Diseases of the canine eyelids

Congenital and developmental eyelid disorders in dogs, including entropion, ectropion, distichiasis, and trichiasis, may be treated by a number of different surgical procedures. Selection of the surgical technique for a particular condition may be influenced not only by the most effective procedure, but also by the experience of the surgeon and the surgical instrumentation available. In older dogs eyelid neoplasms are common. Although a significant percentage of the canine eyelid neoplasms are malignant histologically, local recurrence after surgical excision is infrequent. The majority of canine eyelid neoplasms can be excised by reasonably simple surgical procedures.

Diseases of the feline eyelids

Eyelid surgery in cats is less frequent, but just as challenging. Eyelid agenesis occurs not infrequently in the cat, usually affecting the lateral aspects of the upper eyelid. Several eyelid surgical procedures have been developed to treat this condition. Eyelid neoplasms in cats are usually malignant histologically and clinically. Surgical intervention is often combined with radiation, cryotherapy, or other types of therapy for best results.

Eyelid trauma occurs in the cat, and as one would expect, is most frequent in young animals. The eyelid trauma may be minor or extensive, and fortunately loss of substantial portions of the eyelids is rare. Although traumatized eyelids may exhibit marked swelling and multiple lacerations, the extensive vascularity of the eyelids usually protects against tissue ischemia and necrosis. As a result, excision and liberal trimming of traumatized eyelid tissues prior to repair are unnecessary and discouraged. Reapposition of severely traumatized eyelids usually yields better postoperative results than excision of still attached but lacerated lid tissues and subsequent reconstructive blepharoplastic surgical procedures to repair these defects.

Diseases of the equine eyelids

Developmental lid diseases, such as entropion and ectropion, occur rarely in horses; however, trauma and neoplasia occur not infrequently and are often treated by surgery. Like other animal species, eyelid trauma occurs more frequently in young animals, and maintenance of lid function and preservation of the lid margin are most important. Both squamous cell carcinoma and sarcoid neoplasms commonly involve the eyelids in middle-aged to aged horses, and often surgery is combined with other modalities used to treat these neoplasms and achieve higher success rates.

Diseases of the bovine eyelids

Of the different lid diseases, only neoplasia warrants not infrequent lid surgery in this species. Squamous cell carcinoma is the most frequent neoplasm in cattle, and occurs in cattle directly related to aging. The average age of the Hereford breed affected with squamous cell carcinoma is about 7–8 years. Lack of lid and conjunctival pigmentation is also associated with this tumor.

Adaptations from eyelid surgeries in humans

The early small animal eyelid surgical procedures were adapted from techniques performed in humans. Human eyelids are quite similar to those in domestic animals, with one major difference: in humans the tarsal layer consists of a distinct cartilaginous plate that provides internal support for the eyelids; in domestic animals the tarsal plate has been replaced by a thinner and more flexible fibrous tarsus. As a result, the eyelids of animals have less internal support, and contact with the anterior portion of the globe is more important to maintain their contours and position.

Eyelid skin, cilia (eyelashes), and glands

The skin of domestic animal eyelids is thinner than other parts of the integument system (Fig. 5.2). Eyelid movements require both thin and pliable skin. Fine short hairs normally cover the eyelid skin. The subcutaneous tissues under the eyelid skin are relatively thin and attach the lid skin to the deeper orbicularis oculi muscle. Cilia or eyelashes occur primarily on the canine upper eyelid, usually in two or four irregular rows. These cilia are usually the same color as the adjacent eyelid hair coat. Long tactile hairs (pili supraorbitales or vibrissae) appear as a tuft along the dorsal medial orbital margin in several animal species. Eyelashes are not present in cats, although the eyelid hair next to the dorsal eyelid margin may be considered a substitute. Two types of gland, the glands of Moll and glands of Zeis, are located about the cilia follicles. The gland of Moll is a modified sweat (apocrine) gland and the gland of Zeis is a modified sebaceous gland. These glands can become inflamed and abscessed in young animals, resulting in the formation of a stye or external hordeolum. The margo-intermarginalis represents the free margin of the eyelids.

Fig. 5.2 The different layers of the upper eyelid include: skin (A), orbicularis oculi muscle (B), tarsus (C), insertion of the levator palpebrae superioris muscle (D), meibomian glands (E), palpebral conjunctiva (F), and the cilia (G).

Eyelid muscles

The second layer of the eyelids is the muscle layer which consists of several muscles that either close or open the palpebral fissure. The orbicularis oculi muscle is the predominant muscle involved in closure of the palpebral fissure in domestic animals. This muscle encircles the entire palpebral fissure and is attached by the septum orbitale to the medial and lateral canthi. It is divided into the inner pars palpebralis and the outer pars orbitalis. Both the origin and insertion of the orbicularis oculi muscle is the medial palpebral ligament. This muscle is immediately beneath the skin layer, and surgical procedures for entropion (inversion of the eyelid margin) and ectropion (eversion of the eyelid margin) directly involve this muscle. In large animal species the orbicularis oculi muscle is very powerful, especially when ophthalmic pain is present. In large animals, local nerve blocks of the palpebral nerve branch of the auriculopalpebral nerve (branch of the facial nerve) is often necessary to relax this muscle to adequately examine and even treat a painful eye.

Several muscles are involved in opening the eyelids and increasing the size of the palpebral fissure in the dog and cat. In the canine upper eyelid these muscles consist of (medial to lateral) the levator anguli oculi medialis, levator palpebrae superioris, and the frontalis, and in the lower lid, the pars palpebralis of the sphincter colli profundus (Fig. 5.3). The levator palpebrae superioris muscle has its origin deep within the orbit, along with the rest of the extraocular muscles, and lies immediately above the dorsal rectus muscle that inserts several millimeters from the dorsal limbus of the globe. Fascial attachments between the dorsal rectus and levator palpebrae superioris muscles result in simultaneous upper movements of the globe and retraction of the upper eyelid. The levator palpebrae superioris muscle seems to be the most important muscle for upper lid retraction as damage to this muscle or its insertions into the tarsal layer results in ptosis (or drooping of the upper eyelid). The levator anguli oculi medialis muscle elevates the medial upper eyelid and erects the long tactile hairs of the eyebrow. In some of the large breeds of dogs, the action of this muscle results in a noticeable notch at the junction of the medial and middle one-thirds of the upper lid. In the dog, Müller’s smooth muscle fibers, innervated by the sympathetic nerves, attach to the upper tarsus; in the cat, these muscle fibers also attach to the nictitating membrane. With the release of endogenous adrenaline (epinephrine), or during the fight-and-flight reflex, these adrenergic innervated smooth muscle fibers can immediately increase the size of the palpebral fissure.

Fig. 5.3 Muscles that control the size of the palpebral fissure in the dog include: orbicularis oculi (close), levator anguli oculi medialis (open), frontalis (open), retractor anguli oculi lateralis (open), and pars palpebralis (open). Müller’s muscles (open) and levator palpebrae superioris (open) are not shown.

In the cat, the major muscle for closing the palpebral fissure is the orbicularis oculi. The corrugator supercilii medialis raises the majority of the upper eyelid, but laterally the frontoauricularis helps to elevate the upper lid. Lateral and as a substitute for the lateral canthal ligament, the corrugator supercilii lateralis elongates the palpebral fissure. The lower eyelid is relatively fixed and only the orbicularis oculi is present to close the palpebral fissure.

In the horse, the anatomy of the eyelids follows the other mammalian patterns. The upper lid is considerably larger than the lower lid and contributes most of the lid motility. The lacrimal puncta are about 2 mm in diameter and about 8 mm from the medial canthus. The medial canthus can be prolonged medioventrally and possesses a prominent, occasionally pigmented, caruncle. The upper lid possesses about 40–50 meibomian glands dorsally and 30–35 glands ventrally.

In cattle, the eyelids are quite similar to the horse. They are very thick and strong! The orbicularis oculi is often in small bundles and extends to the edge of the lid margin. The orbicularis oculi is attached medially to the strong medial palpebral ligament and lacrimal bone. Although cattle possess a small and weak lateral palpebral ligament, entropion and ectropion occur rarely in cattle. There are about 32–34 meibomian glands in the upper lid and 26–28 in the ventral lid. The upper fornix measures about 36 mm from the upper lid margin, and the lower fornix is 22 mm from the lower lid margin.

The lateral palpebral or canthal ligament is poorly developed in dogs and cats, and usually consists of an irregular thickened lateral septum orbitale. In the large breeds of dogs the lack of or a poorly developed lateral canthal ligament contributes directly to lateral canthal lid diseases in these breeds. The major component of the lateral canthal support system has been replaced by the retractor anguli oculi lateralis (dog) or corrugator supercilii lateralis (cat) muscle (Fig. 5.4). This results in a somewhat mobile but unstable lateral canthus in dogs, and contributes to the frequent involvement of the lateral canthus and lateral lower and upper eyelids with entropion and ectropion. The lateral one-half of the lower eyelid in the dog also has the pars palpebralis muscle, a subdivision of the sphincter colli profundus muscle, which can depress the lateral lower lid.

Fig. 5.4 Muscles that control the size of the palpebral fissure in the cat include: orbicularis oculi (close), corrugator supercilii lateralis (open), frontoauricularis (open), and corrugator supercilii medialis (open). Müller’s muscles (open) are not shown.

Eyelid tarsal layer and glands

The eyelid skin and tightly adherent muscle layers are easily separated surgically from the deeper two layers, the fibrous tarsus containing the tarsal or meibomian glands and the inner palpebral conjunctiva. The fibrous tarsus provides some infrastructure for the eyelids, but not to the extent that the tarsal hyaline plate does in humans. The fibrous tarsus has fascial attachments to the septum orbitale, resulting in a strong connection to the periosteum of the orbital rim, and a significant barrier for trauma, surgery, and external infectious agents to enter the orbit. The fibrous tarsal layer is also in intimate contact with the medial palpebral or canthal ligament, the base of the nictitating membrane, and the lateral canthus. The medial palpebral ligament is more distinct than the lateral ligament in the dog, and consists of a fibrous band originating from the periosteum of the frontal bone that inserts into the upper and lower tarsal layers. The medial palpebral ligament also serves as the origin and insertion for the circular orbicularis oculi muscle, which undoubtedly assists in the medial movement of tears on the cornea and within the conjunctival sacs. In the middle section of the upper eyelid the levator palpebrae superioris muscle inserts into the tarsal layer. The superior and inferior tarsal muscles are smooth muscles in the dog within the endorbita that inserts into the tarsus. These muscles are under sympathetic innervation and help maintain the palpebral fissure open.

The tarsal or meibomian glands are sebaceous (holocrine) types and produce the important outer lipid or oily fraction of the preocular or precorneal film. The lipid layer of the preocular film prevents evaporation of the thicker aqueous layer and stabilizes the preocular film. The number of tarsal glands in each lid ranges from 20 to 40, with the upper eyelid containing more glands. The orifices of the tarsal or meibomian glands empty onto the center of the eyelid margin. This area is referred to the ‘gray line’ and is an important surgical zone. The tarsal glands are occasionally visible through the palpebral conjunctiva, and extend for 3–5 mm into the lid substance. The tarsal glands seem able to undergo metaplasia and form additional cilia, called distichia. At the medial canthus at the junction of the upper and lower lids, and at the base of the nictitating membrane, is the lacrimal caruncle. Small fine hairs emerge from its surface, which can act as a wick for tears to moisten the medial canthal skin. The lacrimal caruncle also contains small sebaceous glands.

Palpebral conjunctiva

Like the skin and muscle layers of the eyelid, the fibrous tarsus and palpebral conjunctiva are in close contact and difficult to separate surgically. The palpebral conjunctiva starts at the eyelid margin, and continues to the conjunctival fornix to join the bulbar conjunctiva. The palpebral conjunctival surface at the eyelid margin consists of non-keratinized stratified epithelium, but approximately one-third the distance from the lid margin to the conjunctival fornix it changes into pseudostratified epithelium. Once the pseudostratified epithelial layer is established, goblet cells that produce mucin begin to appear. The highest concentrations of goblet cells occur in the conjunctival fornices. Like the cornea, the palpebral conjunctiva is also coated with preocular film to facilitate eyelid movement over the cornea, and to minimize trauma between the bulbar and conjunctival epithelial surfaces.

Anatomy of the equine eyelid

Muscles that open the equine eyelids include the levator palpebrae superioris, Müller’s, levator anguli oculi medialis, frontalis, and malaris. The levator palpebrae superioris muscle, along with the levator anguli oculi medialis muscle, raises the upper eyelid. Müller’s muscle is a smooth muscle that originates from the posterior surface of the levator muscle in the upper eyelid and from the ventral rectus muscle for the lower eyelid. It is innervated by sympathetic fibers that travel with the ophthalmic branch of the fifth cranial nerve. Müller’s muscle inserts on the tarsus and, along with other muscles, keeps the palpebral fissure open. It is the muscle, when innervation is interrupted in Horner’s syndrome, that results in ptosis of the upper eyelid. The frontalis muscle inserts laterally on the upper eyelid. The malaris muscle inserts on the ventral orbicularis oculi muscle and functions to open the lower eyelid. The upper eyelid is the more mobile and larger section of the eyelids. It provides the majority of the blinking function.

Sensory innervation of the eyelids is via the ophthalmic and maxillary branches of the trigeminal (fifth cranial) nerve. The ophthalmic portion branches into the frontal, lacrimal, and nasociliary nerves. The frontal branch passes anteriorly from the orbit through the supraorbital foramen. It then becomes the supraorbital nerve, dividing over the forehead and innervating most of the upper eyelid. The lacrimal nerve innervates the lacrimal gland and the upper eyelid at the lateral canthus. The nasociliary branch gives rise to the infratrochlear nerve, which provides sensory innervation to the medial canthus, caruncle, nictitating membrane, upper and lower conjunctiva, and nasolacrimal puncta and ducts. The remainder of the lower eyelid is innervated by the zygomatic nerve, which is a branch of the maxillary nerve.

Eyelid sensation

Most of the sensation of the animal eyelids is provided by several branches of the trigeminal nerve. Sensation of the lateral two-thirds of the upper eyelids is provided by the trigeminal nerve through its frontal nerve and its branch, the supraorbital nerve, and the medial canthus by the infratrochlear nerve. The medial canthus and medial aspects of the upper eyelids are also served by the nasociliary nerve, the largest branch of the ophthalmic nerve. The sensation for the entire lower eyelid is provided by the maxillary division of the trigeminal nerve through its zygomaticotemporal branch.

Eyelid innervation

The palpebral branch of the facial or seventh cranial nerve innervates the majority of the muscles that control palpebral fissure size, except for the levator palpebrae superioris muscle that, along with most of the extraocular muscles, is innervated by the oculomotor or third cranial nerve. The pars palpebralis muscle of the lateral lower eyelid of the dog is innervated by the dorsal buccal branch of the facial nerve.

In horses, the auriculopalpebral nerve and its branches (branch of the facial nerve), as well as the supraorbital nerve (branch of the ophthalmic division of the trigeminal nerve, sometimes referred to as the frontal nerve), are common sites for local nerve blocks in the horse to provide akinesia (auriculopalpebral nerve) and local analgesia (supraorbital nerve) to the upper lid.

In most animal species the rostral aspect of the palpebral nerve can be blocked a few centimeters caudal of the lateral canthus. In this forward position, movement of the upper lid may persist.

Eyelid blood supply and lymphatics

The blood supply to the eyelids is derived from several sources, but primarily originates from the medial and lateral canthal areas for both eyelids. The lateral aspects of both eyelids in the dog are supplied by the lateral dorsal and lateral ventral palpebral arteries from the superficial temporal artery. Additional blood supply to the lateral canthus and upper eyelid is derived from the lacrimal and dorsal muscular branch arteries, and the lower eyelid by the zygomatic artery, all branches from the external ethmoidal artery. The medial aspects of the canine eyelids are supplied by branches of the malaris artery, a branch of the infraorbital artery, which anastomose with the inferior palpebral and transverse facial arteries, and branches of the external ophthalmic artery. Limited blood supply to the eyelids is also provided from small vessels within the septum orbitale and conjunctival fornices that originate from the deeper orbital blood vessels.

The lymphatic drainage from the eyelids converges at the medial and lateral canthal areas. Lymphatic drainage appears to mainly involve the parotid lymph node. However, some of the same areas may drain to the mandibular lymph nodes. As a result, both lymph nodes need to be accessed clinically if regional metastases from eyelid neoplasms are suspected, particularly in horses and cattle.

Eyelid function

The functions of the eyelids are numerous and include: 1) protection of the eye; 2) entrapment of material before it contacts the conjunctiva and cornea; 3) production of glandular secretions by the tarsal or meibomian glands, a vital component of the preocular film; 4) distribution of the preocular film and tears across the corneal and conjunctival surfaces; 5) medial movements of tears toward the lacrimal puncta for exit via the nasolacrimal drainage apparatus; and 6) provision of the blink reflex to tactile stimuli applied to the cornea, conjunctiva or nictitating membranes, or following a strong light and/or loud noise. When direct stimuli are applied to the eyelids, conjunctival and corneal surfaces, the eyelids blink. This reflex is subcortical, involving the ophthalmic division of the trigeminal nerve (afferent portion) and palpebral division of the facial nerve (efferent portion). A strong light source directed at the eye will not only initiate a light-induced pupillary response (a subcortical reflex), but also a blink response (also a subcortical function).

Permanent tarsorrhaphy

In the permanent tarsorrhaphy procedure the upper and lower eyelid margins are trimmed by curved Metzenbaum scissors 3–5 mm from the lid margins (Fig. 5.6a). This area is usually where the eyelid pigmentation stops and the eyelid hair appears. For complete permanent tarsorrhaphy the entire eyelid margins are excised for 360°, with special care to adequately excise the lid margins at the two canthi. For partial permanent tarsorrhaphy any section of the eyelid may be used, but most often the lateral and medial canthal areas are involved. At a depth of 4–5 mm, the bases of the meibomian glands are excised. The eyelids are usually apposed by two layers of sutures in most dogs (Fig. 5.6b); however, in miniature breeds, a single suture layer may suffice. The deeper tarsopalpebral layer is apposed with either 4-0 to 6-0 simple interrupted or a simple continuous absorbable suture placed submucosally to avoid corneal contact. The eyelid skin–orbicularis oculi layer is usually apposed with 4-0 to 6-0 simple interrupted or interrupted mattress non-absorbable sutures.

Fig. 5.6 For a lateral permanent tarsorrhaphy. (a) The apposing upper and lower eyelid margins are trimmed by Metzenbaum or Mayo scissors at a depth of 4–5 mm. (b) The lids are apposed by two layers of sutures: the tarsoconjunctival layer with 4-0 to 6-0 simple continuous absorbable sutures placed submucosally, and the orbicularis oculi muscle and skin layer with 4-0 to 6-0 simple interrupted non-absorbable sutures.

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