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Protrusion of Third Eyelid in Dogs

By: Dr. Noelle McNabb

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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, with the exception of birds and most reptiles.

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 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 occurs primarily in rural dogs in certain areas of the midwestern United States, such as Kansas, Missouri and Oklahoma.

  • 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 uncommon in the dog.

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

  • 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.

  • In dogs, an optical illusion of TE protrusion can occur when one TE lacks pigmentation and appears pink or white, and the other TE is pigmented dark brown or black along its free edge.

  • Eversion (outward scrolling) of the cartilage within the TE from a congenital malformation may change the position of the TE.

  • Cyst (small sac containing fluid) of the gland of the TE (very rare) can change the position of the TE.

  • Tumors may affect the TE or gland of the TE, such as an adenocarcinoma, hemangiosarcoma, and lymphosarcoma

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

  • Follicular conjunctivitis, an allergy or environmental-related inflammation of the conjunctiva may involve the TE.

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

  • Nodular granulomatous episcleritis or nodular fasciitis is a severe inflammatory disease that can involve the TE and can alter both its shape and position.

  • Plasma cell infiltration (migration of a specific type of white blood cell) of the TE causes thickening, swelling, and depigmentation.

    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, or 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 chronic 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

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