The Third Eyelid: What It Is and Why It Matters
Most cat owners are familiar with the sight of a pale, filmy membrane creeping across the inner corner of their cat's eye. This is the third eyelid — formally called the nictitating membrane or palpebra tertia — and in a healthy, alert cat it should be largely invisible. When it becomes visible, especially persistently or in both eyes simultaneously, it is your cat's body signalling that something needs attention.
The third eyelid serves several important functions. It helps distribute tears across the ocular surface, contributes to immune defence through lymphoid tissue embedded within it, and provides additional physical protection to the cornea. In some situations, it moves across the eye as a reflex response to protect the globe from injury or irritation.
When Third Eyelid Visibility Is Normal
There are circumstances where brief visibility of the third eyelid is perfectly normal and not a cause for alarm.
During deep sleep or when waking from sleep, cats often show some third eyelid as they drift between states of consciousness. If your cat is dozing and you notice the membrane partly visible, this is generally not concerning. Similarly, immediately after blinking, the third eyelid may briefly sweep across the eye.
Some kittens also show their third eyelids more readily than adult cats, particularly when very relaxed or during feeding. As long as the cat is otherwise well and alert when awake, this tends to resolve without intervention.
Causes of Persistent Third Eyelid Visibility
Persistent or pronounced third eyelid protrusion in a fully alert cat is not normal and warrants veterinary investigation. The causes are varied and range from localised eye problems to systemic illness.
Haw's Syndrome
This is one of the more curious causes of third eyelid protrusion in cats, and interestingly, one of the more benign. Haw's syndrome refers to bilateral protrusion of the third eyelids — both eyes simultaneously — in an otherwise apparently healthy cat. It is most commonly seen following gastrointestinal illness, particularly diarrhoea, and is thought to involve autonomic nervous system disruption associated with intestinal parasites or viral gut infections.
The condition typically resolves spontaneously over four to eight weeks once the underlying gastrointestinal issue is addressed. No specific treatment targeting the third eyelid is usually required, though a veterinary examination to identify and treat the gut problem is worthwhile.
Conjunctivitis and Eye Infections
Inflammation of the conjunctival tissue — the moist lining around the eye — can cause the third eyelid to protrude as a protective response. Upper respiratory infections, particularly those involving feline herpesvirus, frequently cause conjunctival inflammation alongside nasal symptoms. The third eyelid may be particularly prominent in the more severely affected eye.
Foreign Bodies and Corneal Injury
A grass seed, grit or other foreign material lodged beneath the third eyelid or on the ocular surface triggers a strong protective response, causing the membrane to advance across the eye. If the protrusion is more pronounced in one eye and appeared suddenly — particularly in a cat that has recently been outdoors — a foreign body should be high on the list of possibilities.
Corneal scratches or ulcers produce the same reflex response. The eye will typically also be squinted, and the cat will show signs of discomfort when the area around the eye is approached.
Horner's Syndrome
Horner's syndrome is a neurological condition resulting from damage to or disruption of the sympathetic nerve supply to the eye and face. It produces a characteristic cluster of signs on one side of the face: third eyelid protrusion, a drooping upper eyelid, a smaller pupil and a slightly sunken appearance of the eyeball. The underlying causes range from otitis media — infection of the middle ear — to bite wounds, trauma and in some cases tumours affecting the nerve pathway. Identifying the cause requires thorough veterinary investigation.
Systemic Illness and Dehydration
A cat that is unwell, dehydrated or experiencing significant weight loss may show bilateral third eyelid protrusion simply because the supportive fat pad behind the eye has diminished, allowing the globe to sit slightly more deeply in the socket. This pulls the third eyelid forward. Conditions associated with muscle wasting or dehydration — including kidney disease, cancer and severe infections — can all produce this appearance.
Cherry Eye
Though far more common in dogs, cats can occasionally develop prolapse of the gland of the third eyelid — what is informally called cherry eye. This appears as a rounded, red or pink mass protruding from the inner corner of the eye. It requires veterinary attention and is typically corrected surgically to preserve the gland's tear-producing function.
What to Do When You Notice Third Eyelid Showing
The appropriate response depends on what else you observe alongside the third eyelid protrusion.
- If both third eyelids are visible and your cat recently had diarrhoea but is otherwise behaving normally, a non-urgent veterinary appointment is a reasonable next step
- If only one eye is affected, particularly with squinting or discharge, arrange a vet appointment within 24 hours
- If you notice signs consistent with Horner's syndrome — unequal pupils, drooping eyelid — veterinary assessment should not be delayed
- If your cat appears systemically unwell alongside the eye changes, the veterinary visit should be treated as urgent
At the appointment, your vet will examine both eyes carefully, assess the cat's general health and may perform additional tests including blood work or imaging depending on what they find. The third eyelid itself is rarely the problem — it is the messenger, reliably telling you that something elsewhere deserves attention. Listening to it promptly is always the right instinct.