Expert opinion: Corneal grafting
The cornea can sometimes become damaged to the extent that a replacement by means of a corneal graft becomes the only way to restore vision.
The damage may be caused by trauma, infection or other disorders and probably the commonest disorder that may result in a corneal graft is keratoconus. This is a bulging (ectasia) of the cornea caused by loss of corneal thickness and hence corneal rigidity. The cornea thins and the intra-ocular pressure causes the cornea to steepen in curvature forming a cone. The irregular shape of this reduces the quality of vision and this may progress to such an extent that it is not correctable with spectacle or contact lenses or there is a risk that the cornea may perforate. This is the stage at which a corneal graft is usually carried out, although in many cases the disorder does not become this severe and only about 10-20% of people affected by keratoconus require a corneal graft.
Estimates of the prevalence of keratoconus vary considerably but it is thought to affect 0.15 to 0.6% of population worldwide, with the risk being equal across all ethnic groups. It appears more common in males than females and often has an early onset, during teenage years with an average age of onset of about 16 years. The disorder usually affects both eyes, though often not to the same extent. There appears to be regional variations in the prevalence of the disease and Bradford is often regarded has having a higher than normal incidence of keratoconus for reasons which are not clear.
Role of the Optometrist
Optometrists have always had an important role in detecting keratoconus as the early signs often as simple as a more rapid than normal change in spectacle prescription. Traditional management would be to monitor the disorder’s progression, correcting the vision with spectacles for as long as possible and contact lenses in later stages. Referral to a hospital eye department would follow if there was contact lens intolerance, poor vision, peripheral thinning of the cornea or a risk of corneal perforation at which point a corneal graft would be considered. The cornea is an unusual tissue in that it shows what is known as immunological privilege – it will tolerate foreign tissue without the immune system rejecting it. This means that it is much simpler to graft a cornea as there is much less need for the tissue matching needed for transplants elsewhere in the body. This means that cornea grafting has a success rate of about 90%.
However there is a chronic shortage of donor corneas. Attempts are being made to grow corneas in tissue culture but these are not yet near to clinical use, so in instance where a cornea needs to be replaced a graft from a donor cornea is the only option. Recent advances in the treatment of keratoconus use a technique known as collagen cross linking. This uses a vitamin (riboflavin) coupled with ultra-violet light to promote the formation of extra links between the molecules in the cornea, increasing the strength of the cornea and arresting the progression of the disorder, so the hope is that in the future the need for corneal grafting may be reduced. Optometrists have a central role to play in this treatment as it needs to be carried out early in the progression of the disorder so early detection and referral for treatment have become more common.
, Director, Eye Clinic reacting to BBC News article on The country that supplies eyes