Chapter 9 Lacrimal System
The gland of the third eyelid, which lies within the stroma of the third eyelid, is partially visible on the inner surface of the third eyelid (see Chapter 8 and Figure 9-2). The tubuloalveolar lacrimal gland is flattened and lies over the superior-temporal part of the globe. In the dog it lies beneath the orbital ligament and supraorbital process of the frontal bone and is related to the medial surface of the zygomatic bone (Figure 9-3). The position is similar in species with a fully enclosed bony orbit.
(Courtesy Dr. Richard R. Dubielzig.)
(Modified from Evans HE : Miller’s Anatomy of the Dog, 3rd ed. Saunders, Philadelphia.)
Lacrimal secretions enter the superior conjunctival fornix from the canine lacrimal gland via three to five microscopic ducts, and from the gland of the third eyelid via multiple ducts opening on the bulbar surface of the third eyelid between the normal lymphoid follicles. The precorneal tear film is distributed over the ocular surface by gravity, blinking, and movement of the third eyelid.
Figure 9-4 A, The openings of the meibomian (tarsal) gland ductules are apparent as multiple, circular, lightly pigmented foci at the eyelid margin in this goat. The sebaceous secretions of these glands contribute to the lipid layer of the tear film. B, Cross-sectional view of the normal canine eyelid. The accessory lacrimal glands of the eyelid margin include the tarsal (meibomian) glands (T), glands of Moll (M), and glands of Zeiss (Z).
(B courtesy Dr. Richard R. Dubielzig.)
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).
Fine mucous threads lie in the superior and inferior conjunctival fornices in the normal animal. These strands are accumulations of mucus derived from the conjunctival goblet cells and exfoliated epithelial cells. The threads migrate nasally in a predictable fashion, collecting debris from the conjunctival sac. Vacuoles within the threads contain debris and exhibit enzymatic activity. Dehydrated remnants of these threads are frequently found on the skin at the nasal canthus in the morning (“sleep” or “sleepy seeds”). These accumulations are normally grayish and translucent and may be quite large in animals with deep conjunctival fornices (e.g., Irish setters, Doberman pinschers). In cats it is not uncommon for the threads to be deep red to black because they are stained with tear porphyrins. In an animal without signs of conjunctival inflammation or epiphora, larger accumulations are normal if they are grayish or translucent (possibly reddish black in cats). Increased quantities are often a sensitive indicator of ocular surface inflammation. A change in color of the mucous thread to green or yellow is a reliable indicator of the presence of inflammatory cells and mandates a careful clinical examination.
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).
(Modified from Evans HE 1993]: Miller’s Anatomy of the Dog, 3rd ed. Saunders, Philadelphia.)
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).
(Modified from Severin GA : Severin’s Veterinary Ophthalmology Notes, 3rd ed. Severin, Ft. Collins, CO.)
Figure 9-8 Diagram of the rabbit nasolacrimal duct. A, The duct bends and narrows at two points, proximally as it passes through the maxillary bone and distally as it bends near the incisor tooth root. This anatomy makes the rabbit prone to dacryocystitis and nasolacrimal duct obstruction. B, Higher magnification of the area outlined in the box in A, showing the single canaliculus and the location of the lacrimal sac.
(From Burling K, et al. : Anatomy of the rabbit nasolacrimal duct and its clinical implications. Prog Vet Comp Ophthalmol 1:33.)
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.
Innervation of the lacrimal gland and control of secretion are complex, and the exact details are undetermined in domestic animals. Fibers from the ophthalmic division of the trigeminal nerve, facial nerve, pterygopalatine ganglion, and sympathetic fibers from the carotid plexus have been traced to the lacrimal gland.
The fluorescein passage test (Jones test) is reliable only when its result is positive. Because of communications between the nasolacrimal duct and the nasal cavity, false-negative results occur even though the duct is patent. If epiphora is due to overproduction of tears, the eye is usually red and the STT values are higher than normal.
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.
Figure 9-9 A, Lateral radiographic view of the skull of a 4-year-old golden retriever with recurrent intermittent episodes of purulent discharge from the right eye. A radiolucent area with sclerotic margins (arrow) is evident dorsal and rostral to the maxillary process of the maxillary bone (dorsal to the upper right fourth premolar). B and C, Dacryocystorhinogram of the right nasolacrimal duct of the same dog before surgery (B) and 9 months after surgery (C). Before surgery, contrast material accumulated in the cyst indicated in A, and the nasolacrimal duct was visible proximal to the radiolucency. After surgery, contrast material was evident in the nasal passage but did not accumulate in the cyst.
(From van der Woerdt A, et al. : Surgical treatment of dacryocystitis caused by cystic dilatation of the nasolacrimal system in three dogs. J Am Vet Med Assoc 211:445.)
Diagnosis is based on clinical signs, especially expression of purulent material from the puncta. The exact site of the obstruction may be determined by cannulation, dacryocystorhinography, or magnetic resonance imaging.
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.
Figure 9-10 Indwelling nasolacrimal duct catheterization for correction of recurring obstruction. A, A monofilament nylon thread (2/0 with a smooth melted end) is passed via the superior punctum to emerge from the nose. If an obstruction is present in the sac, the duct is threaded from the nasal end, and the thread is manipulated to emerge from the superior punctum. B, Fine polyethylene (PE90), polyvinyl, or silicone tubing with a beveled end is passed over the thread. Halsted forceps are clamped behind the tubing, which is pulled from the nasal end by forceps on the thread. In horses, larger tubing is used. C, Care is taken as the tubing enters the punctum. Note: The inferior punctum may also be used if threading via this punctum was used. The tubing is pulled down the nasolacrimal duct, past any obstructions. D, The tube is sutured in place for 2 to 3 weeks. An Elizabethan collar should be considered to prevent the tubing from being dislodged.
(Modified from Bistner SI, et al. : Atlas of Veterinary Ophthalmic Surgery. Saunders, Philadelphia.)
For patients in which obstruction prevents the passage of a catheter, the lacrimal sac may be exposed ab externo, through an incision parallel to the lower lid, followed by removal of the outer surface of the lacrimal bone with a Hall Surgitome bur over the sac, flushing of the sac, and placement of the catheter (Figure 9-11). In some chronically affected animals a cavity may develop in the region of the sac. If the catheter is left in place and antibiotic therapy is continued, the space usually fills with fibrous tissue and a patent duct remains. This may take several months.
Figure 9-11 Dacryocystostomy in the dog. A, An incision is completed ventral to the medial canthus into the lacrimal sac. B and C, The foreign material is removed and submitted for laboratory evaluation. The nasolacrimal duct system is cannulated with a Silastic tube (D), and the incision is closed routinely (E).
(Modified from Grahn B : Disorders and surgery of the canine nasolacrimal system, in Gelatt KN (editor): Veterinary Ophthalmology, 3rd ed. Lippincott Williams & Wilkins, Philadelphia.)
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.
Figure 9-12 Repair of imperforate punctum through the use of pressurized fluid. A, The opposing punctum is cannulated, and pressure is applied via a saline-filled syringe to elevate the obstructing conjunctiva over the other punctum. The use of methylene blue solution aids in the location of the bleb. Some loss of saline occurs down the nasolacrimal duct. B, The tissue is grasped with fine forceps and incised with strabismus scissors or other fine scissors. Antibiotic-corticosteroid preparations are applied for 7 to 10 days to prevent scarring and obstruction. Daily dilation and flushing may be needed for a few days to prevent closure. If the membrane is thick or bleeds when incised the nasolacrimal system may need to be cannulated for 1 to 3 weeks to prevent fibrosis of the newly created stoma.
Figure 9-13 Retrograde probing of imperforate punctum. The nasal meatus of the nasolacrimal duct is probed using a nylon (2/0) thread. The probe is passed up to elevate the obstructing conjunctiva, which is excised. This procedure is most useful for the superior punctum, because it is more difficult to pass a probe into the inferior punctum from the nasal end.
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.
Cystic disorders of the lacrimal system and tissues are uncommon but may affect the lacrimal gland, canaliculi, nasolacrimal duct, gland of the third eyelid, zygomatic salivary gland, conjunctival goblet cells, lacrimal sac, and parotid duct after transplantation. Clinical signs are usually restricted to localized swelling, and treatment consists of careful surgical excision and, sometimes, cannulation (see also section on dacryocystitis). Periorbital epidermoid cysts have also been described at the medial canthus in dogs.
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.
In conjunctivorhinostomy a communication is made from the medial conjunctival sac to the nasal cavity and is kept open with a stent of plastic or silicone tubing until healed (Figure 9-14). The method is most suitable for dogs lacking lacrimal drainage but can be used in cats. In cats the opening tends to become obstructed with scar tissue, and the stent is left in longer (8 to 12 weeks) before initial removal. During the postoperative period the topical antibiotic therapy is continued and the stent is cleaned frequently. Also, the eye is checked weekly to ensure that the stent is not causing ocular irritation (e.g., by pressing on the cornea through the third eyelid).
Figure 9-14 Conjunctivorhinostomy. A, The conjunctiva is removed from the inferior nasal area overlying the lacrimal bone. B, A communication is made from the conjunctival sac to the nasal cavity with a Steinmann orthopaedic pin. The pin is directed toward the contralateral external nares but is advanced only until it enters the nasal cavity. A stent of plastic tubing is sutured in place.