CORNEAL BLINDNESS

 Matt Oliva MD 

Corneas preserved using glycerin or gamma irradiation have an important role to play in the treatment of blinding corneal disease in the developing world, Matt Oliva MD told a session of the World Cornea Congress VII in San Diego, USA.

“There is a huge amount of cornea-related blindness in the developing world, with a 2010 World Health Organization estimate putting the number at 4.9million cases of bilateral cornea blindness that could potentially have vision restored with a cornea transplant. Factor in unilateral corneal blindness, and the number goes up to 10million people who could benefit,” noted Dr Oliva, Associate Medical Director for SightLife Eye Bank.

There are many factors related to the current worldwide shortage of corneal transplants being performed, including lack of surgeon training, availability of supplies in the developing world, and transportation issues in getting corneas to surgeons and patients. However, the biggest issue continues to be the lack of available tissue for corneal surgeons practising in the developed world, he emphasised.

“In the developing world setting, we often need to have corneas we can pull off the shelf for an emergent case in order to save an eye. Long-term storage medium offers an opportunity to stabilise the situation until a fresh cornea becomes available. Although with long-term storage the corneal endothelium is not viable, placing corneas into glycerin or irradiating them also offers us the ability to use corneas that may otherwise go to waste due to expiration. This also can allow high-quality fresh tissue to be used for optical cases in the developing world, where a high proportion of corneal transplants are often emergent therapeutic grafts or tectonic grafts,” said Dr Oliva, an Associate Clinical Professor in the Division of International Ophthalmology, at Casey Eye Institute and Oregon Health Sciences University, USA.

Glycerin, or glycerol, is a colourless, odourless substance with antimicrobial properties. It was first used to store corneas as far back as 1955. Corneas stored in this medium can be stored for up to five years and require no refrigeration. One of the benefits of glycerin processing is that it removes all cells, reducing antigenicity, and potentially leading to less rejection episodes with lamellar grafts. Another benefit is that these corneas can be pre-cut to be carriers for keratoprostheses and work well for glaucoma tube shunt coverage. They also produce good results when used for lamellar keratoplasty, he noted.

“One downside of using glycerin-preserved corneas is the preparation process. It takes about 20 minutes to remove the cornea from the glycerol, and rehydrate it in a saline and antibiotic solution. The cornea becomes leathery and hazy, and it is a little bit tricky to work with. You need to thin it and debride the non-viable epithelium and endothelial cells before it can be used,” Dr Oliva explained.

Gamma irradiation for preservation of corneas using cobalt or electron beam irradiation offers many of the same benefits. This technique produces an acellular sterile cornea, with good maintenance of the stromal architecture. These corneas work much like an optisol-preserved cornea, with no rehydration required, he said, although there is not viable endothelium, as with glycerin-preserved tissue.

INCREASE CORNEA SUPPLY

In the developing world the biggest applications for long-term preserved corneas are for therapeutic and tectonic grafts. They can also be used for limbal dermoids, for therapeutic lamellar keratoplasty, or for optical deep anterior lamellar keratoplasty (DALK). In the developed world these corneas have an important role in glaucoma tube shunt surgery.

“I’m really excited about the use of long-term preserved corneas for optical-quality DALK. There are two retrospective clinical studies comparing DALK performed with glycerin-preserved corneas and optisol-preserved corneas. These showed no difference at one year in terms of thickness, spherical equivalence or best corrected visual acuity (BCVA) between the groups,” he said.

He stressed that it could take a little while for the surgeon to get used to using the glycerin-preserved corneas. They do need to be rehydrated, and they often need to be manually debulked. Since the epithelium can heal very slowly there is a low threshold for tarsorrhaphy in cases.

“If we are going to combat corneal blindness worldwide we need to increase the cornea supply. Long-term preserved corneas allow use of corneas that would otherwise be discarded. There are many benefits to having them on the shelf. While there are subtle differences between glycerin- and gamma irradiation-preserved corneas, both are really useful for increasing transplantation rates,” Dr Oliva concluded.

moliva@cureblindness.org