C: Corneal Dystrophies

Corneal Dystrophies

BASICS

OVERVIEW

Primary, inherited (or familial), bilateral, and often symmetrical condition of the cornea that is not associated with other ocular or systemic diseases.
Three types based on anatomic location—epithelial: associated with dyskeratotic and necrotic epithelial cells, focal absence of epithelial basement membrane, and increased cells in anterior corneal stroma; stromal: lipid deposition within the corneal stroma; and endothelial: characterized by abnormal, dystrophic endothelial cells.

SIGNALMENT

Usually dogs; rare in cats.

Epithelial

Shetland sheepdogs—age of onset 6 months–6 years; slow progression.

Stromal

Usually young adult dogs at age of onset.
Affected breeds—Afghan hound, Airedale terrier, Alaskan malamute, American cocker spaniel, beagle, bearded collie, bichon frise, Cavalier King Charles spaniel, German shepherd, Lhasa apso, mastiff, miniature pinscher, rough collie, Samoyed, Siberian husky, Weimaraner, whippet, and others; inheritance pattern identified in only a few breeds.

Endothelial

Dogs—primarily affects Boston terriers, Chihuahuas, and dachshunds; may affect other breeds; typically middle-aged or older at onset of clinical signs; female predilection suggested.
Cats—affects young animals; described most often in domestic shorthairs; a similar condition that occurs without endothelial disease is inherited in Manx as an autosomal recessive disorder.

SIGNS

All cause some degree of opacity in the cornea.

Epithelial

Can be asymptomatic or have blepharospasm; multifocal white or gray circular to irregular opacities or rings; sometimes associated with multifocal corneal erosions.
Vision—usually not affected.

Stromal

Usually asymptomatic with no associated inflammation.
Central—most common; gray, white, or silver oval to circular opacity of the central or paracentral cornea; with magnification may note multiple fibrillar to coalescing opacities that have a crystalline or ground-glass appearance (crystalline corneal dystrophy).
Diffuse—affects Airedales; more diffuse, dense opacity than with central dystrophy.
Annular—affects Siberian huskies most commonly; doughnut-shaped opacity of the paracentral or peripheral cornea.
Vision—usually not affected; visual deficit possible with advanced or diffuse disease.

Endothelial

Asymptomatic in early stages.
Edema of temporal or inferio-temporal cornea that usually progresses to involve the entire cornea after months to years.
Corneal epithelial bullae (bullous keratopathy) and subsequent corneal erosion ulceration may develop; erosions or ulceration may cause blepharospasm due to pain.
Vision—may be impaired with advanced disease.

CAUSES & RISK FACTORS

Epithelial—result of degenerative or innate abnormalities of the corneal epithelium and/or basement membrane.
Stromal—innate abnormality or localized error in corneal lipid metabolism; may be affected by hyperlipoproteinemia (may increase opacity).
Endothelium—degeneration of the endothelial cell layer; subsequent loss of endothelial cell pump function results in corneal edema.

DIAGNOSIS

DIFFERENTIAL DIAGNOSIS

Epithelial, stromal—other causes of corneal opacity: corneal degenerations, ulcers, scars, inflammatory cell infiltrates.
Endothelial—other causes of diffuse corneal edema: uveitis and glaucoma.

CBC/BIOCHEMISTRY/URINALYSIS

Epithelial, stromal—high concentrations of cholesterol and triglyceride levels may modify the course of the disease but are not the cause.

DIAGNOSTIC PROCEDURES

Stromal—usually does not retain fluorescein stain.
Epithelial or endothelial—may retain fluorescein stain, often in multifocal punctate areas, particularly with advanced disease.
Tonometry—to eliminate glaucoma as cause of corneal edema.

TREATMENT

Advanced epithelial or endothelial disease with ulceration—may require treatment for ulcerative keratitis.
Stromal—usually none required; may perform superficial keratectomy to remove lipid deposits if severe but usually unnecessary and deposits may recur.
Inform client that some corneal dystrophies are inherited.
Advanced endothelial dystrophy—may use therapeutic soft contact lens with or without debridement of redundant corneal epithelial tags; conjunctival flap surgery; thermal cautery of the cornea; penetrating keratoplasty (corneal transplant) may be of benefit but success rates vary (fair to good for cats, poor for dogs).

MEDICATIONS

DRUG(S)

Corneal ulceration—topical antibiotics and possibly atropine (see Keratitis, Ulcerative).
Epithelial—1–2% cyclosporine in oil or 0.2% ointment q8–24h as needed to relieve clinical signs.
Endothelial—topical 5% sodium chloride ointment; palliative treatment; does not markedly clear cornea but may prevent progression and rupture of corneal epithelial bullae.

CONTRAINDICATIONS/POSSIBLE INTERACTIONS

Topical corticosteroids—no benefit to lipid (stromal) dystrophy; of questionable benefit to other forms of dystrophy.

FOLLOW-UP

Reexamination—necessary only if ocular pain or corneal ulceration develops.
Corneal opacity—may wax and wane with lipid dystrophy; unlikely to resolve.
Corneal ulceration—may accompany progression of epithelial or endothelial dystrophy.
Vision—not substantially affected except in advanced cases.

MISCELLANEOUS