What Does It Mean When a Cat’s Third Eyelid Is Showing?

When a Cat’s Third Eyelid Is Showing

You are sitting on the couch and your cat jumps into your lap, turns to look at you, and something about their eyes stops you mid-sentence. There is a white or pinkish film creeping across the inner corner of one or both eyes, partially covering the iris like a curtain that has been drawn halfway across a window. It might be subtle, just a sliver visible at the nasal edge when your cat blinks. Or it might be pronounced enough that a significant portion of the eye is obscured and your cat looks at you through what appears to be a translucent veil. Either way, you know instinctively that you have never seen this before, and the strangeness of it unsettles you in a way that is difficult to articulate.
What you are looking at is your cat’s third eyelid, a structure formally known as the nictitating membrane, and its visible protrusion is one of the most reliable physical indicators that something is happening inside your cat’s body that deserves investigation. Cats possess this third eyelid from birth, tucked neatly into the inner corner of each eye where it normally remains almost entirely hidden from view. Most owners go years without ever seeing it, which is precisely why its sudden appearance triggers alarm. The third eyelid is not supposed to be visible during normal waking hours in a healthy cat. When it is, the cat is telling you something through the only language available to them, and the range of things they might be saying spans from mild and self-resolving to urgent and potentially sight-threatening.
Cat owners throughout the United States, the United Kingdom, Germany, across Europe, Australia, and everywhere domestic cats share homes with families encounter this phenomenon with enough regularity that it consistently ranks among the top feline health searches online. The clinical term for a visible third eyelid is third eyelid elevation or protrusion of the nictitating membrane, and veterinarians encounter it frequently enough that they can often narrow the list of likely causes within minutes of examining the cat.
This guide explains what the third eyelid is and why cats have it, details the full range of conditions that cause it to become visible, walks through the diagnostic process your veterinarian will use to identify the underlying cause, covers the treatments available for each condition, and provides the practical knowledge you need to distinguish a situation that can wait until morning from one that demands immediate attention. Understanding this single physical sign equips you to respond with informed calm rather than uninformed panic, and that distinction matters more than most owners appreciate when time and accuracy both count.

What the Third Eyelid Actually Is and Why Cats Have One

The nictitating membrane is a thin, partially transparent fold of tissue anchored at the inner corner of each eye, positioned between the lower eyelid and the eyeball itself. It is composed of a T-shaped cartilage framework covered by conjunctival tissue, and at its base sits a tear-producing gland that contributes a significant portion of the eye’s total tear film. In cats, this membrane is typically pale white, light pink, or occasionally pigmented in darker-coated breeds.
The third eyelid serves several protective functions that the upper and lower eyelids alone cannot fully provide. When it sweeps across the eye surface, which it does reflexively during sleep and certain protective responses, it distributes a portion of the tear film across the cornea, clears debris and particulate matter, and provides a physical barrier against trauma. It contains lymphoid tissue that participates in local immune defense, essentially functioning as a small outpost of the immune system positioned directly at one of the body’s most vulnerable points of entry. The tear gland embedded within its base, sometimes called the gland of the third eyelid, produces roughly thirty to fifty percent of the eye’s aqueous tear layer, making it far more than a vestigial structure.
In a healthy, alert, well-hydrated cat, the nictitating membrane retracts fully into the inner corner of the eye and remains virtually invisible. You might catch a brief glimpse of it as your cat wakes from a deep sleep and the membrane has not yet fully retracted, or during a slow, drowsy blink, but under normal circumstances it should disappear within seconds as the cat becomes fully alert. Persistent visibility of the third eyelid during normal waking activity, whether in one eye or both, signals that something is causing the membrane to remain elevated beyond its resting position.

The Mechanism Behind Third Eyelid Elevation

Understanding why the third eyelid becomes visible requires a brief look at the anatomy that holds it in place. The nictitating membrane is maintained in its retracted position by a combination of factors. The retractor bulbi muscle, a smooth muscle that sits behind the eyeball, helps hold the globe firmly forward in the socket. Sympathetic nerve input maintains the muscle tone that keeps the membrane retracted. The orbital fat pad that cushions the eye within the skull provides structural support that keeps the globe seated in its proper position. The size of the globe itself relative to the orbit plays a role.
When any of these factors is disrupted, the third eyelid can passively rise. If the eye retracts slightly into the socket due to pain, dehydration, weight loss, or muscle dysfunction, the third eyelid slides upward to fill the space that opens at the front of the eye. If sympathetic nerve supply is interrupted, the muscle tone that actively holds the membrane down is lost. If the orbital fat pad shrinks from weight loss or dehydration, the structural scaffolding that supports the globe’s position weakens. If the eye itself becomes inflamed and swollen, the altered pressure dynamics within the orbit can push the membrane forward.
This is why third eyelid protrusion is such a valuable clinical sign. It does not point to a single diagnosis. Instead, it reflects a disruption in the delicate mechanical and neurological balance that normally keeps the membrane hidden, and the pattern of that disruption, whether the elevation is unilateral or bilateral, whether it is accompanied by other neurological signs, and what additional symptoms are present, guides the diagnostic process with remarkable specificity.

Bilateral Third Eyelid Protrusion and Its Most Common Causes

When both third eyelids are elevated simultaneously, the list of likely causes centers on systemic conditions affecting the entire body rather than problems localized to one eye.

Gastrointestinal illness is one of the most frequent triggers of bilateral third eyelid protrusion in cats, common enough that the association has its own clinical name. Haw syndrome refers to bilateral elevation of the nictitating membranes in an otherwise healthy-appearing cat, typically a young cat under two years of age, occurring in association with or shortly following a gastrointestinal upset such as diarrhea. The exact mechanism is debated but is believed to involve transient disruption of sympathetic nerve input to the smooth muscle that retracts the third eyelid, possibly triggered by viral gastrointestinal infection or intestinal inflammation that affects the sympathetic nerve fibers as they travel through or near the abdominal cavity. Haw syndrome is generally self-limiting, resolving on its own within four to eight weeks as the underlying gastrointestinal condition clears. Some veterinarians prescribe topical phenylephrine eye drops to improve cosmetic appearance during the recovery period, though treatment is not strictly necessary for resolution.
Dehydration causes bilateral third eyelid elevation through a straightforward mechanical pathway. As the body loses fluid, the orbital fat pad that cushions the eye within the skull loses volume. The eye settles slightly deeper into the socket, and the third eyelid passively rises to occupy the space that opens at the front. The degree of third eyelid visibility often correlates roughly with the severity of dehydration, making it a useful clinical indicator during physical examination. Dehydration severe enough to produce visible third eyelid protrusion typically also produces other signs including dry or tacky gums, decreased skin turgor where the skin tents briefly when gently pinched and released, sunken eyes, lethargy, and reduced urine output.
Intestinal parasites have been associated with bilateral third eyelid protrusion in cats, particularly heavy tapeworm or roundworm burdens. The mechanism likely overlaps with the gastrointestinal pathway described in haw syndrome, where parasitic inflammation of the intestinal tract disrupts the sympathetic nerve fibers that control third eyelid retraction. Cats with heavy parasite burdens may also be mildly dehydrated from fluid losses in the stool, compounding the mechanical effect on globe position.
Sedation and anesthesia produce bilateral third eyelid protrusion through direct pharmacological relaxation of the retractor bulbi muscle and reduction in sympathetic tone. If your cat recently underwent a veterinary procedure involving sedation, visible third eyelids during the recovery period are expected and should resolve as the drugs clear the system over hours. Certain oral medications with sedative properties can produce a similar but milder effect.
Systemic viral infections including feline herpesvirus, feline calicivirus, and feline immunodeficiency virus can trigger bilateral third eyelid protrusion through a combination of dehydration, general malaise, and in the case of herpesvirus, direct inflammatory effects on ocular tissues. A cat showing bilateral third eyelid elevation along with sneezing, nasal discharge, oral ulcers, fever, and lethargy is presenting a picture consistent with upper respiratory infection that warrants veterinary evaluation.

Unilateral Third Eyelid Protrusion and What It Signals

When only one third eyelid is elevated while the other remains normal, the diagnostic focus shifts toward conditions affecting that specific eye or the nerves supplying it. Unilateral protrusion narrows the differential list considerably and often points toward more localized pathology.

Horner’s syndrome is one of the most important causes of unilateral third eyelid elevation in cats and deserves particular attention because its presentation is distinctive and its underlying causes range from benign to life-threatening. Horner’s syndrome occurs when the sympathetic nerve pathway supplying the eye is disrupted at any point along its lengthy anatomical course, which runs from the brain through the neck, into the chest, back up through the neck, and finally to the eye and its associated structures. Interruption of this pathway produces a characteristic cluster of signs on the affected side: elevation of the third eyelid, constriction of the pupil (miosis), drooping of the upper eyelid (ptosis), and recession of the eyeball into the socket (enophthalmos). When all four signs are present together on one side of the face, the diagnosis of Horner’s syndrome is clinically straightforward. The far more challenging question is determining where along the sympathetic pathway the disruption has occurred and why.
In cats, the causes of Horner’s syndrome include middle ear infections that damage the sympathetic fibers as they pass through the middle ear cavity, trauma to the head or neck, chest masses including lymphoma that compress the sympathetic trunk as it courses through the thorax, and in a frustrating number of cases, no identifiable cause at all. Idiopathic Horner’s syndrome, where the condition appears spontaneously without a discoverable underlying reason, accounts for a substantial percentage of feline cases and typically resolves on its own over weeks to months. However, the possibility that an occult mass, infection, or other serious pathology is responsible means that cats presenting with Horner’s syndrome should receive a thorough diagnostic workup including otoscopic examination of the ear canals, chest radiographs to evaluate for thoracic masses, and in some cases advanced imaging of the head and neck.
Ocular pain from any cause triggers a protective reflex that retracts the globe and elevates the third eyelid on the affected side. Corneal ulcers, foreign bodies trapped under the eyelid or embedded in the corneal surface, uveitis (inflammation of the internal structures of the eye), glaucoma (elevated intraocular pressure), and conjunctivitis can all produce enough discomfort to trigger this protective response. When pain is the driver, additional signs are usually present: squinting or holding the affected eye partially closed, excessive tearing, redness of the conjunctiva, cloudiness of the cornea, sensitivity to light, or visible changes in pupil size. A cat pawing at one eye or rubbing the side of its face against furniture while that eye’s third eyelid is elevated presents a picture strongly suggestive of ocular pain.
Masses behind the eye, whether tumors, abscesses, or cysts, can physically push the third eyelid forward by occupying space within the orbit and displacing normal structures. Retrobulbar masses may also cause the affected eye to protrude forward (exophthalmos), a presentation that looks dramatically different from the sunken-eye appearance of Horner’s syndrome. Any discrepancy in the apparent prominence of the two eyes, particularly when accompanied by third eyelid elevation on the more prominent side, raises concern for a space-occupying lesion behind the globe.
Nerve damage from trauma, including injuries sustained during fights with other cats, falls, or vehicle accidents, can produce unilateral third eyelid protrusion through direct damage to the sympathetic or motor nerves supplying the eye and its associated structures. The history of recent trauma, even if the specific eye injury was not witnessed, provides important diagnostic context.

Cherry Eye Versus Third Eyelid Protrusion

A common point of confusion among cat owners is the distinction between protrusion of the third eyelid itself and prolapse of the gland within the third eyelid, a condition commonly known as cherry eye. These are different conditions with different appearances and different clinical implications.
Third eyelid protrusion presents as a white or pink membrane partially covering the eye from the inner corner, creating a veil-like appearance. The tissue is flat and membranous. Cherry eye, by contrast, presents as a round, red, swollen mass at the inner corner of the eye. It occurs when the gland of the third eyelid, normally anchored in position by a ligament, loses its attachment and pops out from behind the membrane, creating a red, fleshy protrusion that resembles a small cherry. Cherry eye is considerably more common in dogs than in cats, but it does occur in felines, particularly in Burmese and other brachycephalic breeds.
Cherry eye requires surgical correction to reposition or, in some cases, partially remove the prolapsed gland. Simply pushing the gland back into position provides only temporary relief as it will inevitably prolapse again without surgical fixation. Removing the gland entirely is generally avoided because of its substantial contribution to tear production, and loss of this gland predisposes the eye to chronic dry eye (keratoconjunctivitis sicca) later in life.

The Diagnostic Process at the Veterinary Clinic

When you bring your cat to the veterinarian for visible third eyelid protrusion, the examination follows a systematic approach designed to identify the underlying cause efficiently.
The veterinarian will begin by observing whether the protrusion is unilateral or bilateral, which immediately divides the diagnostic possibilities into two broad categories. They will examine the eyes closely using an ophthalmoscope and possibly a slit lamp to evaluate the cornea for ulcers or foreign bodies, the anterior chamber for signs of uveitis, the pupils for symmetry and appropriate light responses, and the intraocular pressure using tonometry to screen for glaucoma. Fluorescein stain, an orange dye that fluoresces bright green under blue light, may be applied to the corneal surface to reveal ulcers or abrasions that are invisible to the naked eye.
A thorough general physical examination evaluates hydration status, body condition, lymph node size, abdominal palpation for masses or organ enlargement, and auscultation of the chest for cardiac and respiratory abnormalities. The ear canals will be examined with an otoscope to check for middle ear disease, particularly when Horner’s syndrome is suspected.
Blood work including a complete blood count and serum chemistry panel screens for systemic disease, dehydration, infection, and organ dysfunction. Fecal examination may be recommended if intestinal parasites are suspected as a contributing factor. Chest radiographs are indicated when Horner’s syndrome is present to rule out thoracic masses. Advanced imaging with CT or MRI of the head and neck may be pursued in cases where the initial workup does not yield a clear answer or when a retrobulbar mass is suspected.
In cases of suspected Horner’s syndrome, pharmacological testing with topical phenylephrine eye drops can help localize the lesion along the sympathetic pathway. The speed at which the third eyelid retracts and the pupil dilates after application of phenylephrine provides information about whether the disruption is preganglionic (first or second order) or postganglionic (third order), which in turn influences the list of underlying causes to investigate and the prognosis to discuss.

Treatment Based on the Underlying Cause

Treatment for third eyelid protrusion is directed at the condition causing it rather than at the third eyelid itself. The membrane is a messenger, not the message, and pushing it back or ignoring it without investigating the cause misses the point entirely.
Dehydration is corrected through fluid therapy, either subcutaneous fluids for mild cases or intravenous fluids for more severely dehydrated cats. Once hydration is restored, the orbital fat pad regains its volume, the globe returns to its normal position, and the third eyelid retracts. The underlying cause of the dehydration, whether vomiting, diarrhea, kidney disease, or inadequate water intake, must be identified and addressed to prevent recurrence.
Gastrointestinal-associated haw syndrome in young cats typically resolves without specific treatment directed at the eyes. Supportive care for the gastrointestinal upset, including antiparasitic treatment if indicated, probiotics, and dietary management, addresses the root cause. Some veterinarians prescribe topical ophthalmic phenylephrine to hasten retraction of the membranes for cosmetic purposes and owner reassurance, though this treats the symptom rather than the cause.
Intestinal parasites are treated with appropriate deworming protocols based on fecal examination results. Broad-spectrum anthelmintics effective against roundworms and tapeworms are commonly used, and repeat fecal testing confirms clearance.
Corneal ulcers are managed based on their depth and complexity. Superficial ulcers may heal with topical antibiotic drops alone, while deeper or infected ulcers require more aggressive treatment including frequent topical medication, pain management, and sometimes surgical intervention. A protective cone or collar prevents the cat from rubbing the eye and worsening the ulcer during healing.
Uveitis requires identification of its underlying cause, which in cats includes infectious diseases such as feline infectious peritonitis, feline leukemia virus, feline immunodeficiency virus, toxoplasmosis, and bartonellosis, as well as non-infectious causes including trauma and lens-induced inflammation. Treatment involves topical anti-inflammatory drops, pupil-dilating drops to prevent adhesions within the eye, and specific therapy directed at the identified infectious agent.
Glaucoma in cats is less common than in dogs but carries a guarded prognosis for vision preservation. Treatment aims to reduce intraocular pressure through topical and systemic medications, and in refractory cases, surgical options including laser cyclophotocoagulation or enucleation (eye removal) may be necessary.
Horner’s syndrome treatment depends entirely on the underlying cause. Middle ear infections are treated with systemic antibiotics and sometimes surgical drainage. Thoracic masses require oncological evaluation and treatment. Idiopathic Horner’s syndrome is managed with monitoring and topical phenylephrine if desired, with the reassurance that spontaneous resolution occurs in the majority of cases over several weeks to months.
Retrobulbar masses require advanced imaging for characterization and may necessitate fine-needle aspirate or biopsy for definitive diagnosis. Treatment depends on the nature of the mass, whether infectious abscess, benign growth, or malignant tumor.

When Third Eyelid Protrusion Is an Emergency

Most cases of third eyelid protrusion can be evaluated during a routine appointment within a day or two. However, certain presentations demand same-day or emergency veterinary care.
A third eyelid protrusion accompanied by a cloudy, red, or painful eye that the cat is squinting shut or pawing at may indicate a corneal ulcer, acute glaucoma, or penetrating injury that requires immediate treatment to preserve vision. Any delay in treating a deep corneal ulcer or acute glaucoma can result in permanent vision loss or loss of the eye.
Third eyelid elevation combined with severe lethargy, high fever, complete appetite loss, difficulty breathing, or abdominal distension suggests a serious systemic condition that may include feline infectious peritonitis, sepsis, or an internal crisis requiring emergency stabilization.
A cat that has experienced recent trauma such as a fall, vehicle strike, or fight with another animal and subsequently develops third eyelid protrusion along with any neurological abnormality including head tilt, circling, disorientation, or unequal pupils needs immediate evaluation for head trauma, orbital fracture, or internal injuries.
Sudden onset of Horner’s syndrome signs including third eyelid protrusion, small pupil, droopy eyelid, and sunken eye on one side warrants prompt evaluation even if the cat otherwise appears well, because of the need to rule out thoracic masses and other significant underlying conditions.

What You Can Observe at Home Before the Appointment

While awaiting your veterinary appointment, careful observation at home can provide valuable information that accelerates the diagnostic process.
Note whether one or both third eyelids are affected. Check whether the protrusion is constant or intermittent, worsening or stable, and whether it appeared suddenly or has developed gradually over days. Observe whether the pupils are equal in size and respond normally to changes in light. Look for any discharge from the eyes, whether clear, cloudy, yellow, or green. Note whether the cat is squinting, rubbing the face, or showing sensitivity to light. Assess whether the cat is eating, drinking, urinating, and defecating normally. Check the gums for moisture and color. Note any recent changes in weight, appetite, behavior, or activity level. Document any recent illness including vomiting or diarrhea, any medications administered, any known trauma, and any recent changes in the household environment.
Taking a clear photograph of the eye or eyes showing the third eyelid protrusion provides your veterinarian with a visual record that captures exactly what you are seeing, which is particularly useful if the protrusion is intermittent and might not be maximally visible during the examination.

Frequently Asked Questions

Is it normal to see the third eyelid when my cat is falling asleep or just waking up?
Brief visibility of the third eyelid during the transition between sleep and full wakefulness is entirely normal. During sleep, the third eyelid extends across the eye surface to protect and lubricate the cornea. As the cat wakes and the retractor muscles engage, the membrane should retract fully within a few seconds to a minute. If it persists well beyond the waking period or is visible during alert, active states, that crosses into abnormal territory.

My cat’s third eyelids have been showing for a week but she seems perfectly fine otherwise. Should I still see a vet?
Yes. Cats are extraordinarily skilled at concealing illness, and the visible third eyelid may be the only external sign of an internal condition that is not yet producing other obvious symptoms. Persistent protrusion lasting more than a day or two warrants professional evaluation even in the absence of other apparent signs. The sooner a cause is identified, the simpler and more effective treatment tends to be.

Can stress alone cause the third eyelid to show?
Acute stress does not typically cause sustained third eyelid protrusion. However, stress can contribute to gastrointestinal upset, dehydration, and immune suppression, all of which can indirectly lead to third eyelid elevation. If your cat’s third eyelids became visible following a stressful event such as a move, introduction of a new pet, or boarding, the stress may have triggered an underlying condition that is now manifesting through the eyes.

Does the color of the third eyelid matter?
The normal color ranges from white to pale pink. A red, swollen, or inflamed-appearing third eyelid may indicate conjunctivitis, cherry eye, or local irritation. A pigmented or darkly colored third eyelid can be a normal variation in certain breeds or coat colors but can occasionally indicate melanosis or other pigmented lesions that warrant veterinary assessment. Any change in the color of a previously normal-appearing third eyelid should be noted and reported.

Can I push the third eyelid back into place myself?
You should not attempt to manually reposition the third eyelid. It is elevated because of an underlying condition, and pushing it back does not address the cause. Additionally, applying pressure to the eye or surrounding structures without proper technique risks injuring the cornea or other delicate ocular tissues. The only appropriate intervention at home is observation and documentation while you arrange veterinary evaluation.

My outdoor cat came home with one third eyelid showing and the eye looks watery. What is the most likely cause?
In an outdoor cat presenting with unilateral third eyelid protrusion and tearing, the most likely causes include a corneal scratch or ulcer from vegetation or a fight, a foreign body such as a grass seed lodged under the eyelid or on the corneal surface, or direct trauma to the eye or orbit. These presentations warrant prompt veterinary attention because corneal injuries can deteriorate rapidly without appropriate treatment and foreign bodies cause ongoing damage for as long as they remain in contact with the eye.

Is haw syndrome serious and will it come back?
Haw syndrome is generally benign and self-limiting. Most cats recover fully within four to eight weeks without lasting consequences. Recurrence is possible but uncommon. The condition does not cause permanent damage to the eyes or the third eyelids themselves. However, because the diagnosis of haw syndrome is one of exclusion, meaning it is made after other causes have been ruled out, it should only be accepted after appropriate veterinary evaluation has confirmed that no more serious underlying condition is responsible.

Can feline herpesvirus cause the third eyelid to show permanently?
Feline herpesvirus can cause recurrent episodes of conjunctivitis and ocular inflammation that may produce third eyelid protrusion during active flare-ups. Between flares, the third eyelid typically retracts to its normal position. Chronic or severe herpesvirus-related eye disease can occasionally lead to scarring or adhesions involving the third eyelid (symblepharon), which may cause permanent positional changes. Cats with known herpesvirus infections benefit from ongoing veterinary monitoring and prompt treatment during flare-ups to minimize the risk of long-term ocular complications.

Does third eyelid protrusion affect my cat’s vision?
If the membrane covers a significant portion of the pupil, it can partially obstruct the visual field on the affected side. However, the third eyelid is thin and semi-transparent, so even when elevated it does not produce complete visual blockage. The greater concern is not the membrane itself but the underlying condition causing it to protrude, which may independently affect vision through mechanisms such as corneal damage, uveitis, or glaucoma.

At what age are cats most likely to develop problems that cause third eyelid showing?
Haw syndrome and parasite-related causes are more common in young cats under two years of age. Horner’s syndrome, uveitis related to chronic infections such as FIP or FIV, and retrobulbar masses become more frequent in middle-aged and older cats. Traumatic causes and corneal injuries can occur at any age. There is no age at which third eyelid protrusion can be safely dismissed without investigation.