Prolapse of the Nictitans The third eyelid can become dislodged from its normal resting position in the ventromedial orbit. This occur-rence is known as a prolapse of the nictitans.
Colloquially, this is referred to as “cherry eye” because the prolapsed gland looks like a cherry red mass bulg-ing out from beneath the lower palpebra [1, 11, 23, 24] (see Figures 20.4a–d).
Although this prolapse can be initially transient, it fre-quently takes on a permanent abnormal position in which it is visible all the time. Over time, the smooth, shiny, moist surface of the nictitans dries out, and the gland may become secondarily infected [1, 11, 23].
The etiology is unknown [11]. However, certain canine breeds appear to be predisposed to this condition, including Cocker Spaniels, Boston Terriers, Pekingese, Beagles, Bassett Hounds, English Bulldogs, Lhasa Apsos, and Shih Tzus [13, 23–26]. Larger breeds that are over-represented include Great Danes, German Shepherds, Weimaraners, German Shorthaired Pointers, Irish Setters, and Newfoundland dogs [15, 27–31].
Patients with a breed predisposition often develop cherry eye as young adults. Frequently, they are less than two years of age at presentation [13, 24, 26].
One or both nictitating membranes may be involved. It is also possible that unilateral presentations will progress to bilateral ones [1, 11, 24].
Cats are less often afflicted [13, 14, 32–35]. Of the few cases that have been reported, the majority involve Burmese cats or kittens [12, 14, 34]. Isolated cases involv-ing a British Blue, Persian, and Domestic Shorthaired cat also appear in the veterinary medical literature [12, 14].
Before the gland’s role in tear film production was established, surgical correction of cherry eye classically involved excision of the gland. However, subsequent keratoconjunctivitis sicca (KCS) or “dry eye” was the most common sequelae. Since this association was made, surgical techniques have been revised to replace the gland to its normal anatomic position [13]. There is no one right method [13, 26, 32, 36–39]. One of the most commonly employed methods is the conjunctival pocket technique because the rate of postoperative recurrence is low [26].
Thermal cautery has recently been explored as an alternate approach [15]. The theory underlying this tech-nique is that by reshaping the T‐shaped hyaline cartilage, the nictitans may be more likely to drop back into its normal position [15]. Care must be taken not to inad-vertently damage the gland associated with the third eyelid, or else KCS could still result [15].
20.2.3 Neoplastic Disease Involving the Nictitans Neoplasia of the nictitans is uncommonly seen in companion animal practice [13, 16]. When it occurs, neoplastic disease at this location is typically malignant [13, 16, 40–43].
Older patients are most at risk [13].
Few studies explore breed predisposition to disease concerning neoplasia of the nictitans. However, a 2016 study by Dees et al. demonstrated greater frequency in mixed breed dogs, Labrador retrievers, Shih Tzus, Cocker Spaniels, Dachshunds, Beagles, and Golden Retrievers [17]. The same study reported more cases involving domestic short, medium, and longhaired breeds, Siamese, and Maine Coon cats [17].
In both dogs and cats, adenocarcinomas are most common [16, 42, 43]. These are much more concerning in the feline patient because they exhibit potential for metastatic disease, whereas in the dog, they tend to remain localized [16, 42, 43].
Other neoplasia that has been reported in dogs and cats include hemangiosarcoma, mast cell tumor, lymphoma, squamous cell carcinoma, hemangioma, fibrosarcoma, and malignant melanoma [17, 19, 44–51].
Because there is such variety in terms of tumor type, neoplasia of the nictitans does not have a classic appear-ance. However, in general, the neoplastic nictitans is more bulbous (see Figures 20.5a, b).
Neoplasia of the nictitans requires excision of the nic-titating membrane as opposed to replacement of the gland within its anatomic pocket [13, 16, 17, 40, 41, 52].
Recurrence is possible, particularly because complete surgical removal of the nictitans is a challenge. It is diffi-cult to obtain 5–10 mm of normal tissue margins [16].
Cryotherapy and/or radiation therapy may be indicated as adjunct therapy to reduce the risk of recurrence [16, 42, 53].
Figure 20.3 Prominent nictitans in a feline patient that has a corneal ulcer. In this case, the nictitans is elevated in response to ocular pain as a protective barrier between the external environment and the damaged cornea.
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