Diseases of the Cornea and Sclera

Chapter 134 Diseases of the Cornea and Sclera



Thomas J. Kern


Disorders of the cornea are common in veterinary practice. Acquired corneal disorders such as ulcerative keratitis, melanosis, and pannus are leading causes of preventable blindness in dogs and cats. The high frequency of corneal disorders should not lull practitioners into forgetting the importance of rigorous diagnosis and attentive management. Disorders of the sclera are less common. Their diagnosis may be more difficult than corneal disorders and they may be presented at a more advanced stage.



CONGENITAL DISORDERS



Dermoids


Dermoids are islands of skin embryologically misplaced on the cornea (especially the temporal cornea) and conjunctiva and occasionally malpositioned on the eyelids. They contain epidermis, dermis, fat, sebaceous glands, and hair follicles.



Etiology



• In Burmese cats, dermoids are an inherited condition that frequently involves the eyelids.

• Dermoids are considered a spontaneous nonheritable condition in dogs, although some breeds (e.g., German shepherd) appear to be afflicted at a higher frequency than others; thus a genetic basis cannot be completely ruled out.


Clinical Signs


Dermoids’ surface irregularity promotes chronic blepharospasm and epiphora; the hairs cause corneal and conjunctival irritation.



Diagnosis



• On ocular examination, the typical appearance of skin on the cornea or conjunctiva is diagnostic.


Treatment



• Remove by keratectomy (surgical removal of corneal tissue). If removal is complete, corneal scarring is present but minimal.

• If the conjunctiva also must be resected, suture the edges of the conjunctiva to the limbus, using 7-0 Vicryl, to discourage postoperative adhesion of the conjunctiva to the cornea.


Microcornea



Etiology


Microcornea usually is associated with complicated microphthalmos, a condition with both genetic and environmental causes.



• Inherited microphthalmos syndromes have been characterized in the Australian shepherd, collie, Shetland sheepdog, Old English sheepdog, Akita, American cocker spaniel, miniature schnauzer, Doberman pinscher, Samoyed, Cavalier King Charles spaniel, and Lancashire heeler.

• Multiple heritable ocular defects associated with partial albinism and deafness occur in the Great Dane and collie. Teratogenic influences on the dam during early pregnancy (e.g., viral or other illnesses, live virus vaccines, drugs) may be responsible.

• Microphthalmos occurs less frequently in cats than in dogs; its causes are poorly characterized.


Clinical Signs



• The cornea appears abnormally small and often is misshapen.


Diagnosis



• Measure the horizontal and vertical diameters of the cornea and compare with those of the normal contralateral eye, if present, or with the eye of a normal animal of similar breed.


Treatment



• Treatment is neither available nor warranted. If the condition is associated with microphthalmos, vision may be impaired.


Corneal Opacities



Persistent Pupillary Membrane–Associated Opacities



Etiology



• An embryologic cleavage defect results in strands of iris attached to the endothelial surface of the cornea. Where attachment occurs, endothelium is absent and Descemet’s membrane is abnormal, resulting in permanent nonprogressive opacity.

• The condition is inherited in basenjis and probably in other breeds; the mode of inheritance is uncertain (see Chapter 136).


Axial Geographic Subepithelial Opacities in Puppies



Etiology



• The cause(s) of these variably prominent, wellcircumscribed corneal opacities is unknown.

• They are commonly noted during examination of certain breeds (collie, Shetland sheepdog, English springer spaniel) for inherited ocular disorders and thus have an uncertain genetic basis.


Clinical Signs



• Most obvious in the exposure strip of axial cornea, these superficial opacities are usually bilateral, relatively symmetric, painless, and irregular.

• Typical appearance is that of a non-ulcerated geographic axial corneal opacity of variable density.


Diagnosis



• A typical corneal opacity in a dog less than 1 year of age suggests the diagnosis.

• The absence of fluorescein dye retention rules out ulcerative keratitis and lack of corneal vascularization rules out an old corneal scar.


Treatment



• Do not treat. Most opacities substantially disappear by maturity.

• A residual faint opacity may remain near the nasal and temporal limbus.


Colobomatous Defects of the Sclera



Etiology



• Scleral defects occur as part of the spectrum of inherited ocular anomalies in collies, Shetland sheepdogs, and Australian shepherds.
• The collie/Shetland sheepdog anomaly is said to be due to a simple recessive gene with variable expression; a different mode of inheritance has been postulated for the colobomas.

• The Australian shepherd defect is inherited as an incompletely penetrant recessive trait.

• Optic disc and peripapillary colobomas occur in basenjis; inheritance may be autosomal dominant.

• Great Danes that are the progeny of two harlequin parents are afflicted with colobomatous microphthalmos similar to that in Australian shepherds.

• Posterior pole colobomas of the optic disc and surrounding sclera of unknown cause are seen occasionally in domestic short- and longhaired cats.


Clinical Signs



• In collies and Shetland sheepdogs, the only commonly visible external sign of the defect is relative or absolute microphthalmos. Choroidal hypoplasia, scleral ectasia, optic nerve coloboma, and retinal detachment may be present in the fundus. Blindness is present with retinal detachment and/or optic nerve coloboma.

• Affected Australian shepherds (and Great Danes) usually have mostly white coat color and unilateral or bilateral microphthalmos. In many microphthalmic eyes there are large equatorial staphylomas (i.e., areas of pitted and thinned sclera posterior to the ciliary body). Iris anomalies (heterochromia, corectopia, persistent pupillary membranes, pseudopolycoria) often are present, as are cataracts, retinal dysplasia, and retinal detachment.

• Basenjis with optic disc colobomas have abnormal vision and may have persistent pupillary membranes, although the two defects may not be related.

• Cats afflicted with posterior segment optic nerve coloboma and scleral ectasia are blind in the affected eye, the external appearance of which is normal except for tonic mydriasis.


Diagnosis



• Indirect ophthalmoscopy provides panoramic views of the ocular fundus and is the most sensitive and practical means of diagnosis.


Treatment



• Therapy for these defects is neither indicated nor available.

• Discourage breeding of affected animals.


CORNEAL DEGENERATIONS AND DYSTROPHIES




Key Point


Corneal dystrophy is a congenital or acquired, usually bilateral, corneal opacity typically unassociated with neovascularization. Degenerations are often but not invariably unilateral abnormalities, frequently (but not necessarily) associated with neovascularization, which are secondary to other ocular or systemic disorders.


Corneal degenerations and dystrophies may affect epithelium, stroma, or endothelium (epithelial dystrophies are discussed under Corneal Ulceration). In some animals, differentiation between the two disorders is difficult.



Stromal Dystrophy



Etiology



• Stromal corneal dystrophy is a genetic disorder afflicting many breeds of dogs, including the collie, Siberian husky, Cavalier King Charles spaniel, beagle, Airedale terrier, cocker spaniel, Alaskan malamute, bearded collie, bichon frise, German shepherd, Lhasa apso, mastiff, miniature pinscher, Weimaraner, pointer, and Samoyed.

• One line of Manx cats was reported with an inherited bilateral stromal dystrophy.


Clinical Signs



• At least two forms of stromal dystrophy occur in dogs:
• One form is a bilaterally symmetric oval axial or paraxial subepithelial/anterior stromal crystalline opacity.

• The second form, in which a deeper stromal opacity involves more peripheral portions of the cornea, occurs in Siberian huskies and occasionally in other breeds.

• Corneal neovascularization is absent in both forms. Age of onset varies according to breed and individual animals, from 6 months to old age.

• Affected Manx cats developed progressive stromal edema leading to bullous keratopathy and recurrent epithelial erosion.


Diagnosis



• Dogs: The typical crystalline, usually bilateral, corneal opacity (fluorescein dye negative) unassociated with corneal neovascularization or other corneal pathologic changes suggests stromal dystrophy.
• Rule out causes of corneal degeneration, which may appear similar, including keratoconjunctivitis sicca (KCS), lagophthalmos, eyelid defects, and endocrinopathy (e.g., hypothyroidism).

• Cats: Progressive corneal edema in Manx cats suggests the diagnosis.


Treatment



• Treatment in dogs is rarely indicated or necessary. Keratectomy may remove superficial opacities, but recurrence is likely. Do not breed affected animals.


Stromal Degenerations



Etiology


Ocular causes of stromal degenerations include



• Ulcerative and non-ulcerative keratitis

• Corneal exposure

• Dryness secondary to KCS or lagophthalmos



Key Point


Systemic diseases suspected to cause degenerations include primary hyperlipidemia, secondary hyperlipidemia (especially associated with hypothyroidism), and hyperadrenocorticism (implicated in calcific degeneration or “band keratopathy”).



Clinical Signs



• The central (axial) to paracentral opacities are often bilaterally symmetric and range from crystalline to dense yellow-white; they may be indistinguishable from primary corneal dystrophy.

• In calcific degeneration, the cornea feels “gritty” when touched with a cotton-tipped applicator.

• Neovascularization is often although not invariably present.


Diagnosis



• A typical lesion (with or without neovascularization) and/or a history of a previous condition (e.g., ulcer, other keratitis, lagophthalmos) suggests degeneration rather than dystrophy.



Key Point


Do not assume that concurrent primary or secondary hyperlipidemia or hyperadrenocorticism is the cause unless ocular causes of degeneration have been ruled out. Even then, a causal relationship may be uncertain.



• Clinical differentiation of these two conditions frequently is difficult.


Treatment



• Treatment is rarely indicated or necessary.

• Consider keratectomy for large, dense axial lesions that interfere with vision.
• Prognosis is guarded, as recurrence is possible; in addition, postoperative corneal opacity may be significant.

• For calcific degeneration, consider keratectomy for superficial lesions. Topical application of 1% EDTA q4h may be helpful.


Endothelial Dystrophy



Etiology



• Endothelial dystrophy is a breed-related, acquired disease resulting in premature loss of corneal endothelial cells to a level below that required to maintain normal corneal stromal deturgescence.


Clinical Signs



• Dogs: Progressive bilateral corneal edema may affect Boston terriers, Chihuahuas, dachshunds, and occasionally other breeds. It can begin focally or in a diffuse pattern and progress from the peripheral to the central cornea or vice versa.
• Initially painless, the condition frequently results in recurrent corneal erosion when stromal and epithelial edema become extensive. Corneal neovascularization usually is absent.

• Cats: Endothelial dystrophy causing progressive bilateral corneal edema has been reported rarely in domestic shorthaired cats.


Diagnosis



• Endothelial dystrophy is presumptively diagnosed when bilateral corneal stromal edema is present unassociated with signs of uveitis or glaucoma.



Key Point


Determine intraocular pressure (IOP) to rule out uveitis (decreased IOP) or glaucoma (elevated IOP) (see Chapters 136 and 137).



• Corneal ulceration, if present, usually is superficial and insufficient to cause the degree of stromal edema observed.


Treatment



• For severe stromal and epithelial edema, consider treatment with topical 5% sodium chloride (Muro-128, Bausch and Lomb) q4–6h to discourage corneal erosion.

• When erosions occur, debride non-adherent epithelium and administer topical broad-spectrum antibiotics and 1% atropine.


Endothelial Degeneration



Etiology



• Intraocular inflammation or hemorrhage, primary and secondary glaucoma, anterior lens luxation, and corneal injury may cause endothelial degeneration.

• Natural infection with canine adenovirus-1 (infectious canine hepatitis) or vaccination with modified live vaccines containing adenovirus-1 or adenovirus-2 may result in immune-mediated anterior uveitis and endothelial destruction 7 to 10 days following exposure (see Chapter 16).

• The proportion of dogs affected can be high (in natural infection, about 20%), low (adenovirus-1 vaccination), or very low (adenovirus-2 vaccination).


Clinical Signs



• Chronic focal or diffuse corneal edema (giving the cornea a ground-glass appearance) with or without superficial or deep corneal neovascularization characterizes endothelial degeneration. If the edema resolves, only transient endothelial dysfunction occurs.



Key Point


Corneal endothelial cells in many adult domestic animals have little to no regenerative capacity. Repair occurs by hypertrophy and migration of remaining endothelial cells.



• Most dogs with adenovirus-mediated uveitis present with generalized corneal edema after resolution of transient uveitis.
• A small proportion of affected dogs, especially sight hounds and Arctic breeds, quickly develop intractable secondary glaucoma.


Diagnosis



• Diffuse, moderate to severe generalized corneal edema strongly suggests endothelial dysfunction.

• Perform a careful ocular examination to rule out antecedent causes (e.g., inflammation, glaucoma, hyphema, lens luxation).


Treatment



• Direct treatment toward prevention by effectively controlling the causative or attendant conditions (e.g., glaucoma, uveitis, hyphema) before irreversible endothelial degeneration occurs.


Prognosis



• Most dogs with adenovirus-related endotheliopathy recover endothelial function, and the corneal edema resolves within a few weeks.

• Corneal edema secondary to other causes has a variable prognosis for resolution.


CORNEAL ULCERATION


Loss of one or more corneal epithelial layers commonly is termed corneal erosion or abrasion. Full-thickness loss of epithelium with at least some stromal loss is termed ulceration.



• Simple corneal ulceration is that which heals uneventfully in a normal amount of time (3 days).

• Complicated corneal ulceration involves delayed healing associated with infection or other pathologic processes.

• Progressive corneal ulceration involves a deepening or enlarging area.


Etiology



Traumatic Injury


This is probably the most common cause of ulcerative keratitis in dogs and cats.



• Blunt trauma may cause focal or diffuse damage to any or all corneal layers. Corneal laceration may be partial or full thickness. Aqueous loss, anterior chamber collapse, and iris prolapse may ensue.

• Most traumatic corneal erosions and ulcers heal rapidly; however, some become persistent if epithelial basement membrane damage is severe or if a foreign body persists in the conjunctival fornix.

Related posts:

  1. Feline Infectious Peritonitis (Feline Coronavirus)
  2. Borreliosis (Lyme Disease)
  3. Immune-Mediated Dermatoses
  4. Otitis Media and Otitis Interna

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Aug 27, 2016 | Posted by in SMALL ANIMAL | Comments Off on Diseases of the Cornea and Sclera

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