Unequal Pupil Size in Cats

Normally, the pupils expand and contract in unison. When unequal pupils occur, the abnormal eye can be either be 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.

Only one symptom need be present to merit a veterinary examination. If any of the following arise , veterinary attention is advised

• 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

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. Causes include:

• Concussion (bleeding and swelling of the brain from head trauma)
• Injury to optic nerves
• Neurological disorders (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)

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 carried out 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. Specific 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 suspected. 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.

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, reducing the intra ocular pressure and 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 side effects and treatment time 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. 

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 as needed.

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.