Dilated Pupils (Mydriasis) in Cats

The pupil is the opening in the center of the eye. Dilated pupils in cats occur normally in response to decreased light levels and some physiologic conditions, such as during the natural fear response.

A pupil is dilated when the black circle in the center of the eye is enlarged and the colored part of the eye (the iris) appears as a narrow outline around it
Pupils may become abnormally dilated due to many causes including nerve dysfunction, retinal disease, eye conditions, injury, toxicity, medications, and brain conditions
Symptoms of abnormal dilation are pupils that are unresponsive to light or only partially responsive to light; one or both pupils may be affected
Diagnostics include both physical and ophthalmic exams, blood work, and diagnostic imaging
Treatment and prognosis vary widely depending on the underlying cause

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A closer look: Dilated Pupils (Mydriasis) in Cats

The colored part of the eye surrounding the pupil is called the iris. The pupil is the opening in the eye that allows light to reach the retina. Light signals are sent from the retina to the brain, creating the sensation of vision. The iris controls the opening (dilation) and closing (constriction) of the pupil in response to changing levels of light.

Temporary pupil dilation that responds normally to light is very common and is associated with benign physiologic conditions such as low light conditions, fear, alarm, or pain.

Symptoms vary from excessively dilated pupils that are non-responsive to light, excessively dilated pupils that are partially responsive to light (pupils can shrink some in response to light), to dilated pupils that respond normally to light.

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Possible causes

Multiple causes of abnormal pupil dilation (mydriasis) exist.

Some examples include

Glaucoma in Cats Progressive Retinal Atrophy in Cats Retinal Detachment in Cats Eye Cancer (Ocular Neoplasia) in Cats Prolapsed (Luxating) Eye Lens (Ectopia Lentis) in Cats Fungal Infection (Blastomycosis) in Cats Rabies in Cats Feline Infectious Peritonitis (FIP) in Cats Feline Leukemia Virus (FeLV)

Head, brain, or eye injuries

Reaction to medication

Flea and Tick Medication Poisoning (Pyrethroid Toxicosis) in Cats Chocolate Poisoning (Toxicosis) in Cats

Caffeine toxicity

Zinc Poisoning in Cats Lead Poisoning in Cats Botulism in Cats Vitamin B1 (Thiamine) Deficiency in Cats Pit Viper Snakebite Poisoning (Crotalid Envenomation) in Cats

Heaststroke

Hydrocephalus

Brain tumors

Risk factors

Immediately life-threatening causes of dilated pupils are rare. Potentially life threatening associated conditions typically have additional symptoms such as excessive drooling, vomiting, or trouble walking as with toxicities or obvious signs of trauma with head or eye injuries.

Any age, breed, or sex of cat can experience mydriasis. The causes of mydriasis are many and varied. Prognosis depends on the cause. Any cat experiencing abnormal mydriasis needs prompt veterinary attention.

Mydriasis can affect one or both eyes depending on the cause.

Testing and diagnosis

Diagnostics may include

A full physical exam
Ophthalmologic exam
Blood work
Blood pressure
Infectious disease testing
Diagnostic imaging

The degree of mydriasis and the nature of concurrent symptoms helps direct investigation. Additional tests may be necessary.

Similar symptoms

Dilated pupils are self evident and not likely to be mistaken for other symptoms. Mydriasis may be very hard to observe, especially in cats with very dark irises.