Causes of Dilated Pupils in Cats

Last Updated on June 18, 2021 by Julia Wilson

The pupil is the black slit/circular shape in the middle of the cat’s iris (the coloured part of the eye). Pupils control the amount of light entering the eye by dilating (becoming large) and constricting (becoming small/slit-like).

Normal causes of dilated pupils

  • Reduced light: The pupil dilates in poor light to let in more light or will constrict (shrink) in bright light to reduce the amount of light.
  • Emotions: When a cat is angry or aggravated, the pupils constrict when it is happy, excited or scared, they dilate.
  • Certain medications: (Atropine, tropicamide, morphine, clonidine, amphetamine) and plants (such as catnip) can cause dilated pupils in cats. If your cat is on medication and has dilated pupils, contact your veterinary surgery who will be able to advise if this is a common side effect.

Both pupils should be equal in size, and when looked at with a bright light, they should both constrict quickly.

Medical causes

Feline dysautonomia

Feline dilated pupil syndrome or Key-Gaskell syndrome

A rare condition first discovered in 1982 caused by a dysfunction of the autonomic nervous system. This system controls parts of the body not consciously operated such as heartbeat, digestive system, and pupillary response. Young cats under three years old are most commonly affected. The cause is unknown, although some suggest that Clostridium botulinum (botulism) may be a cause.



There is no cure for this condition, and the goal of treatment is to manage symptoms which may include:

  • Elevated feeding position: Feed the cat in an elevated position so that gravity can help to push food down the esophagus. The cat should remain in that position for 10-15 minutes after each meal.
  • Feeding tube: It may be necessary to place a feeding tube in cats who cannot eat in an upright feeding position or who are refusing food.
  • Fluid therapy: To treat cats with dehydration.
  • Artificial tears: To moisten and lubricate the eyes.


A tumour of the beta cells of the pancreas that produces excessive amounts of insulin, leading to low blood sugar (hypoglycemia) which causes neurological problems and weakness.


  • Dilated pupils
  • Increased appetite
  • Weakness, especially in the hind legs
  • Nausea
  • Nervousness
  • Rapid breathing
  • Muscle twitching
  • Mental confusion
  • Seizures


If possible, surgical removal of the tumour otherwise, medical management such as steroids to promote the formation of glucose and dietary management.

Brain tumour

Brain tumours are an abnormal growth of cells that can be benign or malignant and account for 2.2% of all tumours in cats. They can be primary, originating in the brain or its membranes, or secondary, having originated elsewhere. Approximately 70% of brain tumours are primary. There are many types of brain tumours, depending on the cells involved, the most common type of brain tumour in cats is a meningioma, a benign tumour that arises from the meninges, which is the membrane that surrounds the brain and spinal cord.


Can vary on the type of brain tumour as well as its location within the brain, but may include:

  • Dilated pupils
  • Changes in behaviour
  • Seizures
  • Head pressing
  • Circling
  • Head tilting
  • Drunken gait (ataxia)
  • Loss of appetite
  • Disorientation
  • Loss of vision
  • Vomiting


  • The treatment of choice is the surgical removal of the tumour, where possible. However, this is not possible for many brain tumours which are located deep within the brain.
  • Other treatment options include radiation and or chemotherapy to shrink the tumour and slow down its progress.
  • Supportive care to manage symptoms can include medications to control seizures and nausea.

Retinal detachment

Retinal detachment in cats

Retinal detachment (RD) is a common, serious and sight-threatening disorder that occurs when the retina (a thin, transparent layer of light-sensitive tissue that lines the rear of the eye) detaches from the underlying retinal pigment epithelium layer. High blood pressure is the most common cause of retinal detachment, and hyperthyroidism (a benign tumour of the thyroid gland) is a leading cause of high blood pressure. Kidney disease (the kidneys regulate blood pressure by releasing the enzyme renin, which constricts (tightens) blood vessels, increasing blood pressure.

Hyperviscosity syndrome (HVS) in which causes the blood to become thicker than usual due to increased proteins (hyperproteinemia), most often associated with multiple myeloma which can lead to ruptures in the small blood vessels behind the retina.

Other causes include trauma, infection, inflammation, cancers, glaucoma, toxins, and autoimmune disease.


  • Dilated pupils
  • Sudden blindness
  • Other symptoms will vary depending on the underlying cause


The goal of treatment is to address the underlying cause, which may include:

  • Medication to control blood pressure.
  • Radioactive iodine to kill the thyroid tumour or surgery to remove it. Non-surgical options include a prescription diet low in iodine.
  • Low protein diet for cats with chronic kidney disease.

Head trauma

Any head trauma can potentially damage the brain and affect the autonomic nervous system, which as we noted above, is responsible for specific functions cat has no direct control over such as heartbeat, digestive system, and pupillary response. A fall from a height, trauma from a car accident or intentional hit to the head can all potentially cause injury to the brain.


  • Dilated pupils
  • Changes in behaviour


Treatment depends on the severity of the trauma; it will require hospitalisation and supportive care, including fluid therapy, oxygen therapy, medications to control seizures and in some cases, surgery.


Glaucoma in cats

An increase of pressure within the eye (intraocular pressure) can damage the optic nerve, connecting the brain to the eye. The most common cause is a blockage to the drainage system within the eye, leading to a build-up of fluid, which may be due to inflammation or infection, trauma, displacement of the lens, cataract surgery or tumours.


Glaucoma is a sneaky disease and often has few signs until irreversible damage has occurred.

  • Dilated pupil
  • Pain
  • One eye which appears larger than the other
  • Squinting
  • Loss of vision


  • Medications (dorzolamide and or timolol) to bring down the intraocular pressure.
  • Anti-inflammatory drugs to control inflammation.
  • Painkillers to relieve pain.
  • Surgical removal of the eye (enucleation) if blindness has occurred.


Low level of calcium in the blood levels, which impacts many systems of the body, including the heart and nervous system. There are several causes of hypocalcemia which include: Hypoparathyroidism: Low parathyroid hormone levels in the blood, most commonly due to the accidental removal of the parathyroid gland during surgery to remove the thyroid gland in cats with hyperthyroidism. The parathyroid glands are responsible for monitoring blood calcium levels and when they decrease, secreting a parathyroid hormone to stimulate the release of calcium from the bones.

Acute or chronic kidney disease: Damage to kidneys cause a decrease in their ability to filter the blood and remove toxins (via the urine). Kidney disease affects blood calcium in two ways.

  • Increased blood phosphorous pushes blood calcium into bone and other tissues.
  • The kidneys are responsible for the production of vitamin D, and when levels drop, there is a decrease in gastrointestinal absorption of calcium from the food.

Milk fever: This condition occurs in female cats who are nursing kittens.

Other causes of hypocalcemia include acute pancreatitis, sepsis (blood infection), ethylene glycol poisoning, hypoalbuminemia and administration of phosphate enemas.


  • Dilated pupils
  • Changes in behaviour
  • Twitching
  • Stiff legged gait
  • Fever
  • Loss of appetite


The goal of treatment is to find and manage the underlying cause as well as treat symptoms of hypocalcemia.

  • Slow intravenous administration of calcium gluconate 10% solution.
  • Fluid therapy and nutritional support if your cat is anorexic.
  • Vitamin D assists in the absorption of calcium, and therefore may also be given.
  • Once the cat has stabilised, oral administration of calcium supplements. Adjustments may be necessary with both vitamin D and calcium in the first few months.
  • ECG to check for cardiac abnormalities.
  • Regular blood tests to monitor calcium levels to ensure they don’t become too low or too high (hypercalcemia), monthly checks will be necessary for the first six months and then every 2-3 months after that.

Thiamine deficiency

Thiamine (B1) is a water-soluble vitamin that plays a vital role in many bodily functions including metabolising carbohydrates, maintaining a healthy heart and nervous system. Thiamine deficiency is rare in cats and is usually associated with those who eat a home-made diet that is either cooked in water or heat (both of which destroy thiamine) or a diet high in fish.


  • Dilated pupils
  • Drooling
  • Loss of appetite
  • Ataxia (wobbly gait)
  • Loss of righting reflexes
  • Slow heartbeat
  • Behaviour changes
  • Twitching
  • Cervical ventroflexion (necks flexed/rigid, which causes an inability to raise the head, the chin rests near the chest)


  • Feed a nutritionally balanced diet and avoid diets containing large quantities of fish.
  • Thiamine injections.

Venom toxicity

Venom is a form of poison secreted by several animals in defence or to kill their prey. Snakes, scorpions, spiders, and ticks are all common sources of venom toxicity in cats.


Venom affects several body systems, including the blood, nervous system and heart and symptoms can vary depending on the type and amount of venom

  • Dilated pupils
  • Ataxia (wobbly gait)
  • Drooling
  • Confusion
  • Vomiting
  • Diarrhea
  • Bleeding (nose, mouth, anus)
  • Paralysis
  • Difficulty breathing


Where possible, administer antivenom which counteracts the effects of the venom. Intense supportive care is essential and may include oxygen therapy, intravenous fluids, anti-nausea medication and nutritional support.

Anticholinergic drugs and plants

Drugs or plants with anticholinergic effects are substances that block acetylcholine from binding to its receptors on certain nerve cells, inhibiting parasympathetic nerve impulses. These nerve impulses are part of the autonomic nervous system responsible for several functions not consciously controlled, which includes pupil dilation and constriction). Several prescription and over the counter drugs in the following classes have anticholinergic effects, which include:

  • Tricyclic antidepressants (TCA’s)
  • Mydriatics/eye drops which dilate the pupils (atropine, phenylephrine, tropicamide, cyclopentolate)
  • Neuroleptics/antipsychotics
  • Cycloplegic drugs
  • Antihistamines (Benadryl/diphenhydramine)
  • Cold medications (pseudoephedrine and decongestants)
  • Cardiovascular and gastrointestinal medications
  • Parasympatholytic medications

Plants with anticholinergic properties include:

  • Atropa belladonna (deadly nightshade)
  • Brugmansia species
  • Datura species
  • Garrya species
  • Hyoscyamus (henbane)
  • Mandragora officinarum (mandrake)
  • Lycopersicon (tomato)
  • Solanum (nightshade)


  • Hyperthermia (elevated temperature)
  • Drooling
  • Ataxia (wobbly gait)
  • Behavioural changes
  • Lethargy
  • Dilated pupils
  • Sensitivity to bright light
  • Gastrointestinal upset
  • Delayed gastrointestinal emptying (decreased bowel motions)
  • Urinary retention
  • Seizures
  • Decreased heart rate


Gastrointestinal decontamination if ingestion is recent, followed by activated charcoal to bind to any remaining toxins in the gastrointestinal tract.

The antidote for anticholinergic toxicity is Physostigmine, which works by increasing extracellular levels of the neurotransmitter acetylcholine, which counteracts the effects of the anticholinergics.

Supportive care may include intravenous fluids to correct pH disturbances or electrolyte imbalances, oxygen therapy, pilocarpine to reduce pupil dilation, diazepam to control seizures.


Your veterinarian will perform a complete physical examination and may need to perform some diagnostic tests to determine the cause.

He will carefully examine the eyes and obtain a medical history from you.

  • How long have symptoms been present?
  • Have you noticed any other symptoms?
  • Is the cat’s vision affected?
  • Is the cat on any medication?
  • Does the cat have any underlying medical conditions?

Diagnostic workup:

  • Baseline tests – Biochemical profile, complete blood count, and urinalysis to check the overall health of your cat and evaluate for diabetes (glucose in the blood and urine)
  • Blood pressure – This test uses a cuff and doppler to measure the pressure within the arteries to check for high blood pressure.
  • Ultrasound – To evaluate the eyes and pancreas for signs of inflammation or tumours.
  • CT scan – This advanced diagnostic imaging uses an x-ray and a computer to look inside the cat’s brain, in this case, to evaluate for tumours. The procedure is carried out under heavy sedation or anesthesia to ensure the cat remains still.
  • Tonometry – Measurement of the pressure within the eye to check for glaucoma.
  • Ophthalmoscopy – Examination of the back of the eye which includes the optic nerve, blood vessels and retina.
  • Gonioscopy – A gonioscope is used in conjunction with a slit lamp to evaluate the internal drainage system of the eye (anterior chamber) where the cornea and iris meet.

What should you do?

If you notice your cat has dilated pupils, move him to a source of light, or shine a torch in the eyes to see if they constrict (go smaller). If there is no change, despite exposure to light, consult your veterinarian.