The Classic Comprehensive Handbook of Cat Care
What to Do When Your Cat Is Sick: PROLAPSED THIRD EYELID
The third eyelid (nictitating membrane, haw, see Anatomy) often moves from its normal position near the medial corners of the eyes to partially obscure the eye. When this occurs on one side only and intermittently as if blinking, it is often a sign of local irritation to that eye, such as a foreign body or damage to the cornea. When it occurs in both eyes (often described by owners as a “film over the eye”) and for prolonged periods of time (several hours to several days), it sometimes interferes with vision and can have many causes.
The most common cause of this second type of third eyelid elevation (third eyelid prolapse, haws syndrome) in a cat who seems relatively healthy is a gastrointestinal upset. Signs that often accompany this kind of third eyelid prolapse are a change in appetite, a loose stool, or transient vomiting. When the gastrointestinal disturbance is corrected, the third eyelids will return to their normal positions. Third eyelid elevation accompanied by more serious signs of illness (fever, weight loss, complete absence of appetite) should prompt you to have your cat examined by a veterinarian, since serious systemic illness such as autonomic polyganglionopathy (Key-Gaskell syndrome), a nervous system disorder, may be accompanied by third eyelid prolapse.
Third eyelid prolapse that occurs in an apparently healthy cat displaying no signs of illness (not even mild intestinal disturbance) is frequently without apparent cause. It usually disappears spontaneously. If, after performing a physical examination, you feel confident that your cat is healthy you may then choose to wait and watch your cat for a few days in hopes that the problem will quickly correct itself. If prolapse is severe enough to interfere with vision, your veterinarian can supply you with a prescription for eye drops that will provide temporary symptomatic relief, if no evidence of general illness is found when he or she examines your cat.