Diseases and surgery of the third eyelid

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Proceeding of the SEVC Southern European Veterinary Conference
Oct. 17-19, 2008 – Barcelona, Spain

http://www.sevc.info

Reprinted in the IVIS website with the permission of the SEVC www.ivis.org

Reprinted in IVIS with the permission of the SEVC

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Ophthalmology

Diseases and Surgery of theThird Eyelid
Dr.F.Stades
The space between the eyelid margins and the globe is separated from the ambient environment by a thin, transparent mucosa, the conjunctiva. In the ventral medial canthus, there is a conjunctival fold called the nictitating membrane (NM), third eyelid, or (according to the Nomina anatomica veterinaria) plica semilunaris conjunctivae. The firmness of the NM is derived from avery thin T-shaped cartilage, the base of which is embedded in the superficial gland (an additional deep gland is found in swine, rabbits, and chickens). This gland produces more than 30% of the watery fraction of the tears, and in some species it also secretes some mucus for the precorneal tear film. The outer 2-mm zone of the palpebral surface of the free margin of the NM is usually pigmented.In some white or non-pigmented animals, or those having non-pigmented skin around the eyes, the margins of the NM may also be non-pigmented. There are sympathetically-innervated, smooth muscle fibers that actively retract the NM in birds. There are only a few such fibers in cats and almost none in dogs, resulting in an almost passive retraction in these species. In birds, the NM is almosttransparent, originates dorsomedially, and plays a major role in distributing the precorneal tear film and protecting the cornea. Protrusion of the NM follows spontaneous or iatrogenic enophthalmos, or may be the result of retrobulbar swelling. The surgical removal of the entire NM is only indicated in cases of malignancy of this structure that cannot otherwise be resolved. The removal for other reasonsmust, therefore, be considered a professional mistake. In addition, the partial removal of an abnormal gland, e.g. because of hyperplasia, should be avoided until all other therapies have failed and the owner has been informed of the consequences. This is mainly because of the risk of inducing secondary entropion or KCS that will require lifelong treatment. Protrusion of the NM may be caused by manyabnormalities. The NM will protrude over the globe in cases of 1) loss of sympathetic innervation, as found in Horner’s Syndrome or feline dysautonomia; 2) processes which press the globe into the orbit, such as fractures; 3) active or passive enophthalmos, as may be caused by pain in the eye, loss of retrobulbar fat, or atrophy of the chewing muscles; 4) spaceoccupying processes at the base ofthe third eyelid; or 5) swelling or symblepharon of the NM itself. Increased tonus of the muscles, feelings of discomfort, or general illness may, especially in cats, result in a bilateral protrusion of the NM. The diagnostic examination should be aimed at differentiating between these causes. 2. Non-pigmented margin of the NM If the free margin of the NM is non-pigmented it may be extremelysensitive to sunlight, resulting in erythema and edema of the margin. In severe cases, black tattooing of the exposed NM can be tried or the margin may be shortened by 2–3 mm. The knots are embedded subconjunctivally. Removal of the NM for cosmetic effects is not justifiable und must be rejected in favor of the patient’s welfare. 3. Protrusion of the NM Protrusion of the NM may be caused by manyabnormalities. The NM will protrude over the globe in cases of 1) loss of sympathetic innervation, as found in Horner’s Syndrome or feline dysautonomia; 2) processes which press the globe into the orbit, such as fractures; 3) active or passive enophthalmos, as

Proceedings of the Southern European Veterinary Conference & Congreso Nacional AVEPA, 2008 - Barcelona, Spain

Reprinted in IVIS with the...