When a Membrane Covers Your Cat’s Eye — Causes and Suspected Conditions of Third Eyelid (Nictitating Membrane) Exposure

Eye HealthSymptomsMeongsiljang Veterinary Advisory Board

When a cat’s third eyelid becomes visible, it can be caused by a range of issues, from mild lethargy to Horner’s syndrome or even eye pain. The urgency is determined by whether one or both eyes are affected and by any accompanying symptoms.

What Is Nictitating Membrane Exposure in Cats?

A close-up view of a cat's eye showing the nictitating membrane exposed on the inner corner.

The "third eyelid" in cats is a thin membrane hidden in the inner corner of the eye (ventromedial orbit) that emerges to protect the eyeball and spread tears. It is normal for it to be barely visible most of the time. When this pinkish membrane covers from the inner corner toward the pupil, it is called "third eyelid protrusion." If both eyes are affected, it is often a sign related to the cat's overall health rather than the eyes themselves (e.g., Haw syndrome, where both protrude without eye disease). If only one eye suddenly protrudes, you should first suspect nerve abnormalities (Horner's syndrome) or eye pain. The accompanying symptoms are more important than the protrusion itself, so please be sure to check for any other signs.

Normal vs. Exposure — At a Glance

Item	Normal	Mild protrusion	Marked protrusion
Visibility of the membrane	Barely visible	Inner 1/4 visible	Covers more than half the eye
Pattern	Same on both eyes	Both or one eye	More common in one eye
Accompanying symptoms	None	Slight decline in condition	Pain/pupil abnormality/decreased appetite
Response	Observation only	See a vet after 1–2 days of observation	Same-day or next-day vet visit

If the membrane is red and swollen, immediate veterinary care is needed regardless of stage.

Checklist of Accompanying Symptoms

Isolated exposure of the nictitating membrane is rare. If you notice any of the following symptoms alongside it, please make a note and share them with your veterinarian during the visit. - Frequent blinking or squinting: A sign of ocular pain - Unequal pupil size: Possible Horner’s syndrome - Drooping upper eyelid and sunken eye: Four key signs of Horner’s syndrome - Decreased appetite and activity, diarrhea: General decline in health or gastrointestinal issues - Increased tearing, discharge, and redness of the conjunctiva: Conjunctivitis or corneal injury - Bilateral exposure accompanied by weight loss: Possible chronic disease

Eye symptoms that occur alongside feline entropion

Signs That Require Immediate Veterinary Care

If any of the following apply, your pet needs same-day veterinary care, as it could indicate eye pain or a neurological emergency: - One pupil is constricted and the upper eyelid droops (possible Horner’s syndrome) - The eye appears red, swollen, and bulging (possible cherry eye or tumor) - Inability to open the eye or frequent pawing at it (suspected corneal ulcer or foreign body) - Decreased consciousness, ataxity, and bilateral eye exposure (indicative of severe systemic disease) - Refusal of food and water for more than 24 hours, accompanied by bilateral eye exposure

Bilateral Exposure — Haw Syndrome and Systemic Causes

When both third eyelids are exposed simultaneously, Haw syndrome should be the first consideration. Haw syndrome is characterized by the concurrent protrusion of both third eyelids in the absence of any specific ocular disease. In many cases, it is idiopathic, meaning the cause remains unknown, and typically follows a self-limiting course, resolving spontaneously within a few days. Although frequently reported in young cats and sometimes observed alongside gastrointestinal symptoms like diarrhea or general lethargy, these accompanying factors have not been definitively proven as causes in veterinary textbooks. Nevertheless, bilateral exposure serves as a signal that the issue may extend beyond the eyes to the entire body. Therefore, it is essential to assess the cat’s overall condition, including appetite, activity level, and elimination patterns.

Unilateral Exposure — Horner’s Syndrome and Ocular Pain

When the third eyelid suddenly protrudes on only one side, it often points to a neurological issue or a problem within the eye itself. The most classic example is Horner's syndrome, which presents with a combination of signs: constricted pupil (miosis), drooping of the upper eyelid (ptosis), sunken appearance of the eye (enophthalmos), and exposure of the third eyelid. Reported causes include ear conditions such as otitis media, or lesions affecting the sympathetic nerve pathway, such as nerve damage or nerve sheath tumors in the neck region. Another common cause is eye pain from conditions like corneal ulcers, where the third eyelid rises in response to the pain stimulus. If the third eyelid is exposed on only one side and accompanied by these signs, it is best to seek veterinary ophthalmic care as soon as possible.

Comparing Four Ocular Signs of Feline Horner’s Syndrome

Cherry Eye — When the Nictitating Gland Itself Prolapses

Inside the third eyelid lies the nictitating gland (also known as the third eyelid gland), which produces the tear film and helps spread it across the surface of the eye. When this gland prolapses—slipping out of place due to weakened or abnormally developed connective tissue that normally holds it in position—it appears as a smooth, pink-to-red mass at the inner corner of the eye, a condition commonly referred to as "cherry eye." While rare in cats compared to dogs, a predisposition has been reported in the Burmese breed. Unlike simple exposure, cherry eye is characterized by a darker color and a more pronounced, raised appearance. Because the prolapsed tissue can dry out, leading to conjunctivitis, reduced tear production, and secondary corneal damage, surgical repositioning to return the gland to its normal location is recommended. Removal of the gland should be avoided whenever possible.

How Diagnosis Is Performed at the Clinic

Veterinarians perform step-by-step examinations to determine whether the condition is due to simple exposure or another underlying disease. - Slit-lamp examination: Detailed observation of the cornea and conjunctiva - Fluorescein staining: To check for the presence of corneal ulcers - Schirmer tear test: To measure tear production (to rule out dry eye) - Pupillary response and neurological exams: To localize Horner’s syndrome - Blood and fecal tests: To rule out systemic diseases and intestinal parasites Treatment varies depending on the cause, ranging from eye drops and deworming medication to imaging studies and surgery, making accurate differential diagnosis essential.

A veterinarian carefully examining a cat’s eyes.

Actions to Absolutely Avoid at Home

Improper handling by the owner can lead to more severe injuries. - Pushing the third eyelid back in with a finger or cotton swab (risk of corneal damage) - Administering human eye drops or artificial tears without veterinary guidance (potential toxicity depending on ingredients) - Self-applying steroid ointment for yellow discharge or redness - Leaving the condition untreated for over a week under the assumption that it will resolve on its own - Attempting massage or pressure techniques based on online videos

Prevention and Routine Management Tips

While you can’t completely prevent everted nictitating membranes, reducing underlying health issues and catching changes early makes management much easier. - Annual comprehensive checkup: Early detection of systemic diseases - Regular deworming and digestive health care: Minimizes gastrointestinal issues like diarrhea to maintain overall well-being - Ear (middle ear) health care: Checks for problems such as otitis media that may be linked to Horner’s syndrome - Hygiene in multi-cat households: Prevents the spread of ocular surface diseases like conjunctivitis - Weekly photo records: Quickly spot changes by comparing with your cat’s normal eye appearance In particular, when both eyes are affected, it may signal a systemic issue rather than just an eye problem, so please monitor senior cats more frequently.

A pet owner checking their cat’s eye condition at home.

Reviewed by a veterinarian

Dr. Tony — Punnawat Phongkittirak

Veterinarian

A veterinarian who majored in veterinary medicine at Khon Kaen University, Thailand, and completed the IVSA program at North Carolina State University in the United States. Drawing on clinical experience at animal hospitals, he works in the pet healthcare field and is dedicated to building a digital care environment that connects pet parents with veterinarians.

Frequently Asked Questions

If the hymen is visible for a few days, should I take my pet to the vet?

If both eyes are affected and your pet’s overall condition is stable, you can monitor them for 1–2 days. However, if symptoms persist for more than 48 hours, a veterinary visit is necessary. If only one eye is affected, or if there is pain or abnormal pupil response, we recommend seeking veterinary care the same day.

Is it normal to see the third eyelid right after waking up?

It can be normal to notice it briefly right after waking up. However, if it persists for more than 5–10 minutes after waking or appears prominently in both eyes, it should be considered abnormal.

Does Horner’s syndrome resolve on its own?

Haw syndrome is an idiopathic condition characterized by the bilateral protrusion of the third eyelid in the absence of any underlying eye disease. It is often self-limiting, meaning it typically resolves on its own within a few days without any specific treatment. However, because it is important to rule out other potential causes, it is advisable to have your pet examined by a veterinarian if the protrusion persists for an extended period or is accompanied by other symptoms.

Does Horner's syndrome leave any aftereffects?

It depends on the underlying cause. Mild cases, such as idiopathic (cause unknown) conditions, may resolve on their own over time, but if there’s an underlying disease like a tumor or severe trauma, some symptoms may persist. The likelihood and duration of recovery vary widely depending on the individual animal and the cause, so it’s important to identify the root cause through proper diagnosis.

Can cats get cherry eye?

Although less common in dogs, it has been reported in certain breeds such as the Vizsla. In cats, the condition presents similarly to dogs, with a pink to red mass visible at the inner corner of the eye. Leaving the prolapsed gland tissue exposed can cause it to dry out and reduce tear production; therefore, surgical repositioning is recommended over excision.

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References

[1] Little SE. The Cat: Clinical Medicine and Management, 2nd Edition. Elsevier Saunders; 2014. Chapter on Ocular Disease.

[2] Maggs DJ, Miller PE, Ofri R. Slatter's Fundamentals of Veterinary Ophthalmology, 6th Edition. Elsevier; 2018. Third Eyelid chapter.

[3] Chahory S, Crasta M, Trio S, Clerc B. Three cases of prolapse of the nictitans gland in cats. Vet Ophthalmol. 2004;7(6):417-419.

[4] Schoofs SH. Prolapse of the gland of the third eyelid in a cat: a case report and literature review. J Am Anim Hosp Assoc. 1999;35(3):240-242.

[5] Featherstone HJ, Heinrich CL. Ophthalmic Examination and Diagnostics. In: Gelatt KN, ed. Veterinary Ophthalmology, 6th Edition. Wiley-Blackwell; 2021.