1. What is Cornea?
Cornea is the transparent part of the eye that fits like watch glass over the coloured part of the eye called iris. Light passes through the clear transparent cornea to finally fall over retina.
Structure of Eye
2. How many layers are there in cornea?
There are six layers in cornea.
1. Epithelium: It is the outermost layer of about 6 layers of cells. It has strong regenerative capacity, if limbal stem cells are intact.
2. Bowman’s Membrane: It is a tough acellular layer, mainly composed of collagen.
3. Stroma: about 90% of thickness of cornea is comprised of stroma. Stroma consists of regularly arranged collagen fibers intermixed with keratocytes.
4. Dua’s layer: This layer has been recently discovered by Dr Dua. It is very thin (5 to 20 micron thick) layer
5. Descemet’s membrane: It is the thin acellular layer made of Type-IV collagen mainly. Descemets layer serves as the purpose of basement membrane of endothelium.
6. Endothelium: It is the innermost layer of cornea made of single layer of cells and is about 5 micron thick. Unlike corneal epithelium, the endothelial cells do not regenerate. Instead they stretch to compensate for dead cells which reduce the density of these cells. Endothelium is responsible for maintaining fluid and solute transport between stroma and aqueous. If diseased endothelium is not able to maintain fluid balance, stromal swelling occurs and cornea loses its transparency.
3. How does the cornea become hazy or opaque?
Clouding or irregularity of the cornea may be caused by many different kinds of problems. Some of the causes are:
Corneal Scar after Trauma
4. What happens when cornea becomes hazy or opaque?
When the cornea becomes hazy, similar to the frosting of glass, light is not able to pass through the eye resulting in poor vision.
Hazy cornea due to Corneal Decompensation
5. How is vision restored in cases of hazy cornea?
The only way to restore vision is to replace the cornea with donated healthy corneal tissue by a surgery called corneal transplant or keratoplasty. This consists of removal of a central disc of the abnormal cornea and replacing it with a similarly sized piece of normal cornea obtained from a donor eye.
6. Can the whole eye be transplanted ?
It is a misconception that whole eye can be transplanted. Cornea is the only part of the eye that can be transplanted.
7. What is rejection and why does it occur?
Rejection is the immunological destruction of cornea. Rejection is usually acute and many a times it can be reversed with intensive therapy. The term “graft rejection” refers to the specific immunologic response of the host to the donor cornea. Because it is a specific process, it should be distinguished from other causes of “graft failure” that are not immune mediated.
Acute Rejection after Corneal Transplant
8. What are the signs of rejection ?
Signs of rejection can be remembered as an acronym: R.S.V.P.
S: Sensitivity to light
V: Vision loss
9. What should a patient (after corneal transplant) do if he/she has these signs of rejection?
If the patient suddenly experiences increase in redness, pain, watering of the operated eye or a drop in vision in the same eye, he/ she should immediately consult nearest cornea surgeon, preferably on the same day. Early graft rejection and early graft infection both can present similarly, but the treatment for both are exactly opposite.
10. Can corneal transplant cure all kinds of blindness?
No. Corneal transplant can cure only “Corneal blindness”.
11. What is the DALK (Deep Anterior lamellar Keratoplasty)?
In case of penetrating keratoplasty (PK), the diseased cornea is removed with all its layers and replaced by a similar or slightly larger sized donor cornea also of ‘full’ thickness. However in certain diseases such as Keratoconus or in superficial corneal scars, the innermost layer of the cornea (endothelium) is intact and healthy and therefore need not be changed. In these cases, approximately 90% of the thickness is changed, i.e. the endothelium is left intact. Since it is the donor endothelium which is mainly responsible for the rejection, the chances of rejection of this ‘lamellar’ graft are reduced dramatically. However, this procedure requires a more surgical skill than the usual penetrating keratoplasty. It cannot be used in patients who have unhealthy endothelium. It therefore requires careful patient selection.
12. What is DSEK (Descemet Stripping Endothelial Keratoplasty)?
In some cases, only the innermost layer of cornea (endothelium) is diseased / non-functional. Endothelium is responsible for keeping the cornea clear. Endothelium can be compromised in post-cataract surgery cases or some inherited diseases like Fuch’s Dystrophy or ICE Syndrome. DSEK is partial thickness corneal transplants which replace primarily the endothelium (innermost portion of the cornea) rather than the full thickness of the cornea.