The cornea is the clear outer window of the eye and is made up of three layers:

• Epithelium - the outermost layer (bathed by tears)
   
• Stroma - the middle and largest layer. This region of the cornea is made up of precisely arranged layers of collagen fibers. This undisturbed arrangement is critical to the clarity of the cornea and for allowing light to pass through the tissue unimpinged.
   
• Endothelium - the innermost layer adjacent to the fluid in the front chamber of the eye (aqueous humor). In eyes of most species, there is a finite number of corneal endothelial cells, and if damaged or if they die, they cannot replace themselves. In the case of lost endothelial cells, the remaining functioning cells enlarge and 'move' toward each other to fill up the resulting space or defect. 

If either the epithelium or endothelium fail to keep fluid (tears or aqueous humor, respectively) from entering the stroma, the cornea can swell and become edematous (corneal edema).  

Under normal circumstances, the endothelial cells biochemically "pump" fluid that enters the stroma back into the anterior chamber, keeping the arrangement of collagen fibers undisturbed.  As endothelial cells degenerate, as seen in Corneal Endothelial Degeneration, the function and efficiency of the remaining healthy cells decreases, resulting in more and more fluid accumulation in the corneal stroma.

Signs of Corneal Endothelial Degeneration

• Clouding of the cornea (focally or diffuse)
• Blue to greyish color change to the cornea
• Bullae or "blisters" in the cornea - fluid that makes its way into the cornea can accumulate between the stroma and the epithelium, causing the formation of small bullae or "blisters."  This condition is known as bullous keratopathy
  
• Corneal ulcers - rupture or breakage of the bullae lead to the formation of corneal ulcers.  These ulcerations can have a tendency to become recurrent or be slow to heal due to the persistent edema and decreased healing abilities seen in older patients
  

Causes of Corneal Endothelial Degeneration

• Aging - corneal edema due to endothelial cell degeneration is commonly seen in older animals
• Genetics - there are inherited forms of endothelial cell change seen in certain breeds including the Boston Terrier and Airedale
• Trauma - a penetrating injury to the cornea or anterior luxation of the lens can damage the corneal endothelium 

Corneal edema due to Corneal Endothelial Degeneration must be differentiated from other conditions that can precipitate corneal edema including:

• Glaucoma
• Uveitis

Treatment

While there is no "cure" for this condition, medication to "dehydrate" the corneal tissue may be suggested. A hyperosmotic, sodium chloride ointment or solution may be prescribed. In some cases this type of medication can facilitate partial clearing of the cornea and can help in treating or preventing corneal bullae and ulcerations.

In severe cases of corneal edema, surgical procedures may be recommended including procedures that facilitate scarring of the epithelium to the stroma, thus minimizing the potential for bullae to form. In severe cases, a corneal transplant may be a consideration in the right patient.

Prognosis

The presence of corneal edema generally does have not have a significant impact on an animal's vision. Because the cornea is no longer clear, however, the vision experienced is like looking through frosted glass, resulting in a distortion of the objects visualized. The greatest risk this condition poses is the potential for corneal ulcerations to develop.