Lacrimal System

Chapter 9 Lacrimal System


The lacrimal system consists of the following structures:

Precorneal Tear Film

The precorneal tear film covers the cornea and conjunctiva (Figure 9-5). It consists of three layers, differing in composition, and is about 8 to 9 μm thick.

The outer superficial (lipid) layer (0.1 μm thick) is composed of oily materials and phospholipids from the tarsal glands and the glands of Zeis along the lid margin. Its two functions are as follows:

Drugs containing preservatives with detergent properties and commercial shampoos remove this layer and can lead to corneal drying and possibly corneal ulceration. The layer is difficult to appreciate clinically but may be observed as an oil-like film over the ocular surface if the eye is examined with oblique illumination and at high magnification. Alternatively, the layer has been evaluated experimentally with polarized light biomicroscopy.

The middle, or aqueous, layer, consisting predominantly of water derived from the lacrimal gland and the gland of the third eyelid is the thickest layer (approximately 7μm) and serves the following functions:

The inner mucoid layer (1.0 to 2.0 μm thick) consists of hydrated glycoproteins derived from the conjunctival goblet cells. This layer is critical in binding the lipophobic aqueous layer to the lipophilic corneal surface and in preventing the tear film from “beading up” on the corneal surface much like water does on the surface of a freshly waxed car. The mucoprotein molecules are thought to be bipolar, with one end lipophilic (associated with the corneal epithelium) and one end hydrophilic (associated with the aqueous layer). This layer is also difficult to appreciate clinically but may be indirectly evaluated by determination of the tear film break-up time (TFBUT).

Lacrimal Puncta, Canaliculi, and Nasolacrimal Duct

In most domestic mammals the inferior and superior puncta lie on the inner conjunctival surface of the eyelids, near the nasal limit of the tarsal glands (see Figure 9-1). Rabbits possess only one large inferior puncta that is a few millimeters from the eyelid margin.

The lacrimal canaliculi (superior and inferior) lead to a variable dilation in the common nasolacrimal duct—the lacrimal sac. The lacrimal sac varies in size, in some animals consisting only of a slight dilation in the duct. The sac lies within a depression in the lacrimal bone called the lacrimal fossa (Figure 9-6).

From the lacrimal sac, the nasolacrimal duct passes via a canal on the medial surface of the maxilla to open in the nasal cavity (Figure 9-7). In dogs the opening is ventrolateral near the attached margin of the alar fold; in horses it is ventral on the mucocutaneous junction; and in cattle it is more lateral. In cattle and horses the nasal opening is readily visible and can be cannulated, but in dogs it can be seen only after exposure with a speculum or other suitable instrument with the animals under general anesthesia. In dogs the nasolacrimal duct commonly has an opening into the nasal cavity between the lacrimal sac and the nasal opening, although the remainder of the duct is intact. In rabbits the nasolacrimal duct has multiple sharp bends and constricted areas, which may be associated with the frequency of duct obstruction and dacryocystitis in this species. Cannulation is also difficult in most rabbits (Figure 9-8).

Approximately 25% of the precorneal tear film is lost by evaporation. The remainder passes into the puncta and via the canaliculi, sac, and duct to the nasal cavity. A large proportion of the precorneal tear film accumulates in the inferior fornix as the lacrimal lake. Most of this fluid enters the inferior punctum through capillary attraction and movements of the lids. During contraction of the orbicularis oculi, the wall of the sac is tensed, creating lower pressure within the lumen and causing tears to enter; this mechanism is called the lacrimal pump.


The two categories of lacrimal dysfunction are as follows:

Disorders Characterized by Epiphora

The conjunctiva is usually quiet in patients with epiphora due to passive or simple mechanical obstruction of the nasolacrimal system, whereas it is reddened in patients with epiphora due to chronic irritation/inflammation of the cornea, conjunctiva, or lacrimal sac.


Dacryocystitis is inflammation within the lacrimal sac and nasolacrimal duct. It occurs most frequently in dogs and cats and less frequently in horses. Although foreign bodies (e.g., grass awns, sand, dirt, and concretions of mucopurulent material) can be expressed in some patients, the primary cause is often undetermined. Cystic dilations of the nasolacrimal duct causing chronic dacryocystitis in dogs have been described. They are treated by creation of a drainage stoma into the nasal cavity (Figure 9-9). The infected focus within the proximal portion of the duct may reinfect the conjunctival sac, resulting in chronic, unilateral conjunctivitis of apparent unexplained cause. Often the amount of ocular discharge in dacryocystitis is far in excess of what would be expected in view of the severity of conjunctivitis present.


Nasolacrimal Catheterization.

Because of its tendency to recur, definitive surgical catheterization (Figure 9-10) is indicated for dacryocystitis. Although daily flushing and topical medication are effective in some cases, they are less reliable than catheterization and there is a greater chance of recurrence. The tube is left in place for 2 to 3 weeks. The inserted tubes rarely cause discomfort unless they become loose. For the first few days the uncannulated punctum may be flushed daily with a topical ophthalmic antibiotic solution, and topical antibiotic/corticosteroid solution is also applied to the ocular surface. If abscessation of the sac or severe dermatitis is present, systemic antibiotics are added.

Congenital Atresia, Ectopia, and Imperforate Puncta

In dogs, imperforate puncta (usually of the inferior puncta) and punctal aplasia are common, especially in American cocker spaniels, Bedlington terriers, golden retrievers, miniature and toy poodles, and Samoyeds. The condition is congenital and is often characterized by epiphora, although some animals are relatively asymptomatic and epiphora may not become apparent until several weeks of age, when tear production increases. Diagnosis is made by examination of the normal location of the puncta with magnification and from the inability to cannulate or probe the puncta with a small polytetrafluoroethylene (Teflon) IV catheter (minus needle), a lacrimal cannula, or fine nylon thread. In most cases the obstruction consists of a layer of conjunctiva over the lumen, but occasionally obstructions are present in other parts of the nasolacrimal duct. The overlying conjunctiva may be removed with fine scissors after it is elevated with liquid under pressure (Figure 9-12) or through retrograde probing with fine nylon thread (2/0) from the nasal opening (Figure 9-13). Some patients require short-term (1 to 3 weeks) placement of an indwelling catheter to prevent fibrosis of the newly created stoma, especially if the wound bleeds after excision.

In foals (and crias) the obstruction is usually at the nasal puncta instead of the inferior puncta. The nasal puncta may be covered with mucosa, and a variable portion of the nasolacrimal duct may be missing. Additional or abnormally positioned openings may also be present. For treatment, the lumen of the duct is distended with saline via the lacrimal puncta, or with polyethylene tubing, and the nasal mucosa is incised until the lumen is entered. The stoma is cannulated with the tubing, which is sutured in place for 7 to 21 days. Daily application of a topical antibiotic-corticosteroid preparation for 3 to 4 days after removal of the tube is advisable to reduce the chance of postoperative stricture formation. A variety of diverse congenital anomalies of the nasolacrimal duct occur in all species, but all are rare.

In young cats, the most common cause of apparent congenital lacrimal obstruction is cicatrization of the puncta due to feline herpesvirus type 1 conjunctivitis.

Cicatricial Nasolacrimal Obstructions

In cats, especially kittens, scarring and blockage of the puncta or nasolacrimal ducts are common sequela of presumed herpetic keratoconjunctivitis and upper respiratory tract infections. Similar changes due to variety of causes may be seen in any species and frequently accompanies symblepharon. If the puncta and ducts cannot be cannulated, conjunctivorhinostomy or drainage procedures to the oral cavity (conjunctivobuccostomy) are the only remedy. Conjunctivorhinostomy and conjunctivobuccostomy are usually performed by a veterinary ophthalmologist in animals without evidence of active conjunctivitis or chronic respiratory disease. Active, or recurrent, disease increases the chance the newly created opening will scar closed and usually means that surgery will fail to correct the problem. If recurrent respiratory disease is present in cats, a careful examination for evidence of herpetic keratitis is performed and serologic tests for feline leukemia virus, feline immunodeficiency virus, and possibly cryptococcosis should be considered.

Aug 11, 2016 | Posted by in INTERNAL MEDICINE | Comments Off on Lacrimal System

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