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Cat Pupils Dilated All the Time: Hypertension Warning

You notice your cat’s eyes look different. Her pupils are enormous, like two black circles filling almost the entire eye. You shine a light to see if they constrict and…

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You notice your cat’s eyes look different. Her pupils are enormous, like two black circles filling almost the entire eye. You shine a light to see if they constrict and they don’t respond, or respond minimally. This isn’t just in dim lighting or when she’s excited; the pupils stay dilated constantly, even in bright light. When you look closely, the black pupils seem to dominate her eyes in a way that feels abnormal.

Persistently dilated pupils in cats, called mydriasis, isn’t just an odd appearance issue. It’s a medical sign that something is affecting either the eyes themselves or the neurological systems that control pupil size. While pupils naturally dilate in dim light, during play, or when cats are excited or fearful, they should constrict back to normal in bright light. When pupils stay dilated regardless of lighting conditions, this signals a problem.

High blood pressure, or hypertension, is one of the most serious causes of persistently dilated pupils in cats. Hypertension can cause sudden blindness when extremely high blood pressure damages the retina. The pupils dilate because the retina no longer functions properly, and they don’t respond to light because the normal visual pathway is damaged. This is a medical emergency that can cause permanent blindness if not treated immediately.

Other serious conditions also cause persistent pupil dilation, including brain problems, toxin exposure, glaucoma, and certain medications. Understanding what normal pupils look like, recognizing when dilation is abnormal, knowing which causes represent emergencies, and seeking appropriate veterinary care quickly can save your cat’s vision and potentially her life.

This guide explains how pupils normally work, what causes persistent dilation, how to recognize hypertensive emergencies, what other conditions cause similar symptoms, how veterinarians diagnose the problem, and what treatments exist for various causes.

Understanding Normal Pupils

Before identifying problems, it’s important to understand normal pupil function.

Normal Pupil Appearance

In bright light: Pupils constrict to narrow vertical slits. This protects the sensitive retina from too much light and improves depth perception for hunting.

In dim light: Pupils dilate to large, round openings. This allows maximum light to enter, helping cats see in near darkness.

Size changes: The transition from constricted to dilated (and vice versa) happens smoothly and relatively quickly, usually within seconds of lighting changes.

Normal Pupil Responses

Light response (pupillary light reflex): When light shines in the eye, that pupil constricts immediately (direct response). The other pupil also constricts slightly (consensual response). This is a critical reflex controlled by neurological pathways.

Both pupils should match: In normal cats, both pupils are the same size and respond identically to lighting changes. This is called isocoria.

Accommodation: Pupils also adjust based on focusing distance and arousal state, but light is the primary driver.

Normal Temporary Dilation

Excitement or play: Pupils dilate when cats are aroused, playing, or hunting. This is temporary and pupils return to normal afterward.

Fear or stress: Frightened cats have dilated pupils as part of the “fight or flight” response. Again, temporary.

Dim lighting: Obviously normal in darkness or low-light situations.

Waking from sleep: Pupils are often dilated immediately upon waking, then adjust to ambient light within seconds.

What’s Abnormal

Persistent dilation: Pupils that stay large in bright light or don’t constrict when you shine a light directly at them.

Anisocoria: One pupil is dilated while the other is normal. This asymmetry always indicates a problem.

Progressive dilation: Pupils that gradually become more and more dilated over hours to days and don’t return to normal.

Lack of light response: Pupils that don’t constrict at all when exposed to bright light.

Hypertension: The Emergency Cause

High blood pressure is one of the most urgent causes of persistently dilated pupils.

What Hypertension Is

Hypertension means sustained elevated blood pressure. Normal feline blood pressure is approximately 120-140 mmHg systolic. Hypertension is generally defined as sustained pressures over 160 mmHg, with severe hypertension over 180 mmHg.

How Hypertension Affects Eyes

Retinal damage: Extremely high blood pressure damages the delicate blood vessels in the retina. These vessels can leak fluid, bleed, or cause the retina to detach from the back of the eye.

Sudden blindness: When the retina detaches or is severely damaged, cats go blind suddenly. The blindness can occur in one or both eyes.

Pupil dilation: The damaged retina no longer sends proper signals to the brain about light. The pupil dilates because the normal light reflex pathway is broken. The pupil doesn’t constrict in bright light because the eye essentially can’t “see” the light to trigger constriction.

Hemorrhage: Sometimes bleeding visible in the eye (hyphema) occurs, though often damage isn’t visible from the outside.

Why This Is an Emergency

Time-sensitive: If hypertension is treated within 24 to 48 hours of acute retinal detachment, some cats regain vision. Delayed treatment (beyond 48-72 hours) usually results in permanent blindness.

Life-threatening: Severe hypertension doesn’t just affect eyes. It can cause:

Underlying causes: Hypertension rarely occurs in isolation. It usually results from serious underlying diseases requiring treatment.

Symptoms of Hypertensive Crisis

Sudden pupil dilation: One or both pupils becoming persistently dilated, often over hours to days.

Sudden blindness:

Behavioral changes:

Neurological signs:

No response to light: Shining a bright light in the eye produces no pupil constriction or minimal response.

Causes of Hypertension in Cats

Chronic kidney disease: The most common cause. Approximately 20% of cats with kidney disease develop hypertension.

Hyperthyroidism: Overactive thyroid frequently causes elevated blood pressure.

Primary hypertension: Elevated blood pressure without identifiable underlying cause. Less common in cats than secondary hypertension.

Heart disease: Can contribute to blood pressure problems.

Other: Diabetes, certain tumors, or other endocrine disorders.

Risk Factors

Age: Senior cats (over 10 years) are at highest risk.

Kidney disease: Any cat with kidney disease should have regular blood pressure monitoring.

Hyperthyroidism: Cats being treated for thyroid disease need blood pressure checks.

Other Serious Causes of Dilated Pupils

Glaucoma

Glaucoma means increased pressure inside the eye, damaging the optic nerve.

How it causes dilated pupils: The increased pressure damages internal eye structures. The pupil dilates and becomes fixed (doesn’t respond to light).

Symptoms:

Emergency: Glaucoma is painful and causes rapid permanent vision loss. Same-day treatment is essential.

Uveitis

Uveitis is inflammation inside the eye from various causes (infection, immune-mediated disease, trauma, cancer).

How it causes dilated pupils: Inflammation damages the iris and ciliary body, affecting pupil function. The inflamed eye is often painful, causing the pupil to dilate.

Symptoms:

Retinal Degeneration

Progressive retinal degeneration causes gradual vision loss and pupil dilation.

Causes:

Pattern: Usually bilateral (both eyes). Gradual onset over months. Pupils dilate as retinas lose function. Night blindness often noticed first.

Neurological Problems

Brain or nerve problems affecting the pathways controlling pupils.

Causes:

Symptoms:

Toxins

Certain toxins affect the nervous system and cause pupil dilation.

Substances:

Pattern: Usually sudden onset following exposure. Often both pupils dilated. May be accompanied by other toxicity signs: vomiting, tremors, seizures, altered mental state.

Medications

Some medications cause pupil dilation as a side effect.

Examples:

Pattern: Dilation starts after medication administration. Usually temporary, resolving when medication wears off.

Dysautonomia (Key-Gaskell Syndrome)

Rare neurological condition affecting the autonomic nervous system.

Symptoms:

Prognosis: Poor. Often fatal despite treatment.

Feline Leukemia Virus (FeLV)

FeLV can cause various eye problems including persistent pupil dilation through different mechanisms (uveitis, glaucoma, retinal disease, neurological effects).

What to Do at Home

When you notice persistently dilated pupils, immediate action is important.

Test Pupil Response

Flashlight test: In a dim room, shine a bright flashlight directly into each eye from 2 to 3 inches away.

Normal response: The pupil constricts rapidly into a slit.

Abnormal response:

Document: Note which eye(s) affected and degree of response.

Test for Vision

Simple vision tests:

Drop test: Drop a cotton ball or small soft object near your cat while she’s watching. Normal-sighted cats track falling objects.

Maze test: Rearrange furniture slightly. Blind cats bump into changes, sighted cats navigate easily.

Menace response: Quickly move your hand toward your cat’s face (don’t touch, don’t create wind). Normal cats blink or pull back. Blind cats don’t respond.

Important: Do these tests gently without frightening your cat.

Check for Other Symptoms

Look for:

Monitor Closely

Track:

Do NOT Wait and See

Persistently dilated pupils, especially with no light response, require veterinary evaluation. Don’t wait days to see if it resolves.

When to Seek Emergency Care

Go to emergency vet immediately if:

Can schedule regular vet appointment if:

When in doubt: Treat as urgent. Better to be overly cautious than risk permanent blindness or miss a serious condition.

Veterinary Diagnosis

Your veterinarian systematically identifies the cause.

History Taking

Questions:

Physical Examination

General health assessment: Overall condition, heart rate, breathing, temperature.

Neurological examination: Testing for brain or nerve problems that might cause pupil abnormalities.

Ophthalmic Examination

Pupil testing:

Eye inspection:

Ophthalmoscopy: Looking at the back of the eye (retina, optic nerve) with specialized instruments. This shows retinal detachment, hemorrhage, or degeneration.

Intraocular pressure (tonometry): Measuring pressure inside the eye to check for glaucoma.

Blood Pressure Measurement

Critical test: All cats with dilated pupils need blood pressure measurement to rule out hypertension.

Method:

Interpretation:

Bloodwork

Complete blood count and chemistry panel:

Urinalysis

Assessing kidney function and looking for other abnormalities.

Advanced Testing

Depending on findings:

Treatment

Treatment depends entirely on the underlying cause.

For Hypertension

Immediate treatment:

Amlodipine: Calcium channel blocker that lowers blood pressure.

Goals:

Monitoring:

Treating underlying cause:

Prognosis for vision:

For Glaucoma

Medical management:

Surgical options:

Prognosis: Depends on severity and how quickly treated. Many cases progress to blindness despite treatment.

For Uveitis

Anti-inflammatory medications:

Treating underlying cause:

Supportive care:

For Retinal Problems

Taurine supplementation: If deficiency is the cause (rare).

Managing underlying disease: If retinal degeneration is secondary to other conditions.

Supportive care: Helping blind cats adapt to vision loss.

No cure: Most retinal degeneration is permanent and progressive.

For Neurological Causes

Treatment varies widely:

Prognosis: Depends on specific condition.

For Toxin Exposure

Decontamination: If recent exposure, preventing further absorption.

Supportive care:

Specific antidotes: When available for particular toxins.

Living with a Blind Cat

If vision loss is permanent, cats adapt remarkably well.

Environmental Modifications

Safety:

Resources:

Tactile cues:

Interaction

Sound cues:

Approach carefully: Don’t startle by touching unexpectedly. Make noise first.

Supervised outdoor time: Only in safe enclosed areas (screened porch, catio).

Enrichment

Non-visual play:

Smell enrichment:

Quality of Life

Most blind cats live happy, full lives with appropriate accommodations. They navigate using memory, hearing, smell, and whiskers.

Prevention

Regular Checkups

Senior cats: Twice yearly veterinary exams including blood pressure measurement for cats over 10 years or those with risk factors.

Monitoring at-risk cats: Cats with kidney disease or hyperthyroidism need regular blood pressure monitoring.

Early Disease Detection

Bloodwork: Regular screening catches kidney disease and hyperthyroidism early, allowing treatment before hypertension develops.

Know Your Cat’s Eyes

Familiarity: Know what your cat’s normal pupils look like so you notice changes quickly.

Home checks: Periodically test pupil response to light at home, especially in senior cats.

Prompt Attention

Act quickly: Any sudden change in pupils warrants immediate veterinary evaluation. Early treatment dramatically improves outcomes.

Frequently Asked Questions

My cat’s pupils are dilated but she seems fine otherwise. Is this really an emergency?

Persistent pupil dilation even without other obvious symptoms can indicate serious problems like hypertension that may not show external signs until severe. Cats are experts at hiding illness. Just because she “seems fine” doesn’t mean nothing is wrong. Any pupil that doesn’t constrict in bright light or stays dilated constantly needs same-day to emergency evaluation. Vision loss from hypertension can happen suddenly, and early treatment is critical for vision preservation.

Can stress or fear cause pupils to stay dilated constantly?

Stress or fear causes temporary pupil dilation that resolves when the stressor is removed or the cat calms down. Pupils from stress will still constrict in bright light, just perhaps not as much as in a relaxed state. If pupils remain constantly dilated for hours to days regardless of lighting and never constrict normally, this indicates a medical problem, not just stress. Don’t dismiss persistent dilation as anxiety without veterinary evaluation.

One pupil is dilated and the other is normal. Is this more serious than both being dilated?

Yes, anisocoria (pupils of different sizes) is often more concerning than both being dilated. This asymmetry usually indicates a problem affecting one eye or one side of the nervous system specifically. Causes include trauma, glaucoma, uveitis, retinal detachment, or neurological damage. Anisocoria requires immediate evaluation. Even if your cat seems fine, same-day veterinary care is essential.

My cat had her eyes dilated at the vet. How long does this last?

Tropicamide (the most commonly used dilation drop) typically lasts 6 to 24 hours. Atropine (less commonly used) can last several days. If dilation persists beyond the expected duration (more than 24-48 hours after tropicamide, more than 3-5 days after atropine), contact your vet. Additionally, if medication-induced dilation is expected, your vet will have told you this was done. Spontaneous dilation without known medication use is not normal.

Can dilated pupils be a side effect of medications my cat is taking?

Some medications can cause pupil changes, though this is relatively uncommon. If your cat recently started a new medication and developed dilated pupils, contact your vet immediately to discuss whether this is a known side effect or indicates a problem. Don’t stop medication without veterinary guidance. Some medications that can affect pupils include certain pain medications, some sedatives, and specific eye drops. Your vet can determine if current medications explain the symptom.

My cat has kidney disease. Does this mean she’ll develop high blood pressure and go blind?

Not necessarily. While approximately 20% of cats with kidney disease develop hypertension, that means 80% don’t. However, cats with kidney disease should have blood pressure monitored regularly (every 3-6 months or more frequently if pressures are borderline). Early detection and treatment of hypertension prevents vision loss and other complications. With proper monitoring and management, many cats with kidney disease never develop blood pressure problems.

If my cat goes blind from hypertension, will treatment bring her vision back?

This depends on timing. If hypertension is treated within 24-48 hours of acute vision loss from retinal detachment, approximately 30-60% of cats regain at least some vision, though it may not return to completely normal. Beyond 48-72 hours, the chances of vision recovery drop dramatically as permanent retinal damage occurs. This is why immediate treatment of suspected hypertensive blindness is so critical. Even if vision doesn’t return, blood pressure control prevents further complications and damage to other organs.

Can cats with persistently dilated pupils and blindness still have good quality of life?

Yes, absolutely. Cats adapt remarkably well to blindness. With environmental modifications (keeping layout consistent, removing hazards, using sound and scent cues), most blind cats navigate confidently and live happy lives. They use memory, hearing, smell, touch (whiskers), and other senses to compensate. Many owners report their blind cats are so well-adapted that visitors don’t realize the cat can’t see. The key is treating the underlying cause of blindness to prevent other health complications.