Introduction to the Nictitans In addition to the upper and lower palpebrae, dogs and cats have a third eyelid [1, 2]. This structure is called the nictitans or nictitating membrane. Historically, it was known as the haw, and it was erroneously thought to be vestigial, like the appendix or wisdom teeth [3].
In actuality, the nictitans has important structural and functional roles. Located in the ventromedial orbit, it consists of a T‐shaped hyaline cartilage [2]. This cartilage is like a windshield wiper [2]. With each blink, it glides across the surface of the cornea to spread tear film [2].
Movement of the nictitans is characteristically passive [2]. However, cats have some degree of active control over its movement [2].
The cartilage of the nictitans also serves as an addi-tional protective barrier between the environment and the globe [2].
The nictitans is lined by conjunctiva on both sides: the palpebral conjunctiva, on the surface that faces the eye-lids, and the bulbar conjunctiva, on the surface that faces the globe [2]. The bulbar surface contains lymphoid follicles [1, 2]. These pink‐red structures are thought to play a role in ocular immunity [2].
There is also a seromucoid gland at the base of this cartilage [1, 2]. This gland produces anywhere from 25% to 50% of tears [1, 2, 4].
Operating as a single unit, the nictitans is typically out of sight (see Figures 20.1a, b).
The hidden position of the nictitans is maintained by the sympathetic tone of the orbital smooth muscles [1].
These muscles control where the nictitans sits relative to the globe [1]. When sympathetic tone is diminished or absent, as occurs in Horner’s syndrome, the globe is displaced caudally, and the position of the nictitans becomes prominent [1].
Each nictitans should be evaluated at every veterinary visit [5]. In order to bring the nictitans into view, the veterinarian must manipulate the soft tissue over each orbit [5]. Pushing down gently, but firmly, over the upper palpebra will elevate the nictitans for visual inspection [5] (see Figures 20.2a–c).
Patients that present with ocular complaints may have prominent nictitating membranes that do not require the veterinarian’s assistance to bring them into view [5].
A prominent nictitans may indicate systemic illness or generalized unthriftiness [5]. In young patients, particu-larly kittens, visible nictitans have been linked to severe cases of gastrointestinal parasitism [5].
More often, the prominent nictitating membrane reflects ocular pain or irritation, as from a corneal ulcer [6] (see Figure 20.3).
Nictitating membranes may also be prominent if there is a foreign body in the orbit [5]. Plant material, espe-cially seeds, can lodge beneath the nictitans, causing inflammation of the conjunctival lining. This may make the nictitans puffy enough to visualize on its own.
Additional concerns regarding foreign bodies include the potential for corneal abrasion and ulceration [5].
20.2 Primary Clinical Presentations that Involve the Nictitans In addition to the aforementioned scenarios, there are three primary presentations in clinical practice that involve the nictitans:
● Horner’s syndrome [7–10] ● Prolapse of the nictitans [2, 4, 11–15] ● Neoplasia of the nictitans [16–20] 20.2.1 Horner’s Syndrome Recall that Horner’s syndrome results from sympa-thetic denervation to the eye, and is characterized by miosis, ptosis, enophthalmos, and elevation of the nictitans [7, 10, 21, 22]
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