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Haw's syndrome
Haw’s syndrome is a relatively common problem in cats. It is a condition where both third eyelids protrude (or prolapse). Protrusion of the third eyelids can occur for many reasons in cats. When it has a sudden onset, and is associated with diarrhea or other gut conditions, it is
called Haw’s syndrome.
What is the third eyelid?
The third eyelid, also known as the nictitating membrane or nictitans, is a structure that sweeps over the surface of the eye in order to spread tears across the cornea and protect the eye. It is usually only possible to see the edge of the third eyelid which is located in the lower corner of the eye near the nose. It is often pale pink in color but can have dark pigment on the surface.
https://www.youtube.com/watch?v=dWUWNW5rfJU