The cornea is the clear portion of the eye located right out front. It is like a sandwich in that on the outside is epithelium just like covers our skin but without hair and normally without pigment. In the dog the epithelial layer is 8-12 cell layers thick. In the cat, the epithelium is 5-to-7 cell layers thick. The epithelium is water-tight so that neither tears outside the cornea nor fluid from within the eye can get past the epithelium. Lining the cornea is one cell layer of endothelium. The endothelium is similar to the cells that line blood vessels. The endothelial cells are leaky. They allow fluid (aqueous humor) from inside the front of the eye (anterior chamber) to leak into the stroma and then the endothelial cells pump the fluid back into the front of the eye via a cellular pump. Between the epithelium and the endothelium is the thickest layer of the cornea, the stroma.
Corneal dystrophy is a degenerative change of the cornea in which cholesterol is deposited between the epithelium and the stroma of the cornea. Although the deposits are usually right in the center of the cornea, deposits can occur elsewhere depending on the cause of the degeneration.
This dog has a localized cholesterol infiltrate of the cornea. The white arrow shows the cholesterol.
Cholesterol is a type of fat or lipid produced by the body. This material is present throughout the body and is responsible for "hardening of the arteries" or atherosclerosis in humans. When present in the cornea, it appears as a "ground glass" appearance and is usually symmetrical. Many breeds develop corneal dystrophy. These breeds include: Afghan, Airedale terrier, Australian shepherd, Basenji, Bearded collie, Bichon frise, Boston terrier, Boxer, Briard, Cavalier King Charles spaniel, Cocker spaniel, Curly coated retriever, Dachshund, Dalmatian, German shepherd, Golden retriever, Irish wolfhound, Labrador retriever, Miniature pinscher, Nova Scotia duck tolling retriever, Samoyed, Shetland sheepdog, Siberian husky and Vizsla. Other breeds and individuals within breeds can develop corneal dystrophy. Dogs with thyroid problems (hypothyroidism) are more likely to have corneal dystrophy, as well as dogs on high fat diets are more predisposed. In addition "dry eye" or KCS could result in dystrophic change in the cornea.
It is unlikely for corneal dystrophy to result in complete vision loss. It is possible that a small blind spot can develop resulting in slight vision disturbances.
Corneal dystrophy is non-painful in most breeds. In the Shetland sheepdog, changes do occur in the cornea and these pets can become quite painful. Rather than having a single lesion or area of cholesterol infiltrate, shetland sheepdog's develop multiple "coin lesions". In other breeds, if the cholesterol becomes dense enough, it can causes irritation of the cornea. In these patients, blood vessels enter the cornea and pain eventually results. In addition, the patient can develop rather marked vision loss due to the density of the lipids.
For most patients with corneal dystrophy, nothing need be done except determine if an underlying cause is present such as hypothyroidism or high cholesterol in the blood. If the patient is painful, medication containing steroids will be tried initially to try to suppress the inflammation. If this fails, then surgery needs to be performed. The surgery entails surgically removing layers of the cornea until the cholesterol has been completely removed and subsequent use of steroids to prevent return of the lipid.
Corneal degeneration refers to any degenerative change in the cornea which is usually post inflammatory in nature. Pigmentation, blood vessels in the cornea and calcium are different components of corneal degeneration. Of these, calcium is the most troublesome.
Calcium has been deposited in the cornea of this dog. The white arrow indicates the calcium with a corneal ulcer within.
Calcium is the mineral which makes bones strong. Calcium is in solution in all tissues of the body. It is present in the fluid within the eye (aqueous) and is constantly moving through the corneal tissue from the inside of the cornea (endothelium) to the surface of the cornea (epithelium). Under certain circumstances, the calcium "precipitates out" of solution either just beneath the epithelium or within the stroma. In these circumstances, the epithelium and stroma of the cornea can become whitish. Sometimes the calcium deposits will coalesce and break through the surface and little chips of the deposit can slough off. Alternatively, the calcium may become quite dense immediately beneath the epithelium. If the calcium becomes thick enough, the surface epithelium can no longer remain attached and will "peel off." The sloughing of the calcium or the peeling off of the epithelium will result in a corneal ulcer which is quite painful.
If an ulcer is present, the ulcer needs to be treated with topically applied antibiotics to prevent infection while the ulcer heals. Additionally, atropine will be administered topically to dilate the pupil and relax the muscles within the eye which go into spasm when an ulcer is present. Oral Rimadyl® may be given for pain relief to dogs with corneal ulcers. Finally, to help dissolve the calcium, a "chelating agent", ethylenediaminetetraacetic acid (EDTA) will be applied topically several times a day. If the EDTA does not completely remove the calcium, debridement with a diamond burr or a surgical procedure call a superficial keratectomy may be required.
Endotheliopathy literally means: pathology or disease of the endothelial cells. As stated above, the corneal endothelium is a layer of cells which line the inside surface of the cornea. These cells are similar to those which line blood vessels. Unlike epithelial cells that replicate throughout the life of the patient, the endothelial cells have limited regenerative capabilities depending on the species. In most dogs and rabbits, the endothelial cells will replicate replacing damaged or dead cells. In cats, occasional dogs and humans, the endothelial cells do not reproduce. Also, the epithelial cells are "tight" and do not allow fluid to leak between cells whereas the endothelium is "leaky". The endothelial cells allow the leakage of fluid (aqueous humor) from the front of the eye (anterior chamber) into the corneal stroma and then the endothelial cells normally pump the fluid out of the cornea back into the anterior chamber. The cornea is normally clear despite being bathed in tears on the outer surface and bathed with aqueous humor on the inner surface. This clarity is maintained by the function of this endothelial cell layer. If the endothelial cell numbers are reduced or if they cannot perform their function, the aqueous humor will build up and the cornea will become hazy. The build up of fluid is called corneal edema. In severely affected dogs, the cornea contains enough fluid that bullae or vesicles (medical words for "blisters") form. These are similar to tiny blisters on the skin. The condition in people is called Fuch's endothelial dystrophy, named for Dr. Fuch who was the first to describe the condition in medical literature. Fuch's dystrophy is more common in elderly women than men. Edema can also occur from inflammation within the eye (uveitis) or from high pressure within the eye (glaucoma) which interfere with the endothelium performing its normal function. The fluid build up progresses at varying rates in different pets. The edema will eventually lead to blindness. This is a slowly progressive disease, for which there is no cure.
Diffuse haziness to the left cornea of this patient is the symptom of endotheliopathy.
The right eye of the same patient is normal.
Corneal endothelial dystrophy is an uncommon condition, but is seen most frequently in the Standard poodle, Chow chow, Dachshund, Boston Terrier and Chihuahua. Age of onset is generally 7 to 9 years of age. Additionally, a similar condition has been reported in the Manx cat. We have seen this condition in individual dogs of various breeds. These are usually older patients. The cause of the loss of endothelial cells is not known although the breed distribution suggests an inherited predisposition, along with normal loss of the endothelial cells due to age.
As mentioned above, in severely affected individuals, blisters or bullae form within the cornea. These will break resulting in a corneal ulcer which is painful. If a corneal ulcer is present, it will be treated with topically applied antibiotics and atropine to prevent infection and relieve pain. Additionally, application of topical hyperosmotic eye ointment containing 5% NaCl, may help reduce bullae formation, but the corneal edema does not clear. Corneal transplantation is the treatment of choice for Fuch's endothelial dystrophy in humans. Corneal transplants have been tried in dogs, but the success rate is less than desirable until recently. The reason that success rates have been low includes the fact that the dog's cornea vascularizes more readily that the human cornea which promotes rejection (vascularization and scarring) of the graft. Newer medications such as cyclosporine hold promise for increases in the success of corneal transplants in our pets.
Finally, for those pets who do not have a corneal transplant, the fluid build-up will worsen and they will continually develop corneal ulcers. As an alternative to the corneal transplant, a surgical treatment known as thermokeratoplasty is used to make small lesions on the surface of the cornea which in turn causes blood vessels to enter the stroma. These blood vessels help draw out the aqueous fluid, which causes the edema, before bullae or vesicles develop. Once this procedure is performed, a corneal transplant CANNOT BE PERFORMED because the presence of the blood vessels will guarantee failure of a graft.
Have any questions on this subject? Contact
Dennis Hacker via E-mail:
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