Dilated Pupils in Cats – Causes, Symptoms and Treatment

Causes     Diagnosis     Treatment

dilated pupils in cat

About: Dilated pupils are enlarged pupils which do change shape when light is increased. The medical name for this condition is mydraisis.


  • Insulinoma
  • Horner’s syndrome
  • Certain medications
  • Brain trauma
  • High blood pressure
  • Glaucoma
  • Brain tumour
  • Feline dysautonomia

Diagnosis: Detailed examination of the eye along with baseline and specific tests depending on your veterinarian’s index of suspicion.

Treatment: Address the underlying cause of dilated pupils.

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

  • The pupil dilates in poor light to let in more light.
  • The pupil constricts in bright light to reduce the amount of light which enters the eye.

Dilated and constricted pupils in cats

Emotions also play a role in pupil size, when a cat is angry or aggravated the pupils constrict when it is happy or frightened, they dilate.

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


We have already mentioned that the pupils dilate in low light and also during times of stress, anger or fear. In both situations, the pupils should constrict either when light increases or when the cat has calmed down. Permanently dilated pupils can be a symptom of an underlying problem. One or both pupils may be affected, when only one pupil is affected, the condition is known as anisocoria.

Some causes of dilated pupils include:

  • Feline dysautonomia (feline dilated pupil syndrome) – A dysfunction of the autonomic nervous system, a part of the nervous system which operates without conscious control
  • Insulinoma – Tumour of the pancreas which causes it to produce too much insulin.
  • Brain tumour.
  • Certain drugs and medications (ADD medications which contain amphetamine and methylphenidate, atropine eye drops) can have an effect on the functioning of the pupil.
  • Poisoning such as snake bite or tick bite.
  • Retinal detachment – High blood pressure is the most common cause of retinal detachment in cats,  taurine deficiency used to be a cause, but this rare in cats who are fed a commercial diet. Other causes include glaucoma, certain infections (histoplasmosis), certain poisons which can lead to bleeding behind the pupil, and eventually retinal detachment.
  • Head trauma.
  • Glaucoma – An increase in pressure within the eyeball.
  • High blood pressure – A condition in which blood is pumped through the arteries at a higher than normal force.


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. This may include:

  • How long have the pupils been dilated?
  • Have you noticed any other symptoms?
  • Is the cat’s vision affected?
  • Is the cat on any medication?

Diagnostic tests:

  • Baseline tests such as biochemical profile, complete blood count and urinalysis to check the overall health of your cat and evaluate the organs
  • Checking the blood pressure
  • Blood tests to check for diabetes
  • Ultrasound of the eyes to look for lesions
  • Ultrasound to evaluate the pancreas
  • CT scan to check for tumours or brain lesions
  • Tonometry to measure the pressure within the eye
  • Gonioscopy to evaluate the angle of the anterior chamber of the eye
  • Ophthalmoscopy to examine the optic nerve at the rear of the eye


The goal of treatment is to manage the underlying cause and may include:

Retinal detachment:

Treat the underlying cause, such as high blood pressure medication, radioactive iodine to kill the thyroid tumour, low protein diet for kidney disease, surgery to remove tumours, dietary modification and/or insulin for diabetic cats, antibiotics and supportive care for infections.

Feline dysautonomia:

Fluids and nutritional support and managing symptoms.

Brain tumours: 

Surgery to remove or reduce the size of the tumour and/or chemotherapy or radiotherapy.


Medications to bring down the intraocular pressure. Anti-inflammatory drugs to control inflammation.  Painkillers to relieve pain. Surgical removal of the eye if blindness has occurred.

High blood pressure:

Finding and treating the underlying cause as well as medications to help bring down blood pressure and a low sodium diet.


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


If medications or poisons are the cause, the condition may improve once the drugs/toxins are out of the system.

Head trauma:

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.