Cats and people have similar eyelid structures. Both have an upper and lower fold of skin that blinks to cover, protect, and moisturise the eye. But cats have an extra element that people don’t: a third eyelid!
A cat’s third eyelid, officially called the nictitating membrane, is a thin structure mainly made of cartilage. It’s located at the inner lower part of the eye, close to the nose. Usually, you won’t see most of this third eyelid, except for a small, crescent-shaped slit in the corner of the eye.
“Healthy eyelids in cats are smooth and free of hairs or other irregularities at the eyelid margin,” explains Dr. Sandra van der Woerdt, senior veterinarian and the Service Head of Ophthalmology at the Schwarzman Animal Medical Center.
You can generally check the health of your cat’s eyelids with a glance. Cats with some medical conditions may have a more noticeable third eyelid.
Read on for details on a cat’s third eyelid, including its function, what might make it visible, and when to visit the vet.
Why Do Cats Have a Third Eyelid?
Cats aren’t the only animals with a third eyelid.
“Most terrestrial and aquatic vertebrae, birds, and mammals have a third eyelid, which is only rudimentary in humans and primates,” says Dr. Ursula Dietrich, specialist in veterinary ophthalmology recognised by the Royal College of Veterinary Surgeons and an independent Senior Consultant at the London Cat Clinic.
The third eyelid has similar purposes to other eyelids. For instance, it plays a key role in distributing tear film over the surface of the eye to keep things moist, Dr. Dietrich says.
And, while the main eyelids keep dust and dirt at bay, the third eyelid provides another form of protection. It helps provide immune defence for the surface of the eye, Dr. Dietrich says.
Can cats blink their third eyelid?
Your cat’s third eyelid may have a few different functions, but it doesn’t blink. According to Dr. Dietrich, birds are the only species that use their third eyelid to blink.
A cat’s third eyelid is controlled by a different muscle—the retractor oculi—than the other eyelids. Smooth muscles in it also contribute to its movement over a cat’s eye. This movement is a passive and indirect response, Dr. Dietrich says.
Why Can I See My Cat’s Third Eyelid?
Most of the time, you won’t see much of your cat’s third eyelid. But various medical factors can make it stand out more than usual, according to Dr. Dietrich.
Eye infections
“Many eye infections also involve the third eyelid and cause thickening, redness, swelling, and protrusion,” Dr. Dietrich says.
Common eye infections in felines include conjunctivitis, blepharitis, and corneal ulcers. In most cases, the vet will prescribe medications, like eye drops, to help treat the infection.
Other infectious diseases
Infections, such as chlamydia, mycoplasma, and feline herpes, can cause the third eyelid to protrude, Dr. Dietrich explains.
“Those infections are very common in cats. They can cause swelling and redness of the conjunctiva lining the inside of the eyelids and also affect the third eyelid mucous membranes,” she says.
Other symptoms to note include eye discharge and signs of discomfort, like squinting.
Your vet will prescribe antibiotics to treat these infections.
Haws syndrome
This condition causes a cat’s third eyelid to protrude across their eyes. They often won’t have any other symptoms.
The exact cause is unknown, says Dr. Dietrich, although some experts believe a gastrointestinal infection might be involved. She adds that some cats with this condition may have episodes of watery diarrhoea.
There’s no known treatment for the protrusion of the eyelid, but your vet can recommend steps to manage diarrhoea and other stomach symptoms.
Horner’s syndrome
This neurological condition can affect the nerves controlling the eye and eyelid muscles, Dr. Dietrich says. “This can cause a retraction of the globe and passive protrusion of the third eyelid.”
Other symptoms include:
- Drooping upper eyelid
- Smaller pupils, known as myosis
- Sunken-looking eyes
- A squinting appearance
Horner’s syndrome isn’t painful. It only requires treatment if your cat has severe symptoms or an underlying medical condition causing the syndrome.
Loose gland tissue
Dr. Dietrich explains that the third eyelid gland is held in position by connective tissue. If this tissue becomes loose, the gland may become visible.
Called “cherry eye,” this issue is particularly common in some purebred cats, including the Burmese, Persian, and Bengal breeds.
“This condition may spontaneously disappear in some cats but will often require surgical repositioning,” Dr. Dietrich says.
Tumors
Generally, cats get tumours only rarely. But some cats may develop lymphoma or other tumours on the third eyelid. If your cat does have a tumour, Dr. Dietrich says the entire third eyelid will appear abnormally swollen, red, and protruding. You may also notice bleeding areas.
Eyelid cancers can be difficult to treat, although your vet may recommend radiation therapy and surgical removal as possible approaches.
What if my cat is acting normally?
Sometimes, the third eyelid may become visible in cats who have no other symptoms.
For instance, it’s not uncommon for the third eyelid to become more visible as cats age. Protrusion of the third eyelid may also occur in very old cats due to a loss of orbital eye fat, Dr. Dietrich explains.
The third eyelid can also become permanently visible following certain diseases.
For example, Dr. van der Woerdt says feline herpesvirus can lead to conjunctivitis and corneal ulcers. “As the eye lesions heal, the third eyelid can sometimes scar in an abnormal position and become permanently more visible than normal,” she says.
When to Visit a Vet
If you notice anything unusual about your cat’s eyes, it’s always a good idea to get things checked out by a professional.
Dr. van der Woerdt recommends taking your cat to a vet as soon as you notice the problem, ideally within a few days. You’ll want to make the appointment as soon as possible if you notice symptoms like:
- Squinting
- Redness
- Discharge
- Abnormal eyelid positioning, like the eyelid turning in
- Growths or other abnormalities
You’ll want to consult a vet if your cat has any of these symptoms, even if you can’t see their third eyelid!
A cat’s third eyelid suddenly becoming more visible than usual can happen with severe medical conditions, like corneal ulcers, says Dr. van der Woerdt. In these cases, prompt treatment can help prevent permanent vision damage.
Takeaway
Eyes are a delicate and important part of your cat’s body, and it’s natural to be worried if you notice something unusual about them!
Knowing the signs of a potential issue, along with normal variations in your cat’s eyes, can help you stay on top of your cat’s overall well-being.
Another important aspect of cat parenthood: making sure to schedule yearly vet appointments. Vets can recognise signs of potential illness quickly, and regular checkups can help you keep your cat happy and healthy.
