Julie Gionfriddo, DVM
ACVO Genetics Committee/CERF Liaison
Confusion often arises over the use of the term "corneal dystrophy" in dogs. Technically, "corneal dystrophies" are diseases of the cornea that are bilateral, non-inflammatory and inherited(*1,2). The confusion arises because the term "corneal dystrophy" is sometimes used to refer to a disease with similar clinical signs but is not hereditary. A more appropriate term for the non-inherited conditions is corneal degeneration.
In most breeds, corneal dystrophy appears as gray-white, crystalline or metallic opacities in the center of the cornea or close to the periphery. These opacities may affect any layer of the cornea, the epithelium (outer layer), the stroma (the thick, middle layer), or the endothelium (the inner layer). The opacities are usually oval or round and are sometimes doughnut-shaped. The age of onset of the disease varies within and among dog breeds and may range from 4 months in Airedale Terriers, to up to 13 years in Chihuahuas. The opacities usually progress but in some cases they remain static. Their progression may be very slow and may or may not lead to blindness (common in Cocker Spaniels, Poodles, Samoyeds, Siberian Huskies, Pointers, German Shepherds, and Bichon Frises). On the other hand, progression may be rapid and lead to blindness (more common in Airdale Terriers, Boston Terriers, Chihuahuas and Dachshunds)(*.2) The mode of inheritance varies among breeds and in many breeds it is unknown. In the airedale terrier it is thought to be a sex-linked trait(*1,3) and in the Siberian Husky, Corneal Dystrophy has been shown to be a recessively inherited trait with variable expression(*4).
Corneal dystrophies are usually not painful. In a few breeds, however, a dystrophy can lead to secondary breaks in the epithelial (outer) layer of the cornea. When this occurs a painful corneal ulcer develops requiring intense treatment. In other breeds, a painful ulcer may not develop and the dystrophy itself is not treatable. No medication will "dissolve" the opacity. Surgical removal of the dystrophic area may temporarily decrease the opacity in cases of epithelial dystrophy. Often, however the opacities will reform in the healed cornea.
Shetland Sheepdogs have corneal dystrophy which may begin as early as 4 months of age and usually progress throughout life(*5). It usually manifests as small gray or white rings which start in the center of the cornea and later other spots develop peripherally. This condition is an epithelial dystrophy, meaning it is in the superficial layer of the cornea. This corneal dystrophy is inherited but the mode of inheritance is unknown. In Shelties this disease can cause corneal ulcers.
In Beagles, corneal dystrophy may begin as early as 3.5 years of age (*6). Beagles usually have either an anterior stromal opacity or one which involves all layers of the stroma. The opacity progresses from an oval "nebula" (cloud-like lesion), to a racetrack-shaped lesion, to an arc-shaped opacity. In Beagles dystrophy rarely causes corneal ulcers and the mode of inheritance is unknown.
Siberian Huskies have a form of corneal dystrophy which is properly called "crystalloid corneal dystrophy." it is inherited as a recessive trait and appears round or horizontally oval(*4). It begins as a diffuse, gray haze in the anterior stroma and may progress to crystals or gray-brown smudgy deposits in the anterior stroma, or involve the posterior part of the stroma or the entire stroma(*2). This form of dystrophy usually begins between 5 and 27 months of age.
Boston Terriers and Chiuhuahuas have a form of endothelial dystrophy which usually begins later in life (5 to 9 years)(*7). Its mode of inheritance is unknown. This disease begins as a fluid build-up (edema) in the cornea due to the inability of the endothelium to act as a water barrier to keep the fluid inside the eye from percolating into the corneal stroma. The fluid buildup causes the cornea to look white. It begins at the edge of the cornea, progresses centrally and often involves the entire cornea, causing the cornea to appear thickened. The fluid can accumulate under the epithelium and lift it off, thus causing a painful corneal ulcer which is very difficult to treat.
Airdale Terriers have a dystrophy which is presumably sex-linked inherited and affects male dogs as young as 9-11 months of age. It is located in the anterior stroma of the cornea and consists of an infiltration of lipid (fat). This form of dystrophy often progresses to decreased vision by 4 years of age and is not treatable.