The third eyelid of a cat plays an important role in keeping the eye surface healthy and protecting the eye. But sometimes the third eyelid becomes very visible, which may indicate that there is a health problem with your cat.
What is the third eyelid?
The third eyelid or nictitating membrane – as opposed to the first (upper) and second (lower) eyelid – is a fold of tissue at the inside corner of the eye. It is covered in conjunctiva (the same tissue as on the white of the eye), and keeps its shape from a special T-shaped cartilage. It contains part of the tear gland of the eye, and produces a substantial portion of the tear film. The action of the third eyelid in keeping the tear film in contact with the eye, and in moving tears across the eye keeps the eye surface lubricated and healthy and also removes any unwanted debris.
When a cat is alert, most of the third eyelid is hidden in the eye socket and only a small portion is visible at the inner corner of the eye. When relaxed, during sleep or during blinking, however, retraction of the eyeball by a set of muscles causes the third eyelid to passively move across the eye surface.
Movement of the third eyelid is also partially regulated by the sympathetic nervous system and muscle cells within the third eyelid. As for many other animal species, this helps to protect the eye from injury as cats move through tall grass or capture prey. In humans, probably because we have less need for eye protection, the third eyelid has been reduced to a fleshy bump on the inner corner of the eyelids.
What causes the third eyelid to prolapse?
Sometimes a cat’s third eyelids ‘come up’ or ‘prolapse’, causing them to be visible even when the cat is alert. A number of things can cause this, most of which do not relate directly to the eyes. A thorough veterinary examination and in some cases further laboratory tests are required to find the underlying cause of third eyelid prolapse.
Eye disease: If there is infection or inflammation in one or both eyes, this may cause the third eyelid to come over the eye.
High body temperature: Some cats with high fevers may have a prolapsed third eyelid.
Dehydration: Dehydrated or old thin cats may also have third eyelid prolapse through loss of tissue around the eye.
Nerve damage: Damage to nerves in the face and neck usually result in one eyelid prolapsing although it is possible for both eyes to be involved. There are usually other signs to indicate this nerve damage (including change in pupil size, or loss of movement of the face).
Intestinal upset: The most common cause of a bilateral (both eyes affected) third eyelid prolapse in the cat is thought to be from some sort of gastrointestinal upset – such as intestinal worms or other parasites, food intolerance, or a viral or bacterial gut infection.
If your vet has ruled out other reasons for third eyelid prolapse, they may suggest treating your cat for worms, and checking that they do not have other symptoms such as diarrhoea, vomiting, or being off their food. The cat may respond to a special diet and medication, or other tests may be required (including faecal and blood tests). Even after the cause of the upset has been diagnosed and treated, the third eyelid may remain elevated for four to six weeks in many cats, before eventually returning to normal.