What is it?
The normal cornea is the clear watch-glass like window overlying the coloured part of the eye, the iris.
The cornea allows light to enter the eye and its curved surface helps bring the light to a focus inside the eye. Any alteration in the transparency or smoothness of the cornea affects vision.
There are 5 layers in the cornea:
- The surface epithelium
- Bowman’s membrane
- The corneal stroma
- Descemet’s membrane
- The endothelium
There are four main types of corneal disease:
- Secondary surface changes
- Change in shape
- Change in transparency
Secondary surface changes
Several conditions lead to damage of the surface of the cornea. These include:
- Dry eye
- Abnormal eyelid position and function
Change in corneal shape
The most common abnormality of corneal shape is keratoconus, also known as ‘conical cornea’.
Changes in corneal transparency
There are many diseases in which the cornea becomes less transparent. The most common are:
- Keratoconus or conical cornea
- Fuchs’ corneal dystrophy
- Pseudophakic bullous keratopathy
- Corneal dystrophies
Keratoconus is a disease in which the surface of the cornea becomes progressively steeper and irregular, and the corneal stroma thinner and scarred.
The cause of keratoconus remains obscure. It usually begins in the teenage years and is more common in people with multiple allergies.
In most cases vision can be corrected with glasses or contact lenses, but in severe disease corneal transplantation is required.
Fuchs’ corneal dystrophy
This common disease is essentially a premature aging of the corneal endothelium. The main function of the endothelium is control of the amount of water in the cornea.
Failure of the endothelium leads to swelling and loss of transparency of the cornea. In severe cases the cornea becomes so swollen that blisters or ‘bullae’ form.
Pseudophakic bullous keratopathy (PPBK)
PPBK occurs when the endothelium of the cornea is damaged; usually during cataract surgery. As in Fuchs’ corneal dystrophy, endothelial loss leads to swelling and loss of corneal transparency.
Corneal dystrophies are conditions in which parts of the cornea lose transparency. There are multiple causes and patterns. The most common diseases are:
- Reis-Bucklers dystrophy
- Granular corneal dystrophy
Many infectious agents can cause corneal disease. The most important by far is the Herpes Simplex virus.
Herpes eye disease has two common forms:
- Epithelial ulceration, known as a dendritic ulcer
- Sterile inflammation of the stroma, known as disciform keratitis
How is it diagnosed?
Corneal disease is diagnosed by observing and analysing the cornea with specialised equipment.
Many techniques are used to examine the cornea. These include:
- Slit lamp biomicroscopy
- Orbscan® topography
- Specular microscopy
Slit lamp biomicroscopy
The slit lamp is a high powered microscope that is used to examine and photograph the living tissues of the eye.
Most corneal disease is first diagnosed by observation at the slit lamp. Detection and diagnosis of disease may be aided by the use of dyes such as fluorescein which stain the tears and highlight changes in the corneal surface.
Individual endothelial cells imaged by the specular microscope.
The Orbscan® is an instrument that provides a surface ‘map’ of the cornea, in much the same way a topographic map represents a geographical landscape.
Orbscan analyses both the overall shape of the cornea and local variations.
Measurement of the thickness of the cornea is known as pachymetry. Corneal thickness is assessed by either ultrasound or an optical probe.
This photographic technique images the endothelium, the single layer of cells on the inner surface of the cornea. Details of the size, shape and density of cells are obtained.
Specular microscopy plays a key role in decision making in some corneal diseases. Dr McKellar purchased the first specular microscope in private practice in New Zealand.
Many corneal diseases can be be managed medically.
Goals of medical therapy
Depending on the nature of disease therapy aims to treat the:
- Optical qualities of the cornea
- Appearance of the eye
- Patient comfort
For in many cases more than one problem will be present and treatments will have more than one effect.
Common medical therapies
The mainstays of medical treatment of the cornea are:
- Removing irritants
- Lubrication and rinsing
- Treatment of blepharitis
- Eradication or control of infection
- Management of inflammation
- Corneal smoothing with contact lenses
Removal of irritants
All eyedrops, in particular those which contain preservative, can be toxic to the cornea. Treatment of corneal disease may simply require stopping well intentioned therapy.
The corneal epithelium is affected by drying and toxic agents. Artificial tears and serum eye drops can provide additional lubrication and rinsing. For more information see dry eye medical treatment and dry eye surgical treatment.
Treatment of blepharitis
Blepharitis is a frequent cause of damage to the corneal epithelium. More information can be found at blepharitis medical treatment and blepharitis surgical treatment.
Eradication or control of infection
The commonest cause of corneal infection is the Herpes Simplex virus. Recurrent epithelial disease is the most common manifestation. Herpes is impossible to eradicate but in most cases infection can be managed with acyclovir (Zovirex®) ointment. In severe cases oral acyclovir tablets may be used.
Management of inflammation
Inflammation of the cornea may require the use of anti-inflammatory drugs, of which steroid eye drops are the most effective.
Corneal smoothing with contact lenses
Contact lenses can be used to cover an unhealthy cornea, essentially replacing the diseased irregular tissue surface with a smooth ‘plastic’ surface.
Some corneal diseases can only be treated surgically.
Types of corneal surgery
There are three principal types of corneal surgery:
- Phototherapeutic keratectomy
- Penetrating keratoplasty, in which the whole cornea is transplanted
- Lamellar keratoplasty, where selected layers of the cornea are transplanted
The excimer laser can be used to smooth and reshape a diseased cornea.
The commonest indications are:
- Recurrent erosion sydrome
- Superficial scarring
Penetrating or full thickness corneal transplantation. Courtesy Alcon.
Penetrating surgery is performed when the whole thickness of the cornea is affected by disease. The most common condition treated in this fashion is keratoconus, or ‘conical cornea’.
Individual layers of the cornea can be transplanted. The commonest surgeries are:
- Anterior lamellar keratoplasty – replacement of the corneal stroma
- Posterior lamellar keratoplasty – replacement of the corneal endothelium
Anterior lamellar keratoplasty
Anterior lamellar surgery can be performed when disease is isolated in the corneal stroma.
Advantages of this technique include:
- Only the diseased part of the cornea is replaced
- The likelihood of graft rejection is lower
There are however several disadvantages:
- The surgery is much more technically difficult
- Intraoperative complications are common and often require the surgeon to proceed with a full thickness transplant
- The visual results are probably not as good
Posterior lamellar transplantation
Many corneal diseases affect just the endothelial cell layer. At present it is not possible to replace just this single cell layer. We can however transplant the endothelium attached to Descemet’s membrane together with a thin layer of posterior stroma. The most common operation of this type is known as Descemet’s Stripping Endothelial Keratoplasy or DSEK.
In DSAEK (Descemets Stripping Automated Endothelial Keratoplasty) diseased endothelium is stripped from the back surface of the patient’s cornea and a donor cornea disc is placed against this ‘raw’ surface. A bubble of air is used to hold the grafted tissue in place for the first 24 hours until healing begins. No sutures are required. Patients need to lie flat on their back during the first 24 hours to allow the air bubble to keep the new graft in place.
The advantages of this technique are:
- The eye is structurally sound
- Recovery is quicker
- There is probably less likelihood of rejection
- The smooth front surface of the eye remains unaltered
The disadvantages are:
- Surgery is more technically difficult and expensive
- Slightly higher failure rates
- Early dislocations requiring re-bubbling
- The need to posture the eye toward the ceiling for 24 hours
All corneal transplantation uses human tissue from individuals who have donated their organs at death.
Risks of corneal transplantation
Corneal transplantation is a major undertaking. Risks include:
- Infection at the time of surgery
- Dislocation of the donor button. This is a unique complication of posterior lamellar surgery and usually occurs within the first 48 hours. Most dislocated corneas can be refloated.
- Graft failure. Occasionally a transplanted cornea fails to survive the transplant process.
- Rejection. The host eye can reject the transplanted cornea. The chances of this depend on many factors including the underlying eye problem, the type of surgery and the number of grafts previously performed.
- Weakness of the eye. The eye is never as robust following penetrating surgery. A blow could rupture the eye and lead to blindness. Contact sports are not permitted after penetrating keratoplasty.
- Astigmatism. The surface of a transplanted cornea is always somewhat irregular. Many patients require futher surgery or a rigid contact lens to obtain good vision.
- Scar tissue. This can occur in the host (patient) cornea, the donor cornea and between the two corneas.
- Cataract formation
- Recurrence of the disease being treated in the transplanted cornea
- Transmission of disease from donor tissue. This is unbelievebly rare. All donors are screened for diseases such as hepatitis and HIV/AIDS. It is estimated that at present rates of tranplantation in New Zealand one person will develop an infection from donor tissue every 200 years.
- Rare complications such as retinal swelling or detachment, glaucoma and chronic inflammation.
- Very rare complications include loss of vision.
Having Corneal Surgery
The following is intended to provide an overview for patients having corneal transplant surgery under Dr McKellar’s care at Christchurch Eye Surgery.
The surgery itself
All transplantation is performed in a hospital theatre. Most surgery is performed under local anaesthesia. One night’s stay is usually required. The operation is not very painful but the eye is often watery and sensitive to light for several weeks. It is reasonable to take a week off work.
Early post operative care
There are often some niggly problems in the first week or two. Leaks, loose sutures and pressure problems are the most common.
Most patients will require eye drops for life.
Long term care
Transplanted corneas need to be watched very carefully. Expect to see Dr. McKellar 10 times in the first year and yearly from then on.
Over the first few months one or more sutures may need removal. There may be pressure rises that can lead to glaucoma. This is usually managed with additional eye drops.
The most common major complication is rejection. In most cases rejection can be treated successfully with intense eye drops and intravenous drugs. Rare complications include infection and failure of the graft to survive.
The cornea is very slow to heal so the stitches remain in place for approximately 18 months. Suture removal is a simple process that is performed at the rooms.
The vision will usually be quite poor while the sutures are in place. It is common to only see the first two lines on the eye chart in the first 18 months.
The risk of corneal grafting depends primarily on why the surgery is required. Patients with keratoconus and corneal scarring have an excellent prognosis. Grafting is less successful in patients with Herpes eye disease.
Final visual outcome
It takes two years to achieve the final outcome. Most patients require glasses in order to see well. Often the very best vision is achieved only with a contact lens.
Some patients only need the back layer of the cornea replaced. This layer is known is Descemets membrane and the procedure is termed Descemets Stripping Automated Endothelial Keratoplasty (DSAEK). In this procedure a disc of donor tissue is slipped into the eye and ‘sticks’ to the back of the patients own cornea. A bubble of air is left in the eye to help the tissue adhere. Patients need to lie on their back with their eye facing the ceiling for 24 hours to keep the bubble in the correct position.
FAQs and Links
Can I personally thank the family of the person who donated their cornea to me?
No, all donation is an anonymous gift.
How do I become a cornea donor?
You need to make it very clear to your family that you wish to be a donor. Registering your wishes on your driver’s licience and in your will are important, but ultimately it is your family who give consent for organ donation.