TL;DR: A visible third eyelid combined with lethargy indicates your cat is unwell and requires a veterinary evaluation within 12 to 24 hours to address potential pain or systemic illness.
What is my cat's third eyelid and why is it visible?
The third eyelid, or nictitating membrane, is a protective layer located in the inner corner of your cat's eye. Normally, it remains tucked away and is hardly visible. When it becomes prominent (a condition sometimes called 'cherry eye' or Haw's Syndrome), it is often a non-specific sign that your cat is feeling unwell, experiencing pain, or suffering from a systemic issue.
Is a visible third eyelid and lethargy in my cat an emergency?
- Urgency Level: Medium.
- While rarely an immediate life-threatening emergency, this combination indicates that your cat's immune or nervous system is struggling.
- Aim to have your cat evaluated by a veterinarian within 12 to 24 hours.
- Priority increases if your cat is also refusing food or water.
How does a photo of my cat’s eyes help with veterinary triage?
- Allows professionals to assess membrane color (pale, red, or yellow) and the degree of protrusion.
- Identifies the presence and type of any accompanying eye discharge.
- Helps determine if the issue is localized to the eye or a sign of serious internal illness.
- Ensures your cat receives the appropriate level of priority care in a telehealth setting.
Clinical Context (Merck Veterinary Manual)
In cats presenting with lethargy and a visible third eyelid (nictitating membrane), systemic diseases affecting the eye and associated structures should be considered. The pathogens that commonly cause infectious diseases in cats, such as feline herpesvirus 1, Chlamydia, and Mycoplasma, frequently present as acute and recurrent conjunctivitis. Feline infectious peritonitis, toxoplasmosis, feline immunodeficiency virus, and feline leukemia virus may manifest with ocular signs including anterior and posterior uveitis, retinal detachment, and secondary glaucoma.
Chapter: Ophthalmology, Infectious Disease, Dermatology, Immunology
Source: The Merck Veterinary Manual, 11th Edition (Page 505)