Dilated Pupils in Cats – What Does It Mean?

We know that cats have a sharper sense of sight at night compared to us. This is because their eyes have additional features including larger lenses as well as light-reflective tapetum lucidum that makes their eyes also “glow” in the dark.

Their pupils are also highly adjustable from round and dilated to slits which allows them to rapidly reduce and expand the amount of light entering their eyes.

But do you know that emotions and amount of light aside, your kitty’s pupils will also tell you its state of health?

Cat dilated eyes

Normal Dilated Pupils

Normal dilation of pupils is affected by the amount of light in the environment as well as your kitty’s mood. A well-lit room or a stay outdoors on a sunny day will make your cat’s pupils narrow into slits. During the evenings in a dimly-lit room, however, your kitty’s eyes will dilate in order to receive more light and help it see in the dark.

Your feline’s eyes are narrowed when it is aroused or highly interested – like when it is playing with catnip or diving at its favorite toy or even playing with your feet. The same happens when it is in a happy and contented state; sometimes coupled with kneading or purring or both.

A cat’s pupils also become slits when it is angry or when it shows aggression towards another cat or animal. This is accompanied by snarling, growling, hissing, and spitting. Cats squint their eyes instinctively. In the animal and cat world, fights break out and the narrowing of your kitty’s eyes help it protect its orbs from sharp claws, teeth, and other natural “weapons.”

In contrast, a cat’s pupil dilates out of excitement and fear. The excitement here, however, is not the same excitement your kitty gets from sniffing and playing with catnip. It is also different from its “hunting mode.” Rather, a tabby’s pupils widen when it is surprised or, on a more negative take – frightened, such as a sudden crack of thunder or when you place cucumber behind it.

Anxiety can also cause your kitty’s pupils to dilate. Unlike anxiety in relation to aggression, your kitty’s pupil will widen, its back hunched and tail close to it – as if it wishes to curl up and maybe say something like “leave me alone please!” This usually happens when your cat is taken to the vet, or – when it is not feeling well.

If your kitty is of the British Shorthair breed, then there is a possibility that your cat will exhibit semi-dilated and fully dilated pupils more often than other cats. This is natural for the kitties since they are noted for their large elliptical and dilated eyes. Reviewing other symptoms for any underlying sickness would be a better way to check this friendly feline breed.


Is your kitty currently undergoing medication? There may be a chance that the pupil dilation of your feline friend is due to the medicine. Medications like atropine, amphetamine, clonidine, morphine, and tropicamide tend to do that. If your kitty is taking medication not included in the ones mentioned but is still having dilated pupils, please get vet advice.


Pupil dilation may be equal in size, or unequal. Anisocoria is a variation in pupil size of each eye of a cat which can be a symptom of an underlying medical condition.

When you see anisocoria in your pet puss you must take it to the vet immediately as the poor little fella might be suffering from injury, inflammation or disease related to the inner eye. Worse, it could be indications of cancer, feline leukemia or glaucoma. It could also be a brain injury due to a fall or for being hit by heavy objects (such as getting hit by a vehicle).

Other possible causes for anisocoria are eye problems like scarring between the lens and the iris, iris tissue decline, and other congenital defects.

Treatments for kitties with anisocoria vary according to the vet’s diagnosis. Kitties found with cancer, for example, will have to be subjected to radiation, surgical removal or chemotherapy depending on the location of the tumor.

Feline leukemia is untreatable but secondary problems like infections and anemia can be treated with a prescription of antibiotics and blood transfusions. Others, like glaucoma, may need eye drop medications, anti-inflammatory steroids, and even surgical removal of the affected eye.

Common Causes of Excessive Pupil Dilation

If your kitty has prolonged pupil dilation and it has nothing to do with its bright surroundings, you may need to check for other symptoms such as the excessive pupil widening which could be a tell-tale sign of an underlying medical condition. Of these conditions, the two most common are Key-Gaskell Syndrome and Feline Hypertension.

An important thing to note about excessive pupil dilation is the response time of your kitty’s pupils as well as the degree of dilation. It takes a matter of split seconds for the pupils to constrict for a normal kitty when the intensity or amount of light increases. On the contrary, pupils of cats with excessive dilation may take longer; or the pupils do not narrow as much as they are expected.

For young cats under the age of three, excessive and non-responsive dilated eyes may be a tell-tale sign that the cat has Key-Gaskell Syndrome or feline dysautonomia. Key-Gaskell has no known underlying cause but its symptoms include protrusion of the third eyelid, coughing, aversion to light, vomiting, depression and weakness, loss of appetite, and urination problems.

Feline dysautonomia does not have any direct cure but since it often involves young kitties, treatment includes elevated feeding position, use of feeding tube, fluid therapy, and use of artificial tears. Other treatments are given to counter other symptoms of the ailment.

Feline hypertension, or high blood pressure, is another medical condition that is indicated by excessive dilated eyes. Unlike humans, feline hypertension has nothing to do with stressful lifestyles. Instead, feline hypertension is related to an underlying kidney or thyroid problem.

High blood pressure may be characterized by an affected sense of sight due to a tendency for the retina to get detached. Other possible symptoms include agitation, compulsive grooming, restlessness, and thirst, and changes in appetite and weight.

If your kitty has high blood pressure, it is advisable to go to the vet as soon as possible since the vet will prescribe medication that would help control its blood pressure. Follow the vet’s instructions regarding the application and duration of the treatment, as suddenly stopping the medication could suddenly increase the blood pressure and endanger the cat instead.

Please do not give your cat human hypertension medicine. Humans and cats do not have the same anatomical characteristics and human medication could poison cats.

scared cat with dilated pupils

Other Causes of Pupil Dilation

  • Internal tumors and head injuries

Other possible medical conditions cats may have that include excessive pupil dilation among its symptoms are the presence of tumors in the brain and the pancreas and head injuries. If your kitty experiences or exhibits nausea, nervousness, seizures, vomiting, loss of appetite, changes in behavior or drunken way of walking, consult your vet immediately.

The earlier the health condition is diagnosed, the easier the treatment will be since it may involve hospitalization, medication, and even surgery. For tumor-related causes, treatment may include chemotherapy and radiation.

  • Deficiencies in Nutrients

Deficiency in calcium (in the kitty’s blood) can lead to thyroid and kidney trouble which causes hypertension and excessive pupil dilation.

Calcium deficiency in lactating mama cats may manifest through milk fever. Aside from the dilated pupils due to lack of calcium, milk fever has symptoms such as restlessness and nervousness, lethargy, stiff-legged walk, increased breathing, fever, convulsions, loss of appetite, and muscle twitching.

Treatment for blood calcium deficiency is usually through intravenous (IV) drip infused with calcium. Your kitty will also be subjected to diet and fluid therapy. Vitamin D may also be given but it will have to be vet-prescribed since it needs close monitoring. Excessive Vitamin D can be toxic for your kitty.

You can also prevent calcium deficiency by giving your feline friend well-balanced kitty kidney-friendly cat food like Royal Canin (vet-prescribed), Blue Buffalo, and Fancy Feast brands.

Lack of Vitamin B1 or Thiamine can also cause excessive pupil dilation to your kitty. Thiamine helps in bodily functions and maintenance, particularly with the heart and the nervous system.

Aside from dilated pupils, lack of Vitamin B1is indicated by symptoms such as loss of appetite, lower heartbeat, twitching, behavioral changes, unstable gait and late reflex, drooling, and lowered head (with chin always near the chest area) due to a rigid neck.

Deficiency in Thiamine is often due to certain fish diets, particularly those that contain thiaminase which disables thiamine, as well as home-made cat food improperly cooked in heat and water which tends to reduce Vitamin B1 content in your kitty’s meal.

To avoid lack of Thiamine, your kitty has to be fed with a well-balanced meal and if you do, opt to make it at home and opt for raw fish. Make sure that the fish does not contain thiaminase, and do not use any heat processing method. Avoid canned foods which use sulfur dioxide and sulfur-based preservatives as well as additives that will alter the acidity levels, as well as heat processing method.

Should your kitty be diagnosed with Vitamin B1 deficiency, the doctor will also give Thiamine injections aside from prescribed high-Thiamine diet.

  • Poison

Human medications, plant poisons, and venom from snakes, scorpions, spiders, and ticks can give a variety of symptoms, among them – dilated pupils.

If you notice or get reports of your kitty having wobbly gait, confusion, vomiting, diarrhea, indications of bleeding from the nose, mouth, or even anus and fecal discharge, difficulty in breathing, drooling, and even paralysis, bring your pet to the vet right away. Chances are, the poor puss has been bitten by a venomous critter and any delay from treatment could put the poor cat in further danger.

The good thing about venom is once the vet has established the type of poison in your feline’s system, antivenin may be administered. Other treatments include medication, IV drip, and fluid flushing, as well as oxygen treatment and a modified diet.

If your kitty is poisoned from indoor and outdoor plants or human medication, other indications of its poisoning aside from dilated pupils may include gastrointestinal problems (constipation, and upset stomach), drooling, lethargy, light sensitivity, decreased heart rate, seizures, and wobbly way of walking.

As a pet parent, it would be good to gather evidence of this kind of poisoning. You may have seen it playing with a leaf of a poisonous plant or you may have seen it mess with some cough drops. A household member may have used some human food supplement on it. Whichever poisoned the poor cat – if you can identify it, make sure to present the container, the portion of the torn leaf, and any other evidence if possible.

Even better than helping your vet diagnose the poison problem is prevention of such occurrences. Make sure that all types of poison in the house are inaccessible to your kitty. Involve everyone in the house by informing them that kitties should not be given any human medicine or food supplement.

Make sure that all poisonous plants have labeled pots and placed in areas where the cat would not be able to access them or make your cat avoid accessing these (for example, have the pot sit beside another pot with a citrus plant).

There may still be an unavoidable case, however, and when it happens, have your feline friend taken to the vet right away. The earlier the kitty is taken, the better since the vet may use induced vomiting treatment and gastrointestinal tract flushing if the poison has not yet spread outside the tract. Some poisons may also have counteragents and antitoxins which the vet can produce.

As a further solution, the vet may provide oxygen treatment, IV drip and prescribed diet changes and medications to counter other side effects like seizures, pupil dilation, and difficulty in breathing.

shorthair domestic cat with dilated pupils


Our cats are strong and graceful creatures – strong and prideful enough that they may tend to “hide” the symptoms from us. It is in their nature; for the ancestors of these little tigers have learned that showing weakness to enemies – their predators – is equal to being eaten.

Today, cats may no longer be in danger of these gigantic predators. But we, as their pet parents should be sensitive enough to spot any health problems. By being sensitive Moms and Dads, we can easily correct any kitty problems, reduce its anxiety, and grant our pets healthy and happy lives.

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