Why is my old cat's third eyelid showing? 9 causes and what to do

You are petting your senior cat when something catches your eye. A thin, pale membrane is creeping across the inner corner of one or both eyes, partially covering the iris like a translucent curtain being drawn from the side. It might be subtle, just a sliver of white or pinkish tissue visible when your cat blinks. Or it might be dramatic, with the membrane covering a third or even half of the eye, giving your cat a glazed, unsettling appearance that makes your stomach drop.

What you are seeing is your cat's third eyelid, also called the nictitating membrane. Every cat has one in each eye. In a healthy, alert cat, the third eyelid is usually invisible, tucked away in the inner corner of the eye where it rests behind the lower lid. When it becomes visible, it means something has caused it to move out of its normal resting position, and in a senior cat, that something almost always deserves veterinary attention.

A visible third eyelid is not a disease in itself. It is a symptom, a signal that your cat's body is fighting something. The underlying causes range from mild and easily treated to serious and urgent. Understanding what the third eyelid is, why it protrudes, and what the different presentations mean will help you make informed decisions about your senior cat's care.

This guide will walk you through the most common reasons a senior cat's third eyelid becomes visible, how to distinguish between situations that can wait for a regular vet appointment and those that require emergency care, and what you can do to help your aging cat.

What is the third eyelid and why does it matter?

The third eyelid is a remarkable structure that serves as a built-in protective shield for your cat's eyes. Formally known as the nictitating membrane, it is a thin fold of tissue located in the inner corner of each eye, closest to the nose. It contains cartilage that gives it structure, a gland that produces a significant portion of the eye's tear film, and lymphoid tissue that functions as part of the local immune defense.

In a healthy, awake cat, the third eyelid retracts neatly into the inner corner, out of sight. It sweeps briefly across the eye during blinking, spreading tears and clearing debris, but it moves so quickly that you rarely notice it. You might catch a glimpse of it when your cat is very relaxed or drowsy, slowly blinking as she falls asleep. This brief, partial appearance during drowsiness is normal.

What is not normal is a third eyelid that remains visibly elevated when your cat is awake and alert. When the membrane stays elevated, it means something is pushing it up from behind, pulling it out of position, or interfering with the nerve signals that normally keep it retracted. In senior cats, persistent third eyelid protrusion is almost always a sign of an underlying health problem that needs investigation.

The third eyelid can appear in one eye or both. One-sided protrusion often points to a local problem affecting that specific eye, such as an injury, infection, or growth behind the eye. Bilateral protrusion, where both third eyelids are visible, more commonly suggests a systemic issue: dehydration, gastrointestinal illness, nerve dysfunction, or general debility from an underlying disease.

9 common causes of third eyelid protrusion in senior cats

1. Dehydration

Dehydration is one of the most common and most overlooked causes of bilateral third eyelid protrusion in senior cats. As cats age, their risk of dehydration increases significantly. Chronic kidney disease, which affects up to 30 percent of cats over 15, causes the kidneys to lose their ability to concentrate urine, leading to excessive water loss. Hyperthyroidism, diabetes, and even decreased thirst drive from cognitive decline can further compromise hydration.

When a cat becomes dehydrated, the fat pad that sits behind each eye shrinks slightly. This fat pad normally helps hold the eyeball in a forward position, which keeps the third eyelid retracted. As the fat pad loses volume from fluid loss, the eye sinks back slightly in the socket, reducing the pressure that keeps the third eyelid tucked away. The membrane then slides forward, becoming visible across the inner corner of the eye.

Signs that dehydration may be causing the third eyelid to show:

• Both third eyelids are visible, not just one
• The skin on the back of the neck, when gently pinched and released, stays tented for a second or longer instead of snapping back immediately
• Dry, tacky gums instead of moist and slippery
• Decreased urination or smaller urine clumps in the litter box
• Lethargy and decreased appetite
• Sunken-looking eyes
• Known kidney disease, hyperthyroidism, or diabetes

Mild dehydration may respond to encouraging more water intake: water fountains, multiple water stations, adding water to food, or offering low-sodium broth. But significant dehydration in a senior cat, particularly one with known kidney disease, often requires subcutaneous fluid therapy, which your vet can teach you to administer at home. If your cat's third eyelids are visible and she seems lethargic, is not eating, or has been vomiting or having diarrhea, see your vet promptly. Severe dehydration can become life-threatening quickly in an elderly cat.

2. Gastrointestinal illness

There is a well-documented but not fully understood connection between gastrointestinal problems and third eyelid protrusion in cats. Veterinarians have long observed that cats suffering from intestinal parasites, inflammatory bowel disease (IBD), gastrointestinal infections, or general stomach upset frequently present with visible third eyelids, sometimes before any other symptoms are apparent.

This phenomenon is sometimes referred to as "Haw's syndrome" when it occurs bilaterally in association with gastrointestinal upset. The exact mechanism is believed to involve the sympathetic nervous system: GI distress triggers a reflex that reduces sympathetic tone to the smooth muscle that normally keeps the third eyelid retracted. When that muscle relaxes, the membrane slides forward.

In senior cats, IBD is a particularly common cause. IBD involves chronic inflammation of the intestinal walls that can cause intermittent vomiting, diarrhea, weight loss, and decreased appetite. The third eyelid protrusion may come and go in conjunction with flare-ups of the intestinal inflammation.

Signs that a GI problem may be behind the third eyelid showing:

• Both third eyelids are elevated
• Vomiting, diarrhea, or both
• Decreased appetite or refusal to eat
• Weight loss, especially if gradual over weeks or months
• Gurgling stomach sounds
• Changes in stool consistency or frequency
• The third eyelid appearance coincides with episodes of GI upset

Treatment depends on the underlying GI condition. Intestinal parasites are treated with deworming medication. IBD may require dietary changes (novel protein or hydrolyzed diets), anti-inflammatory medications, or immunosuppressive drugs like prednisolone. GI infections may need antibiotics or supportive care. In cases of Haw's syndrome where the third eyelid protrusion is primarily cosmetic and the GI issue is mild, the membranes typically retract on their own within four to eight weeks as the underlying problem resolves.

3. Eye infection or inflammation

Infections and inflammation of the eye itself are common causes of unilateral third eyelid protrusion, where only one eye is affected. Conjunctivitis, the inflammation of the thin membrane lining the eyelids and covering the white of the eye, is particularly common in cats and can cause the third eyelid to become swollen, red, and prominently elevated.

In senior cats, eye infections can result from viral reactivation. Many cats carry feline herpesvirus (FHV-1), acquired as kittens, which lies dormant in nerve cells and can reactivate during periods of stress or immune suppression. As the immune system weakens with age, herpesvirus flare-ups become more frequent and more severe. A reactivated herpes infection can cause conjunctivitis, corneal ulcers, and significant third eyelid protrusion in the affected eye.

Uveitis, inflammation of the internal structures of the eye, is another serious cause of third eyelid elevation in senior cats. Uveitis can result from infections (including FIP, toxoplasmosis, and fungal diseases), immune-mediated conditions, or cancer. It causes pain, redness, light sensitivity, and often prominent third eyelid protrusion.

Signs that an eye infection or inflammation is the cause:

• Only one eye is affected, though both can be involved in some infections
• Redness, swelling, or discharge from the affected eye
• Squinting, excessive blinking, or holding the eye partially closed
• Watery, mucoid, or yellowish-green discharge
• Pawing at the eye or rubbing the face against furniture
• Light sensitivity: the cat avoids bright areas
• Cloudiness or visible color changes within the eye
• Pain: flinching or pulling away when you touch near the eye

Eye infections and inflammation require veterinary diagnosis and treatment. Do not attempt to treat your cat's eye with over-the-counter drops or home remedies, as some conditions (like corneal ulcers) can worsen rapidly with the wrong treatment. Your vet will use specialized instruments to examine the eye, check for ulcers with a fluorescein stain, and measure eye pressure to rule out glaucoma. Treatment may include topical antibiotic or antiviral eye drops, anti-inflammatory medications, pain relief, and treatment of any underlying systemic disease. Prompt treatment is important: untreated eye infections and uveitis can lead to permanent vision loss.

4. Horner's syndrome

Horner's syndrome is a neurological condition that produces a distinctive triad of eye symptoms, and it is one of the most recognizable causes of third eyelid protrusion in cats. It occurs when the sympathetic nerve pathway to the eye is disrupted. This pathway runs from the brain, down through the spinal cord, into the chest, back up the neck alongside the carotid artery, through the middle ear, and finally to the eye. Damage or disruption anywhere along this long and winding route can cause Horner's syndrome.

The sympathetic nerves are responsible for keeping the third eyelid retracted, the pupil appropriately dilated, and the eyelids fully open. When these nerves stop functioning, three characteristic signs appear simultaneously in the affected eye: the third eyelid protrudes, the pupil constricts to a tiny pinpoint, and the upper eyelid droops, making the eye appear smaller than the other.

In senior cats, the most common causes of Horner's syndrome include:

• Middle ear infections or polyps (the most common identified cause)
• Trauma to the head, neck, or chest
• Tumors affecting the nerve pathway, including lung tumors and lymphoma
• Idiopathic: in many cases, no underlying cause is found

Signs of Horner's syndrome:

• Third eyelid protrusion in one eye
• A noticeably smaller pupil in the affected eye compared to the other
• Drooping of the upper eyelid on the affected side
• The affected eye appears slightly sunken
• All three signs appear together on the same side

If you notice these three signs together, see your veterinarian. The vet will perform a neurological examination and may use phenylephrine eye drops to help localize where along the nerve pathway the disruption is occurring. Further diagnostics may include an otoscopic exam of the ears, skull X-rays, CT scan, or MRI. The prognosis depends on the underlying cause. Idiopathic Horner's syndrome often resolves on its own within eight to twelve weeks. Cases caused by middle ear infections typically improve with appropriate antibiotic treatment. Cases caused by tumors have a more variable prognosis depending on the type and location of the growth.

5. Eye injury or foreign body

Even indoor senior cats can sustain eye injuries, and the third eyelid often protrudes as a protective response. Corneal scratches from rough play with another pet, irritation from dust or litter particles, or accidental pokes from household objects can all cause the third eyelid to elevate over the injured eye as a natural defense mechanism. The membrane acts as a biological bandage, covering and protecting the damaged cornea.

Foreign bodies lodged under the third eyelid or behind the lower lid are another cause. A small piece of plant material, a grain of litter, or a fragment of dried discharge can become trapped and cause intense irritation. The third eyelid may protrude both because of the mechanical presence of the foreign body and because of the inflammatory response it triggers.

Signs that an injury or foreign body may be the cause:

• Sudden onset: the third eyelid appeared quickly, not gradually
• Only one eye is affected
• The cat is squinting, blinking rapidly, or holding the eye shut
• Excessive tearing or watery discharge
• Pawing at the affected eye
• Visible redness or swelling of the eyelids
• The cat is reluctant to let you examine the eye

Do not attempt to remove a foreign body from your cat's eye yourself, as you risk pushing it deeper or causing additional damage. If your cat is squinting and the third eyelid suddenly appeared, see your vet within 24 hours. Corneal ulcers can develop rapidly from untreated injuries or foreign bodies, and they can progress from minor to sight-threatening in a matter of days. Your vet will examine the eye under magnification, stain the cornea to check for ulcers, and remove any foreign material safely. Treatment typically involves antibiotic eye drops to prevent infection and pain relief while the cornea heals.

6. Cancer or growths

In senior cats, masses or tumors in or around the eye are an important cause of third eyelid protrusion that should not be overlooked. Growths can affect the third eyelid itself, the tissue behind the eye (retrobulbar space), or the surrounding orbital structures. As a mass grows, it can physically push the third eyelid forward, displace the eyeball, or compress nerves that control eyelid position.

Squamous cell carcinoma is one of the most common cancers affecting the eyelids and third eyelid in cats, particularly in white or lightly pigmented cats. Lymphoma, the most common cancer in cats overall, can also affect the eye and surrounding structures. Other tumors that can cause third eyelid protrusion include melanoma, fibrosarcoma, and adenocarcinoma.

Retrobulbar tumors, growths that develop in the space behind the eyeball, deserve special mention because they are more common in older cats and their initial signs can be subtle. As the tumor grows behind the eye, it gradually pushes the eyeball forward (a condition called exophthalmos), and the third eyelid may be one of the first visible signs of displacement.

Warning signs that a growth may be involved:

• Third eyelid protrusion that is gradually worsening over weeks
• A visible lump, thickening, or discoloration on the third eyelid itself
• One eye appears to bulge forward compared to the other
• Difficulty closing the eyelids completely over one eye
• Chronic discharge from one eye that does not respond to antibiotics
• Changes in the shape or position of the pupil
• Facial swelling or asymmetry around one eye
• Weight loss, decreased appetite, or general decline

Any persistent or progressive third eyelid protrusion in a senior cat should be evaluated for possible cancer. Your vet may recommend a fine needle aspirate of any visible mass, imaging (X-rays, ultrasound, CT, or MRI) to evaluate for retrobulbar growths, and blood work to check for systemic disease. Treatment options depend on the type and location of the tumor and may include surgical removal, radiation therapy, chemotherapy, or palliative care. Early detection generally offers the best prognosis, which is why new or worsening third eyelid protrusion in an older cat should always be investigated.

7. Chronic kidney disease

Chronic kidney disease (CKD) can contribute to third eyelid protrusion through multiple mechanisms, and it is worth highlighting separately because of how common it is in senior cats. CKD affects an estimated one in three cats over 15, making it one of the most prevalent conditions in the aging feline population.

The connection between kidney disease and third eyelid protrusion works in several ways. First, CKD causes chronic dehydration because the failing kidneys cannot adequately concentrate urine, leading to excessive water loss. This dehydration causes the retrobulbar fat pad to shrink, allowing the eye to sink back and the third eyelid to slide forward, as described earlier. Second, CKD can cause nausea and GI upset, which can trigger the sympathetic nerve response that relaxes the third eyelid. Third, the accumulation of uremic toxins in advanced kidney disease can affect overall muscle tone and neurological function, further contributing to third eyelid laxity.

Signs that kidney disease may be contributing:

• Both third eyelids are visible
• Increased thirst and increased urination
• Weight loss, especially muscle wasting along the spine
• Decreased appetite, nausea, or vomiting
• Bad breath with an ammonia or chemical odor
• Lethargy and withdrawal from social interaction
• Poor coat quality
• Known history of kidney disease or elevated kidney values on blood work

If your senior cat's third eyelids are showing and she has been diagnosed with kidney disease, talk to your vet about optimizing her hydration. Subcutaneous fluid therapy given at home, typically every one to three days, can make an enormous difference in how a CKD cat looks and feels. The third eyelids often retract noticeably after a fluid session. Your vet may also adjust your cat's kidney diet, add anti-nausea medication, and manage blood pressure if needed. While CKD is not curable, good management can maintain quality of life for months to years and often improves or resolves the third eyelid protrusion.

8. Sedation, medication effects, or post-anesthetic recovery

If your senior cat recently had a veterinary procedure involving sedation or anesthesia, visible third eyelids are a normal and temporary side effect. Many anesthetic and sedative drugs reduce sympathetic tone, which allows the third eyelids to relax forward. Gabapentin, which is increasingly prescribed for pain management and pre-visit anxiety in senior cats, can also cause temporary third eyelid protrusion.

Some long-term medications can contribute to third eyelid visibility as well. Tranquilizers, certain anti-anxiety medications, and opioid pain medications can all reduce the muscle tone that keeps the third eyelid retracted. In senior cats who are on multiple medications for various age-related conditions, the cumulative sedating effect can make the third eyelids consistently visible.

Signs that medication is the likely cause:

• The third eyelids appeared after starting a new medication or after a vet procedure
• Both eyes are equally affected
• The cat appears drowsy, relaxed, or sedated
• There are no other signs of illness: no discharge, no squinting, no pain
• The third eyelids retract when the cat is fully alert and active

Medication-related third eyelid protrusion is usually harmless and does not require treatment in itself. If your cat recently had anesthesia, the third eyelids should return to normal within 24 to 48 hours as the drugs clear her system. If a daily medication seems to be causing persistent third eyelid protrusion, mention it to your vet at the next visit. They may adjust the dose or timing, but usually the benefit of the medication outweighs this cosmetic side effect. However, do not assume the third eyelids are from medication without ruling out other causes, especially if the protrusion appeared before the medication was started or is accompanied by other symptoms.

9. General debility and systemic illness

Sometimes visible third eyelids in a senior cat are a nonspecific sign that the cat is simply unwell. When the body is fighting any significant illness, from a severe infection to uncontrolled diabetes to advanced cancer, the physiological stress can reduce sympathetic nervous system tone and cause the third eyelids to appear. Veterinarians sometimes describe prominent third eyelids in a sick cat as a sign that the cat is "not doing well overall."

In very elderly cats, general age-related decline can also contribute. A cat who has lost significant body weight, including the fat reserves behind the eyes, may develop chronically visible third eyelids simply because there is less tissue supporting the normal eye position. Combined with decreased muscle tone and reduced overall vigor, the third eyelids may become a persistent feature in the very oldest cats.

Signs that general illness or debility may be involved:

• Both third eyelids are visible
• The cat appears generally unwell: lethargic, not eating, withdrawn
• Significant weight loss or muscle wasting
• Fever or abnormally low body temperature
• Other symptoms such as vomiting, diarrhea, labored breathing, or changes in behavior
• The third eyelid protrusion developed alongside a general decline in condition

When third eyelid protrusion accompanies signs of general illness, the priority is diagnosing and treating the underlying condition. A comprehensive veterinary workup including blood work (complete blood count, chemistry panel, thyroid levels), urinalysis, and possibly imaging will help identify what is making your cat sick. The third eyelids will usually improve as the underlying condition is treated and the cat begins to feel better. If your senior cat has visible third eyelids along with lethargy, appetite loss, or other signs of illness, do not wait: schedule a vet visit promptly.

When to see the vet: emergency vs. routine

Not all third eyelid protrusion is equally urgent, but all cases deserve veterinary evaluation in a senior cat. Here is how to gauge the urgency:

See the vet within 24 hours if:

• The third eyelid appeared suddenly and your cat is squinting, tearing, or pawing at the eye
• There is discharge, especially yellow or green
• The eye appears red, swollen, or cloudy
• You can see a lump or growth on the third eyelid or around the eye
• Your cat seems to be in pain: hiding, not eating, vocalizing
• You notice the classic Horner's syndrome triad: third eyelid up, small pupil, droopy upper lid

See the vet urgently (same day) if:

• Your cat's eye appears to be bulging forward
• There is sudden vision loss: bumping into things, dilated unresponsive pupils
• Your cat is severely lethargic, not eating or drinking, and appears very unwell
• There is bleeding from or around the eye
• The third eyelid protrusion is accompanied by difficulty breathing, seizures, or collapse

Schedule a routine appointment if:

• Both third eyelids are mildly visible but your cat seems otherwise well
• The protrusion comes and goes
• You recently started a new medication and suspect it may be the cause
• Your cat has known kidney disease and you are wondering if the third eyelids are related

What to expect at the vet visit

When you bring your senior cat in for third eyelid protrusion, your vet will likely perform several assessments to determine the cause:

A thorough physical examination is the starting point. Your vet will check hydration status, feel for masses, listen to the heart and lungs, palpate the abdomen, and assess overall body condition. Many causes of third eyelid protrusion can be suspected based on the physical exam alone.

An ophthalmic examination focuses specifically on the eyes. Your vet will examine the third eyelid itself, check for foreign bodies, look at the cornea and internal eye structures, and test reflexes. A fluorescein stain may be applied to check for corneal ulcers. An instrument called a tonometer may be used to measure the pressure inside the eye to rule out glaucoma.

Blood work is important for senior cats because so many causes of third eyelid protrusion are systemic. A complete blood count, chemistry panel, and thyroid level will screen for kidney disease, hyperthyroidism, diabetes, infection, and other metabolic conditions.

Additional diagnostics may be recommended depending on initial findings. These could include blood pressure measurement, urinalysis, imaging (X-rays, ultrasound, CT, or MRI), fine needle aspirate of any masses, or referral to a veterinary ophthalmologist for specialized evaluation.

Helping your senior cat at home

While veterinary treatment addresses the underlying cause, there are several things you can do at home to support a senior cat with third eyelid protrusion:

Keep the eyes clean. If there is discharge, gently wipe it away with a soft, damp cloth or cotton ball, using a separate cloth for each eye to avoid spreading infection. Wipe from the inner corner outward. Do not use any eye drops or solutions unless specifically prescribed by your vet.

Optimize hydration. Since dehydration is such a common contributor, encourage water intake by providing a cat water fountain, placing water bowls in multiple locations, adding water or low-sodium broth to food, and offering wet food if your cat is eating primarily dry. If your cat is on subcutaneous fluids, maintain the schedule your vet has recommended.

Monitor and document. Take photos of your cat's eyes daily so you can track whether the third eyelid protrusion is improving, stable, or worsening. Note whether it is more prominent at certain times of day, whether it correlates with eating or medication timing, and whether one eye is more affected than the other. This information is valuable for your vet.

Prevent self-trauma. If your cat is pawing at her eye, an Elizabethan collar (cone) may be necessary to prevent her from scratching the cornea or worsening an injury. Your vet can provide one that fits properly and is comfortable enough for an elderly cat.

Maintain routine. Senior cats rely heavily on routine for security. Keep feeding times, sleeping spots, and daily interactions as consistent as possible. A stressed cat is a cat whose body is under additional strain, and maintaining calm, predictable surroundings supports overall health and healing.

When multiple causes overlap

In senior cats, it is common for multiple factors to contribute to third eyelid protrusion simultaneously. A cat with kidney disease may be both dehydrated and nauseous, with both mechanisms contributing to the visible third eyelid. A cat with hyperthyroidism may develop secondary hypertension, which can affect the eyes through yet another pathway. An elderly cat with cognitive decline may not be eating or drinking adequately, leading to dehydration and weight loss that make the third eyelids more prominent.

This is why a comprehensive veterinary evaluation is so important for senior cats. Treating only one contributing factor may produce partial improvement but leave the third eyelids still visible because of an untreated second cause. Blood work, a physical exam, and an eye exam together give the most complete picture and allow your vet to develop a treatment plan that addresses all contributing factors.

The good news is that many of the conditions causing third eyelid protrusion in senior cats are treatable. Dehydration responds to fluid therapy. GI problems respond to dietary management and medication. Eye infections clear with appropriate treatment. Kidney disease, hyperthyroidism, and hypertension can all be managed effectively. Even Horner's syndrome often resolves on its own. With the right diagnosis and a comprehensive treatment plan, many owners find that their cat's third eyelids retract and the cat looks and feels significantly better.

This article provides general information about third eyelid protrusion in senior cats. Every cat is different, and the underlying causes and appropriate treatments vary widely. If your senior cat's third eyelid is visible, please consult your veterinarian for a thorough evaluation including an eye exam, blood work, and hydration assessment. Do not use over-the-counter eye drops or attempt to push the third eyelid back into position. Sudden third eyelid protrusion with eye pain, vision loss, or signs of serious illness should be treated as an urgent matter requiring prompt veterinary care.