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 precise 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

Signs of Corneal Edema

• clouding of the cornea (focally or throughout)
• blue to grey color change of the cornea
• bullae or "blisters" on the corneal surface
• corneal ulcers - can develop due to rupture or breaking of the bullae or "blisters"

Causes

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

• Corneal epithelial defects - A corneal ulcer, especially a chronic or long-standing defect (refractory or indolent corneal ulcer), can lead to an accumulation of fluid in the cornea. Typically, when the corneal ulcer heals, the associated corneal edema will resolve as well.
   
• Corneal endothelial defects - endothelial cells can become damaged or die as a function of age, trauma, inflammation or degenerative conditions (Corneal Endothelial Degeneration).  These cells cannot be replaced nor can they repair themselves. Although the remaining functioning cells enlarge and 'move' toward each other to fill up the space or defect left by lost endothelial cells, their function may not be sufficient to keep fluid pumped out of the cornea.
   
• Glaucoma - The continual production of fluid (aqueous humor) in the normal eye creates pressure within the eye (intraocular pressure). This is maintained in steady state by the concurrent drainage of that fluid from the eye. This pressure gradient causes is a constant forward movement of the fluid into the cornea. Under normal circumstances, the endothelial cells "pump" the fluid back into the anterior chamber, keeping the arrangement of collagen fibers undisturbed. However, with increased pressure (glaucoma), the efficiency of the endothelial cells function decreases, resulting in more fluid entering the corneal stroma. The collagen fibers then swell and the cornea becomes edematous and cloudy in appearance.
   
• Uveitis - Inflammation in the eye causes increased levels of protein, inflammatory cells and other debris in the aqueous humor. This tainted fluid can be "toxic" to the endothelial cells, leading to a decrease in their function, resulting in corneal edema. Typically, once the inflammatory process is controlled, the corneal edema may dissipate.

Treatment

While there is no specific treatment or "cure" for this condition, it may be controlled with medications that "dehydrate" the corneal tissue. A hyperosmotic Sodium Chloride ointment or solution may be prescribed, particularly in patients that have had issues with corneal ulceration. This may help to prevent the recurrence of the ulcerations, and occasionally facilitates partial clearing of the cornea.  

If the corneal edema is secondary to a treatable underlying condition, it may resolve with treatment of the inciting cause.   

Prognosis

The presence of corneal edema generally does not affect the animal's vision. However, because the cornea is no longer clear, the vision experienced is similar to looking through frosted glass, causing a distortion of the objects visualized.  

The greatest risk that this condition poses is the potential for corneal ulceration.  

As fluid makes its way into the cornea it can accumulate and cause the formation of small bullae or "blisters." This is called bullous keratopathy.  If the blisters break or rupture, a corneal ulcer will result.