A normal healthy cornea has 5 layers and the normal thickness of the cornea is 500-600 microns.
The cornea is transparent and acts like a convex lens through which light passes into the eye.
The cornea contributes to almost 80% of the total refractive power of the eye.
Cornea can become opaque due to
Infections – Bactrial, fungal, viral keratitis or corneal ulcersInjuries – Crackers, chemicals, Road traffic accidents etc.
Lack of proper post-operative care – After any cataract/ glaucoma / retinal surgeries
Malnutrition – Especially Vit A deficiency in childrenCongenital or Hereditary reasons- Corneal dystrophies.
Age related – Corneal degenerations.
This loss of the vision is referred to as "corneal blindness". Corneal blindness can affect children and adults and can affect both eyes making them blind.
A corneal transplant will be successful only if the decrease or loss of vision is purely due to a corneal problem. All the other structures of the eye like the Lens, Retina and the Optic Nerve should be normal.
To visualize these structures through a total corneal opacity is difficult and hence we need to do a B scan Ultrasonography of the eye to ascertain that the structures within are normal.
Corneal opacities could involve all the five layers when it is called a Leucomatous Corneal Opacity or it could involve only the superficial layers when it is called a Nebular / Macular grade of corneal opacity.
If all the layers of the cornea are affected the entire cornea is replaced and this is known as Penetrating Keratoplasty.
If only the superficial layers of the cornea are affected, all the layers need not be replaced and this is known as Lamellar Keratoplasty.
Present day Corneal Transplants are quite different from what they used to be in the past. Almost every layer of the cornea can be replaced separately. There are various Terminologies for these procedures
DALK ( Deep Anterior Lamellar Keratoplasty ) where only the stroma is transplanted and deeper layers ie the corneal endothelium and Descemets Membrane are left intact.
DSAEK (Descemet's Stripping Automated Endothelial Keratoplasty) where only the innermost layer of cornea that is the endothelium is transplanted and this layer is taken from the donor cornea with the help of an automated keratome.
DMEK (Descemet's Membrane Endothelial Keratoplasty). This is similar to DSAEK but does not involve the use of a keratome.
DSAEK and DMEK involve peeling the diseased endothelial layer from the back of the cornea and leaving the healthy remainder intact (approximately 95%). Then, healthy corneal tissue (called a donor disc) is placed inside the eye through a small incision and positioned with an air bubble in the place of the diseased layer.
Both DSAEK and DMEK do not involve sutures and visual recovery is very fast.
The Donor Corneal Graft is taken only after death of a person. The Donor Corneal graft is evaluated for its viability by means of a Specular Microscope which does an Endothelial Cell Count. Normal Endothelial Cell count is 5,000 cells per square millimetre which reduces with age .
Once the Donor Corneal button is evaluated and deemed fit for transplantation it is preserved in a storage medium. These corneas can be preserved for 72 hours. There are storage media for long term preservation too.
Procedure Of Corneal Transplant
A circular portion of the opaque cornea is removed from the center of the recipient cornea. A matching circular area is removed from the center of a healthy, clear donor cornea and fixed into position with sutures. These sutures are of 10/0 Nylon and are very fine. The sutures could be interrupted or continuous and the knots are buried within the cornea so that they do not cause any irritation to the patient.
Success Rates Of Corneal Transplants and Rejections
The human cornea does not have any blood supply, hence the risk of rejection is low. With the advent of better instrumentation and diagnostic tools for evaluation of Donor Corneas the success rates of Keratoplasties are quite high. Rejection, if it does occur can be suppressed by timely medication. In general, the chances of success are greater than 80%.
At Dr Kodkany’s Eye Centre, all these corneal transplant procedures both penetrating ( full thickness ) and Lamellar ( partial thickness ) grafts are being done.
The permission to do these procedures has been given by Government of Karnataka under the HOTA ( Human Organ Transplant Act ).