Corneal disease is the second leading cause of blindness. Worldwide, there are an estimated 4.9 million bilaterally corneal blind persons who could potentially have their sight restored through corneal transplantation. Aside from disease of the cornea, trauma is another common cause of corneal blindness. U.S. eye banks provide nearly 70,000 corneas for transplant each year. For people suffering from corneal blindness, the only cure is a cornea transplant. Cornea transplants have a 98% success rate.
The cornea is the clear, front part of the eye where a contact lens would sit. The cornea is responsible for 75% of the focusing power of the eye. Because there is no direct blood supply to the cornea, there are very few disease processes in a donor that would rule out cornea donation. The cornea has 5 layers, but two of the most important are the epithelium and the endothelium.
The epithelium forms the protective, outer covering of the cornea. Think of it like your skin, protecting from the elements. Like your skin cells, corneal epithelial cells are continually sloughed away and replaced. The biggest danger to a donated cornea is the time between death and preservation. The eye ceases to produce tears after death. It does not take long for the epithelium to dry out, slough away, and expose the inner layers to infection.
Slit-lamp micrograph of a healthy, donated cornea. Notice how it is translucent and smooth.
Slit-lamp micrograph of a cornea that has had a long death-to-preservation time. Note how a large area of the epithelium in the lower half has completely sloughed away. The folds you see are evidence of swelling. Finally, the white haze in the lower right corner is from the infiltration of white blood cells. This cornea cannot be transplanted.
The endothelium is a single-cell layer on the back of the cornea. These cells' only job is to pump water out of the cornea, thus keeping it clear. When these cells fail, water builds up in the cornea causing it to become opaque. These cells are critical to a successful transplant because they do not regenerate. What you are born with is what you get! While the average number of endothelium cells is around 5,000/mm2 at birth, you lose about 10% by puberty. As cells age and die, the others get bigger to fill in the gaps. A cornea must have at least 2,000 cells/mm2 to be transplanted. High cell counts will typically be matched with younger recipients.
These endothelial cells continue to be viable for several hours after death. Delays beyond 18 hours render the cornea non-transplantable in most cases.
Specular micrograph of an endothelial layer of a donor cornea. These living cells are critical to a successful transplant, as they do not regenerate and must last the lifetime of the recipient. Extended death-to-preservation times can put the endothelium in danger.