Third Eyelid Conditions in Cats: Welfare Guide
Third Eyelid Overview
The nictitating membrane (third eyelid) is a fold of conjunctival tissue at the medial canthus that provides ocular protection and contributes to tear film production (contains a lacrimal gland). In healthy cats it is not prominently visible. Protrusion of the third eyelid ('haw') can be a sign of ocular, systemic, or neurological disease, and requires veterinary assessment. Conditions affecting the third eyelid include: Haw syndrome; prolapsed nictitans gland (cherry eye); and plasmacytic conjunctivitis.
Haw Syndrome
Haw syndrome (bilateral third eyelid protrusion) is a classic sign in cats, often associated with gastrointestinal disease (particularly Campylobacter infection) causing a sympathetic nerve dysfunction. Other causes: Horner's syndrome (unilateral third eyelid protrusion with ptosis and miosis — from sympathetic trunk disruption); dehydration; weight loss; and systemic illness. Bilateral protrusion in a cat with diarrhoea or weight loss strongly suggests Haw syndrome; treatment of the underlying GI condition resolves the protrusion.
Prolapsed Nictitans Gland
Prolapsed nictitans gland (nictitans gland prolapse, 'cherry eye') occurs when the gland of the third eyelid prolapses over the free margin, appearing as a pink-red protrusion at the medial canthus. It is less common in cats than dogs. Surgical repositioning (pocket technique) is the treatment of choice — the gland should not be excised as it contributes significantly to tear film production; excision causes dry eye (keratoconjunctivitis sicca). Surgical success rates are good.
Plasmacytic Conjunctivitis
Plasmacytic conjunctivitis (lymphoplasmacytic conjunctivitis, follicular conjunctivitis) in cats involves chronic immune-mediated inflammation of the third eyelid and conjunctiva. The third eyelid becomes thickened, pink, and irregular. It is associated with FCV, FHV-1, and Chlamydophila felis infection in some cases. Treatment: topical corticosteroids and/or topical ciclosporin reduce inflammation. Long-term management is usually required; the condition recurs without ongoing treatment in many cats.
Squamous Cell Carcinoma
Squamous cell carcinoma (SCC) of the third eyelid and conjunctiva occurs in cats, particularly white cats or those with low periocular pigmentation (UV radiation exposure is a risk factor). It presents as a pink, proliferative, ulcerative mass. Early surgical excision of the third eyelid provides good local control. Advanced SCC involving the orbit requires more extensive surgery. UV protection (keeping white cats indoors during peak UV hours) and early detection of suspicious lesions are welfare-positive preventive measures.