Canine Cornea and Sclera

8
Canine Cornea and Sclera


Developmental and Congenital Defects


Dermoids


Dermoids, also known as choristomas, are normal tissue in an abnormal location. They occur infrequently in puppies and are usually noticed at weaning or shortly thereafter because of the development of ocular irritation. Dermoids can be inherited in certain breeds such as the Dachshund, Dalmation, Doberman Pinscher, German Shepherd, and St. Bernard and are either unilateral or bilateral. Upon examination a mass of variable size and pigmentation with coarse hairs protruding from its surface is present at corneoscleral limbus and bulbar conjunctiva (Figure 8.1). Treatment is local excision; a superficial keratectomy is indicated if the dermoid extends onto the corneal surface. Incomplete excision usually results in regrowth.

Photo displaying the eye of a St. Bernard puppy with corneoconjunctival dermoid affecting the lateral limbus with few long and coarse hairs.

Figure 8.1 Corneoconjunctival dermoid affecting the lateral limbus in a St. Bernard puppy. A few long and coarse hairs are evident.


Microcornea


Normal size globes with smaller than normal corneas are rare. Microcornea is nearly always associated with microphthalmia and other ocular anomalies. Breeds affected include Australian Shepherd (merle ocular dysgenesis), Collie, Miniature and Toy Poodles, Great Dane, Old English Sheepdog, and St. Bernard (Figure 8.2). Affected individuals usually have a shortened palpebral fissure and a prominent nictitans which partially obscures the globe.

Photo displaying bilateral merle ocular dysgenesis in an excessively white Great Dane puppy with the globes not the same in size.
Photo displaying a Boston Terrier mix puppy with unilateral microcornea and microphthalmia with the opposite globe enlarged from congenital glaucoma.

Figure 8.2 (A) Bilateral merle ocular dysgenesis in an excessively white Great Dane puppy. Note the globes are not the same size. (B) Unilateral microcornea and microphthalmia in a Boston Terrier mix puppy. The opposite globe is enlarged from congenital glaucoma. Multiple ocular anomalies are probably also present.


Microcornea usually refers to a cornea that is <12 mm in horizontal diameter. Occasionally, central corneal mineralization and other ocular anomalies are present. Vision can be reduced or absent. There is no treatment available.


Persistent Pupillary Membranes


Corneal opacities and persistent pupillary membranes (PPMs) are congenital corneal and iridal anomalies (Figure 8.3). The pupil is formed during the last one‐third of gestation and continues to develop for 4–8 weeks postnatally. If pupillary formation is incomplete or delayed, very fine white to pigmented remnants or strands of iris tissue originating from the iridal collarette or minor iridal arteriolar circle protrude off the surface of the iris and extend to other areas of the iris, the posterior cornea, the anterior lens capsule, or some combination thereof. If the cornea is involved, there will be deep round to oval endothelial opacities, sometimes containing pigmentation. Lens opacities involve only the anterior lens capsule and outer anterior cortex. Occasionally, corneal or lens opacities exist without the PPM attachment if the strand has previously atrophied. Generally, no treatment is necessary. Many breeds have been reported with PPMs; however, the Basenji is the only breed in which mode of inheritance has been established.

Photo displaying eye of a mixed-breed dog with persistent pupillary membranes pigmented originating from the middle of the iris forming a pigmented circle on the posterior cornea.
Photo displaying the eye of a Rottweiler puppy with PPM concentrating in lateral posterior cornea with 2 dense corneal opacities and congenital pigment on the anterior lens and a focal anterior cortical cataract.

Figure 8.3 (A) Persistent pupillary membranes (PPM) in a mixed‐breed dog. Note the PPMs are pigmented, originate from the middle of the iris, and form a pigmented circle on the posterior cornea. (B) PPM in a Rottweiler puppy. The pigmented PPMs are concentrated in the lateral posterior cornea with two dense corneal opacities. Note also the congenital pigment on the central anterior lens capsule and a focal anterior cortical cataract.


Corneal Ulcerations and Inflammation


Spontaneous Chronic Corneal Epithelial Defects


Spontaneous chronic corneal epithelial defects, corneal erosion, recurrent erosion, persistent epithelial erosion, Boxer’s ulcer, indolent ulcer, refractory ulcer, or rodent ulcers are all synonyms to describe a clinical syndrome of recurrent superficial corneal ulcers that heal very slowly in the dog, and often recur. At least 24 breeds are predisposed to these ulcers. The corneal epithelium and its basement membrane appear abnormal, which accounts for the development of the erosion as well as its delayed healing (attachment) period. Most affected dogs are middle aged (5–7 years), and the Boxer breed accounts for a significant number of these ulcers (Figure 8.4).

Photo displaying corneal erosion in a Boxer dog stained with topical fluorescein with a loose “lip” of epithelium and some corneal vascularization from the lateral limbus.
Photo displaying dog’s eye with corneal erosion, stained with topical rose Bengal,  featuring a loose epithelium surrounding the defect.
Photo displaying dog’s eye with corneal erosion with healing of the ulcer started with corneal vascularization of its medial margin.

Figure 8.4 (A) Corneal erosion in a Boxer dog stained with topical fluorescein showing a loose “lip” of epithelium and some early superficial corneal vascularization from the lateral limbus. (B) Corneal erosion, stained with topical rose Bengal, demonstrates retention of stain by both the base of the erosion as well as loose epithelium surrounding the defect. (C) In this corneal erosion, healing of the ulcer has started with corneal vascularization of its medial margin.


On examination the affected cornea demonstrates a superficial corneal ulcer with an overlying lip of non‐adherent loose corneal epithelium. Often, fluorescein or rose Bengal applied to these corneas will ooze or leak into stroma adjacent to the ulcer proper where the epithelium is not attached. Conjunctival hyperemia and a mild secondary iridocyclitis are often present.


Treatment consists of removal of the loose epithelium under topical anesthesia with a dry cotton swab, followed by debridement of the exposed stroma to facilitate attachment. Diamond burr debridement is used often in preference to grid or multiple punctate keratotomy. Chemical debridement or superficial keratectomies are less commonly employed now. Recurrence in the ulcer in the same eye or the fellow eye is common given that these ulcers are caused by an abnormality of the epithelium and anterior stroma.


Corneal Ulcers


Corneal ulcers are characterized by the loss of corneal epithelium and varying degrees of stromal loss, which determines the depth of the wound. Application of topical fluorescein to the bulbar conjunctiva and cornea results in the dye uptake by the ulcer (Figure 8.5). As fluorescein rapidly diffuses in the stroma and even the anterior chamber, observation of the stain should occur immediately after application. Topical fluorescein, adequate illumination, and some magnification are essential aids in the diagnosis and monitoring of corneal ulcers. Corneal ulcers follow trauma, shampooing, cat scratches, exposure (as with enlarged globe, impaired blink, or corneal sensitivity), keratoconjunctivitis sicca (KCS), and other causes. In the brachycephalic breeds, exophthalmia, lagophthalmia, incomplete blinks, and other factors contribute to corneal ulcerations (see Appendix B). Corneal ulcer may progress in size and depth (with or without therapy), and changes from a shallow to a deep stromal ulcer, descemetocele, and even to corneal perforation. Iris prolapse can occur within several hours.

Photo displaying dog’s eye with a superficial corneal ulcer stained with topical fluorescein.
Photo displaying dog’s eye with a central corneal ulcer extending to about the middle of the stroma with corneal vascularization approaching the wound bed.
Photo displaying Shih Tzu dog’s eye with deep stromal ulcer extending approximately 80% into the posterior stroma.
Photo displaying eye of a mixed-breed dog with deep stromal and infected corneal ulcer with the ring of perilimbal vascularization approaching central wound and axial cellular infiltrate and soft malacic edges.
Photo displaying a dog's eye with a melting corneal ulcer. There is soft corneal tissue contacting the lower eyelid margin.
Photo displaying a dog's eye with axial deep stromal, near descemetocele with malacic edges, with secondary uveitis as evidenced by miosis and hypopyon in the ventral anterior chamber.
Photo displaying a dog's eye Same eye placement of a conjunctival graft for vascular and structural support. There is dilation in the pupil and resolution of the hypopyon and intracameral fibrin.
Photo displaying a dog's eye with a conjunctival pedicle graft that has healed and is well‐incorporated into the native cornea.
Photo displaying dog's eye immediately following the trimming of a graft pedicle.

Figure 8.5 (A) A very large but superficial corneal ulcer stained with topical fluorescein. The dog’s eye had developed corneal ulceration after being treated with topical corticosteroids for keratoconjunctivitis sicca. (B) Large central corneal ulcer extending to about the middle of the stroma. Note the corneal vascularization approaching the wound bed. (C) Deep stromal ulcer in a Shih Tzu dog. This ulcer extends approximately 80% into the posterior stroma, but not all the way to Descemet’s membrane. (D) Deep stromal and infected corneal ulcer in a mixed‐breed dog. Note the ring of perilimbal vascularization approaching the central wound and the axial cellular infiltrate and soft malacic edges. (E) Melting corneal ulcer in a dog. Note the soft, gray, corneal tissue contacting the lower eyelid margin. (F) Axial deep stromal, near descemetocele with malacic edges. There is considerable secondary uveitis as evidenced by miosis and hypopyon in the ventral anterior chamber. (G) Same eye as in part F 3 weeks after placement of a conjunctival graft for vascular and structural support. Note the dilated pupil and resolution of the hypopyon and intracameral fibrin. (H) Conjunctival pedicle graft in another dog that has healed and is well‐incorporated into the native cornea. (I) Same eye as in part H immediately following trimming of the graft pedicle.


Corneal ulcers in the dog will often become infected by opportunistic or pathogenic bacteria, with Staphylococci and Streptococci species the most commonly isolated. Viral ulcers associated with canine herpesvirus (CHV‐1) have been documented in the dog, and fungal ulcers are uncommon (usually related either to a corneal foreign body or in an immunosuppressed patient).


The main goals of medical therapy are: (i) to eliminate or prevent infection of the ulcer; (ii) to control or prevent the proteolytic and enzymatic digestion of corneal stroma; and (iii) to address the reflex uveitis that invariably accompanies the ulcer. Additionally, if the ulcer has perforated or is likely to because of its deep nature or the presence of keratomalacia, surgical stabilization can be warranted.


Corneal Ulcerations in Brachycephalic Breeds


The brachycephalic breeds are predisposed to central corneal ulcers that have the tendency to progress into the deeper stroma as well as recur (Figure 8.6). These ulcers must be under close clinical monitoring as they can rapidly deteriorate. Corneal exposure, lagophthalmia, reduced central corneal sensitivity, nasal fold trichiasis, distichiasis, inadequate blink rate, quantitative or qualitative tear deficiencies, and increased central corneal epithelial turnover contribute to the high prevalence of corneal ulcerations in brachycephalic breeds (see Appendix B).

Photo displaying a deep central corneal ulcer in a dog's eye. It has been stained with topical fluorescein.
Photo of a Pekingese dog’s eye displaying deep corneal ulcer with corneal vascularization surrounding the ulceration.
Photo displaying a Pekingese cornea with two ulcers: descemetocele and superficial ulcer with corneal vascularization.
Photo of a dog’s eye displaying perforated central corneal ulcer with recent iris prolapse with hemorrhage.

Figure 8.6 (A) Deep central corneal ulcer has been stained with topical fluorescein, the retention of which within the wound bed suggests some stroma is still present. (B) A deep corneal ulcer in a Pekingese. A portion of the ulcer is deeper than the rest (descemetocele). Corneal vascularization completely surrounds the ulceration. (C) Pekingese cornea with two ulcers: descemetocele and superficial ulcer with corneal vascularization. (D) Perforated central corneal ulcer with recent iris prolapse with hemorrhage.


The history usually includes lacrimation, blepharospasm, eyelid swelling, conjunctival hyperemia, and a corneal opacity. Because brachycephalic patients have decreased central corneal sensation (compared with mesocephalic and dolichocephalic breeds), they often will not exhibit pronounced evidence of pain. An ophthalmic examination reveals a central or paracentral corneal ulcer that retains fluorescein. The corneal stroma can be grayish, irregular, and malacic (soft or friable). When the corneal stroma is completely lost within the ulcer, descemetocele occurs (characterized with fluorescein as a circle of dye staining the sides of the ulcer wall but not its bed). With rupture of the very thin descemetocele, iris prolapse follows.


Therapy of the brachycephalic corneal ulcer includes topical antibiotics, autogenous serum, atropine (1%) or tropicamide (1%) for the secondary iridocyclitis, and systemic anti‐inflammatories and analgesics. Both topical antibiotics and serum are often instilled very frequently (sometimes hourly) for the first few days, to halt ulcer progression and promote healing. For those ulcers that progress (larger and/or deeper), immediate surgical intervention is recommended (usually a pedicle conjunctival graft).


Fungal Keratitis


In contrast to the horse, dogs rarely develop fungal keratitis. The history is usually one of a corneal ulcer that persists in spite of intensive topical (and systemic) antibiotic therapy. There may be a history of trauma or a previous corneal foreign body. Affected dogs are often systemically or locally immunocompromised. Corneal ulceration is the most common clinical manifestation, but the occasional fungal corneal abscess occurs in dogs (Figure 8.7). Diagnosis is made based upon index of suspicion and the results of corneal cytology or biopsy (to demonstrate the presence of fungal elements) and culture. Therapy includes routine corneal ulcer therapy with the addition of topical antifungals, removal of the foreign and infected site, and, if necessary, a pedicle conjunctival graft. Prognosis is usually fair to good with appropriate therapy.

Photo of a dog’s eye displaying mycotic keratitis with ulceration and secondary iridocyclitis.
Photo displaying fungal keratitis in a mixed-breed dog’s eye.
Photo displaying fungal keratitis in an aged dog’s eye.

Figure 8.7 (A) Mycotic keratitis with ulceration and secondary iridocyclitis in a dog. Fungal organisms were identified on cytology. (B) Fungal keratitis in a mixed‐breed dog. (C) Fungal keratitis in an aged dog that was systemically immunocompromised.

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Jul 24, 2020 | Posted by in INTERNAL MEDICINE | Comments Off on Canine Cornea and Sclera

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