and neurologic function, and see what might be the cause of the problem. I hope that all goes well for him.

July 21, 2020

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Obie

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Tabby Cat

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3 Years

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Mild severity

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0 found helpful

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Mild severity

Has Symptoms

Uneven Pupils
Puffy Tail Constantly
Overly Dilated Pup
Appetite Changes

I took my cat in a month ago to have his vaccines done and now his pupils are either really dilated or they are different sizes and his tail becomes puffy all the time there is something wrong with him

Sept. 8, 2018

Obie's Owner

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Mr Dave

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Short Haired Mackarel Tabby

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9 Years

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Moderate severity

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0 found helpful

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Moderate severity

Has Symptoms

Uneven Pupils

My cats eyes have odd pupils. One is sometimes slightly larger that the other, they both react normally to light and such, contracting/dilating as normal. He has no other symptoms, and completely normal for him. Eating fine. Just the slightly different pupil sizes sometimes.

Sept. 3, 2018

Mr Dave's Owner

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Bailey

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Domestic shorthair

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2 Years

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Fair severity

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3 found helpful

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Fair severity

Has Symptoms

Unequal Pupil Size

About a week ago I noticed my cat had one super large pupil and one very small pupil. She was acting completely normal (kept getting excited for a treat when I called her name to look at her and was running around) but I kept an eye on her pupils and they returned to normal after about 15-20 minutes. I didn't remember to look it up until four days later and saw this could be a sign of something super serious. It's been about a week now and since then her pupils have been normal and she seems to be acting normal as well. I keep looking for a cloudy eye or drooping eyelid and I think I'm just being paranoid but I want to be sure she's okay!

Aug. 31, 2018

Bailey's Owner

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Milo

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Main Coon

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12 Years

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Mild severity

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0 found helpful

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Mild severity

Has Symptoms

Not Eating
Depressed
Dilated Pupil

Brief history of my cat: 12 years old; Siamese/Main Coon mix; long, thin build; very vocal and affectionate in his older age; mainly indoor cat with some outside time every day; will occasionally hunt/kill/eat birds/mice/rats/bunnies while outside; minimal vaccines and exclusively raw diet fed. Milo came home one day with one dilated pupil. He wasn't acting weird or didn't seem in pain/discomfort other than one pupil was much larger than the other and he didn't open the affected eye all the way, just kinda half closed at times. It was so dilated that I could barely see any blue in his eye! He walked fine (jumping on furniture too), ate fine (whined for his dinner as usual), acted completely normal including purring loudly and whining to be let outside every morning. He had no injuries that we could find. I took him to the vet the next day to get checked out and everything was normal- she couldn't find anything out of the abnormal, aside from his eye, so I decided on the "wait and see" method to see if things improved/worsened. At this point we had stopped letting him outside so we could keep an eye on him and prevent any possible injuries. About 5 days after the discovery he wasn't acting normal any longer. Milo stopped purring loudly, he walked much slower and preferred to sleep all day. He also stopped "meowing" completely... our very vocal cat completely ceased being vocal. He even stopped whining to be let out every morning, which we stopped doing after the discovery of his dilated pupil. At this point I was starting to get worried. He seemed depressed. A few more days passed and he stopped eating. He didn't stop completely but he ate so very little and for a thin cat, I was worried. I tried feeding him several different things and most of the time he would just take a nibble and be done. He was anxious to try everything I was feeding him but never really wanted it. It was about this time I noticed he wasn't sleeping as much.. still not purring or meowing but not as sleepy. A week later he escaped the house and returned hours later. And to our astonishment, his eye returned to normal! Over the following days we noticed he was acting normal again (purring loudly, whining for his food and to be let outside). Three weeks after the discovery of his dilated pupil and he now seems completely back to normal, including eating his regular raw diet.

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Talullah Belle

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Russian blu mix

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11 Years

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Moderate severity

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0 found helpful

pill-rating-filledpill-rating-filledpill-rating-filledpill-rating-filledpill-rating-filled

Moderate severity

Has Symptoms

Dilated Pupil And Siezures

I’ve witnessed my 11 year old cat have seizures, now one of her pupils is dialated and she seems afraid and is hiding under the dresser. Are these related symptoms or is this a separate thing?

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Flash

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Main Coon

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5 Years

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Moderate severity

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1 found helpful

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Moderate severity

Has Symptoms

Weight Loss
Weakness
Lazy
Pupil Dilation

My cat was an outdoor cat When he was younger but hasn’t been since he was about a year old. now he’s about five years old. about three months ago I started letting him outside again. Recently he has been very into Killing rabbits. Tearing them apart and just devouring them. All within the last week. About 2 days ago my boyfriend came in telling me our cat was aggressively killing a rabbit and was covered in blood so we kept him out for an extra day. He has been losing weight but I’ve assumed it was because he’s been going outside instead of being lazy inside. Today I hadn’t seen my cat at all which was very weird and I knew he was inside so I went downstairs and found him laying down when I looked at his eyes he has one small or regular pupil and one large pupil and he is acting kinda out of it. I brought him a bowl of food and water but he was not interested. I’m wondering if maybe while rabbit hunting he was aggressively kicked or something idk. I’m just freaking out about it and it’s the weekend now so I can’t take him in till Monday and don’t know what to do.

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Miss Kitty

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tabby

dog-age-icon

Three Years

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Moderate severity

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0 found helpful

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Moderate severity

Has Symptoms

No Other Symtoms

Today, While I was walking my dog; my cat Miss Kitty usually follows along. When I turned to check on Miss Kitty, I happened to look at her eyes and noticed that her right eye is completely dilated. She seems to be in no pain at all. Miss Kitty is three years old. Her overall health is good. (although she could afford to lose a couple of pounds)I do have pictures

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Jeff

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Bombay

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1 Year

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Fair severity

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0 found helpful

pill-rating-filledpill-rating-filledpill-rating-filledpill-rating-filledpill-rating-filled

Fair severity

Has Symptoms

Anisocoria

My 1.5 year old kitten seems to have temporary anisocoria every so often. He eats fine and drinks well, normal young cat energy. However, every so often we'll be playing and one pupil will dilate slower than the other. When When this happens and the light changes, his eyes quickly even out. It's not every day and it doesn't seem to bother him. Occasionally it's slightly drippy (barely noticeable though) or squinty (again, barely noticeable). Any idea what might slow his one eye down?

Unequal Pupil Size Average Cost

From 370 quotes ranging from $200 - $4,000

Average Cost

$1,000

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