Sudden blindness in a cat: blood pressure, retinal detachment, and the signs that come first

Quick answer

In a bright room the pupils should be narrow. Wide black pupils in good light, in an older cat, are the sign that gets missed.

A cat that goes blind overnight has almost never been ill overnight. In most cases the blood pressure has been high for weeks or months, and the retina has finally lifted away from the back of the eye.

The reason this matters more than most eye problems is that it is partly reversible if the pressure is brought down quickly, and permanent if it is not. The clock is measured in days.

• The commonest cause of sudden blindness in an older cat is systemic hypertension, usually secondary to kidney disease or an overactive thyroid.
• The earliest sign an owner can see at home is pupils that stay wide open in bright light.
• Waving a hand at a cat's face does not test vision. The cat feels the air with its whiskers and appears to respond.
• Blood pressure is not measured at most routine appointments unless someone asks for it.
• Any cat with chronic kidney disease, hyperthyroidism, or a new heart murmur should have its blood pressure checked, and so should any cat in its senior years.

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Sudden blindness in a cat is an emergency appointment, not a next-week appointment.

If the retina has detached because of high blood pressure, lowering the pressure within days can allow it to reattach and some vision to return. After longer, the retina degenerates and the blindness is permanent.

The same high pressure is simultaneously damaging the brain, the heart and the kidneys, and a cat that presents with blindness may be days away from a seizure or a bleed.
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Decide in 60 seconds

Why blood pressure blinds a cat

Where the pressure lands.

Arterial pressure that stays high damages small vessels in the organs with the richest blood supply. Veterinary medicine calls these the target organs: the eye, the brain, the heart and the kidney.

The eye is the one you can see into. It is also the one where damage produces an immediate, obvious disability, which is why hypertension in cats is so often discovered by an owner rather than by a blood test.

How the retina comes off.

The vessels behind the retina, under sustained pressure, leak plasma. Fluid accumulates between the retina and the layer it sits on, and lifts it away. Haemorrhage into and behind the retina often happens at the same time.

This is an exudative detachment, driven by fluid pressure, rather than a tear. That distinction matters because it is the reason the process can reverse. Take away the pressure, the fluid is reabsorbed, and the retina can settle back down.

Why the window closes.

Retinal photoreceptors depend on close contact with the layer beneath them for nutrients and for the recycling of visual pigment. Separated, they begin to die. Reattaching a retina that has been detached for a long time restores the anatomy without restoring the vision.

This is the whole argument for treating sudden blindness in a cat as urgent rather than as sad but settled.

What is driving the pressure.

In cats, hypertension is usually secondary. Chronic kidney disease and hyperthyroidism between them account for most of it. Less commonly, an adrenal tumour producing aldosterone raises blood pressure while dropping potassium, which shows up as weakness and a neck that will not hold up.

Some cats have primary hypertension with no underlying disease found. That does not make it less dangerous, and it still needs treating.

What you can see at home, in the order it appears

The home monitoring kit for an older cat. Weight, notes and dated observations are what turn a vague impression into an appointment that gets somewhere.

Stage one: the pupils.

Wide pupils in a well lit room. Owners usually describe the cat's eyes as looking darker, blacker, or unusually beautiful. The reflective layer behind the retina also becomes more visible, so the eyes seem to glow more in low light.

Photograph the cat's face in daylight, without flash, once a month. Comparing this month's photograph with one from six months ago is far more reliable than trying to remember.

Stage two: navigation errors in changed conditions.

A cat with failing vision copes perfectly in a familiar room and struggles the moment something is different. Watch for hesitation at the top of a jump, misjudged landings, feeling for the edge of a surface with a paw before committing, and bumping into a bag or box left somewhere new.

Dusk is when this shows first, because the cat is running out of both light and retina.

Stage three: the behaviour changes people misread as ageing.

Staying in one room. Startling when touched from behind. Vocalising more, especially at night. Reluctance to go outside. Walking with the head lowered and the whiskers forward, which is a blind cat's way of reading the space ahead.

These are commonly attributed to arthritis, deafness or cognitive decline, all of which are also common in the same cats, which is exactly why vision gets missed.

Stage four: obvious blindness.

Walking into walls, wide fixed pupils, and no response to a silent moving object.

Testing vision at home without fooling yourself

A cat that reaches accurately for a hand is using sight, smell, hearing and whiskers together. Separating them is the point of a proper test.

The pupil check.

In a bright room, look at both pupils. They should be narrow slits or small ovals. Then look again in a dim room, where they should widen. Pupils that stay fully dilated in bright light, or that are unequal, are abnormal.

Do not use a phone torch shone directly into the eye repeatedly. Ordinary room lighting is enough and does not distress the cat.

The cotton wool test.

Take a small ball of cotton wool or a scrap of tissue, hold it above and to one side of the cat's head, and let it drop through its field of vision. Do it without speaking and without any hand movement near the face.

A sighted cat tracks it with its eyes or head. A blind cat does not react at all, because the object is silent, scentless and light enough to move no air.

Why the usual test fails.

Waving a hand at a cat's face produces a blink in most blind cats. The whiskers and facial hairs detect the air current, and the cat responds to touch, not to sight. Owners routinely conclude their cat can see on the strength of this, and the appointment gets delayed by weeks.

The same applies to the cat that walks confidently around the house. Cats hold a detailed spatial map and use it. Confidence at home proves familiarity, not vision.

Do not test by rearranging the room.

It is tempting to move a chair and see what happens. Do not. If the cat is blind, you have removed the map it is living by, and it will be injured or frightened.

The other causes of a suddenly blind cat

Retinal degeneration from taurine deficiency.

Cats cannot make enough taurine and lose it continuously. A diet that is unbalanced, home prepared without formulation, or intended for dogs, produces central retinal degeneration. The loss is gradual but is often noticed suddenly, and the same deficiency damages the heart.

Inflammation inside the eye.

Uveitis from feline leukaemia virus, feline immunodeficiency virus, toxoplasmosis, feline infectious peritonitis, or a fungal infection such as cryptococcosis. The eye usually looks abnormal from the outside: cloudy, red, or with a changed iris colour, and the cat squints. Untreated uveitis can raise pressure inside the eye and cause glaucoma.

Inherited retinal disease.

Progressive retinal atrophy occurs in Abyssinian, Somali, Persian and some other lines. It is gradual and painless, and typically begins younger.

Drug toxicity.

Enrofloxacin can cause acute retinal damage in cats at higher doses, which is a species-specific effect and the reason feline dosing is capped. If a cat has recently started an antibiotic and loses vision, say so immediately when you call.

Disease behind the eye.

Optic nerve inflammation, a brain tumour, or hypertensive damage to the visual pathways in the brain. Here the eye itself looks normal and the pupils may still respond, which is a useful distinguishing feature.

Thiamine deficiency.

Uncommon but reversible if caught. It comes with neck flexion, wobbliness and abnormal pupil signs, and it is a diet problem, particularly with all fish diets or badly stored food.

The separating questions are always: does the eye look normal from the outside, do the pupils respond to light, how fast did it happen, how old is the cat, and what else is wrong with it.

What the vet will do

Measure blood pressure properly.

Expect it to take time. The cat should be in a quiet room, allowed to settle, with the owner present, and several readings taken and averaged. The first reading is nearly always the highest, and the difference is not trivial.

Practices use either a Doppler probe or a high definition oscillometric machine, with a cuff on a leg or the tail. The figure that matters is the systolic pressure. Most practices act on sustained readings from around 160 mmHg upwards, and act immediately at higher levels or where there is already damage to the eye.

A single high reading in a frightened cat is not a diagnosis. A high reading in a cat with a detached retina is.

Look inside the eye.

An ophthalmoscope shows the retina directly: haemorrhage, oedema, tortuous vessels, and detachment. This is the examination that confirms the eye damage is hypertensive rather than something else, and it takes a couple of minutes.

Find the cause.

Blood tests covering kidney values and thyroid, electrolytes including potassium, and glucose. A urine sample, both to interpret kidney values properly and to measure protein loss, which is itself driven by hypertension. Sometimes an ultrasound of the abdomen or an echocardiogram.

Treat the pressure.

Amlodipine is the usual first line drug in cats and often works quickly. Where protein loss is significant, a second drug acting on the renin angiotensin system may be added. The underlying disease is treated alongside, since controlling an overactive thyroid can lower pressure by itself.

Treatment is lifelong in most cases, with repeat blood pressure checks to confirm the dose is right. Ask what the target is and when the next check is due.

The routine that catches this early

What never to do

Do not wait to see whether vision comes back. In hypertensive detachment, waiting is what makes it permanent.

Do not test vision by waving a hand. It produces a false reassurance in most blind cats.

Do not rearrange the furniture, move the litter tray, or change feeding stations for a cat you suspect cannot see.

Do not let a cat with any degree of vision loss outside unsupervised, and do not assume a cat flap is still safe.

Do not accept "she is just getting old" for wide pupils. Pupil size is not an ageing change.

Do not treat suspected high blood pressure with human medication, salt restriction, or supplements. Feline hypertension is drug treated on measured readings, and the drugs used in people are not interchangeable.

When to get help

The aim of blood pressure monitoring in an older cat is that nothing happens at all: the eyes stay working and the disease behind them stays controlled.

Same day, treat as an emergency:

• Any sudden loss of vision, or a cat walking into things
• Pupils that stay wide open in bright light
• Blood inside the eye
• Seizures, circling, sudden disorientation or collapse in an older cat
• Sudden vision loss in a cat that has recently started any new medication

Within the week, book an appointment and ask for blood pressure:

• A cat with kidney disease or hyperthyroidism that has not had a pressure check
• Any cat over the senior threshold that has never had one
• A new heart murmur, a gallop rhythm, or unexplained weight loss
• Hesitation at jumps, night time vocalising, or a cat that has stopped going out after dark

Take with you: the dated photographs, your pupil and vision test results, the weight log, the current diet, every medication and supplement with start dates, and a note of when you first noticed anything different.

Expect the appointment to involve a settling period before any measurement, several readings, an examination of the back of both eyes, blood tests and a urine sample. Ask what the systolic reading was, what the target is, and when the recheck is booked, and write those three answers down before you leave.