TL;DR: A visible third eyelid in cats is a clinical sign of an underlying health issue, such as dehydration or inflammation, and typically warrants a veterinary appointment within 24 to 48 hours.
What is my cat’s third eyelid and why is it showing?
The third eyelid, officially known as the nictitating membrane, is a thin, protective layer located in the inner corner of your cat's eye. Unlike humans, cats have this extra layer to provide additional protection and help keep the cornea moist. Usually, this membrane is hidden. When it becomes visible constantly, it is a sign that your cat's body is reacting to an underlying issue, such as inflammation, dehydration, or a general feeling of being unwell.
Is it an emergency if I can see my cat’s third eyelid?
The urgency level for a visible third eyelid is classified as Medium. While it is rarely a life-threatening emergency on its own, it is a clear clinical sign that something is wrong. You should take the following actions:
- Monitor your cat closely for other symptoms like discharge, squinting, or changes in behavior.
- If both eyelids are showing and your cat is acting otherwise normal, schedule a veterinary appointment within 24 to 48 hours.
- Seek veterinary care sooner if the eye is painful, red, or if your cat is lethargic.
How can taking a photo help the vet triage my cat’s eye issue?
Because the appearance of the third eyelid can change depending on your cat's stress level or environment, capturing a clear, high-resolution photo is incredibly helpful for your veterinary team. A photo helps the team by:
- Assessing the color of the membrane and the presence of any swelling.
- Evaluating the symmetry between both eyes.
- Determining if your cat needs an immediate urgent care slot or if the issue can wait for a standard appointment.
Clinical Context (Merck Veterinary Manual)
In cats, the visible appearance of the third eyelid, or nictitating membrane, can be associated with several factors. Neoplasia of the conjunctiva or nictitating membrane should be considered, as these are frequent primary ophthalmic neoplasms in cats, often malignant and more difficult to treat than in dogs. Squamous cell carcinomas, more common in white cats with nonpigmented eyelid margins, can involve the eyelids, conjunctivae, and nictitating membrane, appearing as pink, roughened, irregular masses or thickened ulcerations. Other less frequent neoplasms include adenocarcinomas, fibrosarcomas, neurofibrosarcomas, and basal cell carcinomas.
Chapter: Ophthalmology
Source: The Merck Veterinary Manual, 11th Edition (Page 490)