Elevation, prolapse or protrusion of the third eyelid (membrana nictitans, nictitating membrane) refers to the abnormal elevation of the smooth inner eyelid that is located between the cornea and the inside corner of the eyelids closest to the nose. The third eyelid (TE) is usually retracted beneath the eyelids and barely noticeable. The ability to move the TE is involuntary in most animals; however, cats have some control over the movement of this membrane.

Numerous disorders affecting the eye, the tissues in the orbit behind the eye, and the neurologic functions around the eye can result in TE protrusion. In addition, certain systemic diseases and medications can also cause this condition. Therefore, TE protrusion in an animal represents a nonspecific symptom that warrants further diagnostic evaluation by a veterinarian to determine its exact cause.

Neurologic causes of prolapse of the third eyelid include the following:

Horner's syndrome is a neurologic disease that arises from dysfunction of the sympathetic nerves to the eye. The sympathetic nervous system controls many glandular and involuntary functions in the body. Prolapse of the third eyelid is one of four clinical signs that develop with Horner's syndrome. Horner's syndrome usually affects only one eye at a time.

Damage to cranial nerves III or VI that control muscles around and behind the eye may result in TE protrusion on the affected side, but damage to these nerves is rare.

Dysautonomia is another extremely rare neurologic disease that causes widespread dysfunction of the autonomic nervous system resulting in bilateral dilation of the pupils, decreased tear production and protrusion of the TE, in addition to serious systemic symptoms. This disease was first described in the cat in England, and cats in the United States are rarely diagnosed with the condition.

Tetanus (lockjaw) is an infection with Clostridial bacteria that produce a neurotoxin. These bacteria usually enter the body through a wound of some sort. This neurotoxin causes severe muscle rigidity, seizures and paralysis. The most noticeable ophthalmic (eye) sign is bilateral protrusion of the third eyelids. Fortunately, tetanus is very rare in the cat.

Systemic causes of protrusion of the third eyelid include the following:

Bilateral protrusion of the third eyelids can occur spontaneously in cats and is known as (Haws syndrome). Although the exact cause of Haws syndrome is unknown, it is believed to be related to some mild gastrointestinal problems, such as rotavirus or parasite infestations. These intestinal problems may affect the sympathetic nerves to the eye, resulting in partial or extensive protrusion of the TEs. This condition often resolves spontaneously within 3 to 4 weeks.

Tranquilization with certain medications can result in bilateral TE protrusion.

Pets with serious physical illnesses may develop TE protrusion from relaxation of the muscles around the eye, or dehydration and weight loss with sinking of the eye deeper into the socket.

Diseases of the third eyelid that may result in protrusion include the following:

Prolapse/protrusion of the tear gland of the TE (also called "cherry eye") changes the position and shape of the third eyelid.

Tumors affecting the TE or gland of the TE, such as lymphoma or squamous cell carcinoma, may result in enlargement and protrusion of the TE.

Injury or lacerations of the TE may produce inflammation and swelling of the TE.

A foreign body lodged within or behind the TE may cause inflammation, infection and pain, with protrusion of the TE.

Diseases that cause protrusion of the TE due to ocular pain include the following:

Entropion (inward rolling of the eyelids) or abnormal growth of eyelashes causing irritation of the conjunctiva and cornea

Corneal ulceration, foreign body, laceration or penetrating injury

Injuries to the eyelids and structures around the eye        

Keratoconjunctivitis sicca (dry eye syndrome) resulting eye discomfort

Iritis and anterior uveitis (inflammation of the iris and nearby tissues)

Acute glaucoma (sudden elevation in pressure in the eye)

Anterior lens luxation (dislocation of the lens into the front chamber of the eye)

Disorders that affect the size or position of the eye and result in protrusion of the TE include the following:

Shrinkage of the eyeball over time from a long-standing, chronic disease within the eye is called phthisis bulbi. As the eyeball shrinks, the TE passively moves upward over the eye.

Microphthalmia (congenitally small eye) that is present at birth

Severe dehydration or weight loss causing relative shrinkage of the soft tissues behind the eye, resulting in sinking of the eye within the orbit

Reduction of orbital fat and muscle related to advanced age or some sort of orbital disease resulting in a deeper-set eye

Tumors or cysts growing within the orbit that move the eye and TE forward

Abscesses or infections of the soft tissues with swelling within the orbit and forward movement of the eye and TE