Overview of Third Eyelid Protrusion in Dogs
Protrusion, prolapse, or elevation of the third eyelid 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 dog’s eyelids and barely noticeable. The third eyelid is also sometimes called the membrana nictitans or nictitating membrane.
Numerous disorders affecting the eye, TE and soft tissues behind the eye can result in TE protrusion. Therefore, TE protrusion represents a common yet nonspecific symptom of ophthalmic disease that warrants further diagnostic evaluation.
The causes of protrusion of the TE fall into one of several categories:
Decreased or loss of function of the nerve supply to the muscles of the TE and those surrounding the eyeball, from certain neurologic diseases
Relaxation of the muscles around the eyeball (that work to keep the TE in a retracted position) from the use of tranquilizers, from poor physical health, etc.
Weakening of the ligament of the gland of the third eyelid with secondary glandular enlargement and prolapse (also known as cherry eye)
Tumors, cysts or inflammatory diseases of the TE
Any source of ocular (eye) pain that stimulates retraction of the eye deeper into the orbit (bony cavity in the skull or eye socket)
Any cause of settling of the eye deeper into the bony socket, such as from dehydration, weight loss, or changes in the structures behind the eye
Abnormally small sized eye. Small eyes may occur as congenital birth defects or may arise from shrinkage of the eye following severe trauma or inflammation.
Presence of a mass, such as a tumor, cyst, infection or inflammation within the orbit, and pushing the eye and TE forward
What to Watch For
Increased prominence and elevation of the smooth inner membrane located at the inside corner of the eyelids
Other signs are dependent upon the cause of the prolapse. They may include squinting, tearing, changes in pupil size, alterations in the size or position of the eyeball, discoloration of the third eyelid, and deformities of the third eyelid.
The condition may affect one or both third eyelids.
Diagnosis of Third Eyelid Protrusion in Dogs
Diagnostic tests may include one or more of the following:
Complete medical history and physical examination
Complete ophthalmic examination including testing of pupillary light reflexes, Schirmer tear test, fluorescein staining of the cornea, tonometry to measure the pressure within the eye, and examination of the interior of the eye under magnification. Your veterinarian may refer your dog to a veterinary ophthalmologist for further evaluation using specialized instrumentation.
The third eyelid itself may be examined with a forceps after application of a local anesthetic.
Neurologic examination to assess the presence of neurologic disease
Complete blood count (CBC) and serum biochemistry tests to evaluate the underlying cause and identify any related problems
Skull radiographs X-rays to determine the presence of a bony orbital or sinus problem
Ultrasound examination of the eye and soft tissues within the orbit behind the eye
Specialized imaging tests such as computed tomography (CT) scan and magnetic resonance imaging (MRI) of the eye, orbit and brain
Treatment of Third Eyelid Protrusion in Dogs
Successful treatment relies solely upon obtaining an accurate diagnosis.
No symptomatic therapy can be initiated until the precise cause of TE elevation is identified.
Home Care for Dogs with Third Eyelid Protrusion
Vision is not usually compromised unless the third eyelid covers more than 1/2 of the eye. The underlying cause of the prolapse may affect vision, however. Confine your pet to a safe area until the cause of the problem is determined.
Do not administer human over-the-counter medications, such as Visine® or other ophthalmic remedies intended to “reduce eye redness” or irritation, because these medications rarely help the problem and may make diagnosis of the cause more difficult.
Information In-depth on Canine Third Eyelid Protrusion
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.
Causes of Canine Third Eyelid Protrusion
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