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The cornea is the clear window of the eye. It is a remarkable structure that is both exceptionally strong as well as being transparent. It normally contains no blood vessels and obtains nutrition from fluid circulating in the eye, known as aqueous. The cornea also relies on oxygen from the atmosphere. The cornea is made up of 5 layers with different functions. Diseases of the different layers of the cornea can cause discomfort and pain, irritation, and can affect vision.
Keratoconus [ top ]
Keratoconus is a disease where the corneal structure is weak and cannot maintain a normal spherical shape. The result is corneal sagging, which can affect vision. It is sometimes associated with allergic eye disease and eye rubbing is known to worsen keratoconus, and so excessive eye rubbing is discouraged.
Keratoconus related vision problems can initially be treated with spectacles and later with contact lenses. In those who progress further, corneal transplantation may be necessary to restore vision. Whilst most people with keratoconus never require surgery, about 10-15% will require a corneal transplant to improve vision. Keratoconus is the most common condition for which corneal transplants are performed.
New Treatments for Keratoconus - Collagen Cross Linking [ top ]
Keratoconus is a corneal ectasia occurring due to a structural weakness of the cornea. Collagen cross linking with Ultra Violet A light and riboflavin is thought to mirror the cross linking known to occur naturally with age in the human body. The cross linking process results in increased intrafibrillar and interfibrillar chemical bonds being formed in the cornea and is thought to explain why keratoconus progression slows with age. A trial being conducted through Sydney Eye Hospital by John Males which is designed to investigate a new treatment to hopefully slow and possibly reverse the progression of corneal ectasia, and therefore slow or reverse visual deterioration. This treatment involves applying riboflavin (Vitamin B2) drops to the eye and applying ultraviolet - A light over a 30 minute period. This treatment has been shown to produce stiffening of the cornea in studies performed overseas. It is the first treatment for keratoconus that has the potential to reverse the disease process.
It is hoped this treatment will reduce the need for corneal transplantation and allow those with keratoconus to remain in their spectacles and contact lenses for longer. However, this treatment on its own is unlikely to give those with moderate or advanced keratoconus good unaided vision.
Early results from the trial are very promising. No adverse outcomes have been reported to date and patients have experienced reductions in maximal keratometric steepness. Patients are actively being recruited into the trial at this time.
New Treatments for Keratoconus - INTACS Corneal Intrastromal Rings [ top ]
INTACS intrastromal corneal rings represent an innovation in the treatment of keratoconus for patients who otherwise require a corneal transplant. INTACS are solid semicircular rings implanted into the cornea to alter the corneal shape and flatten the cornea which is steeper than normal in keratoconus.
It is a technique which is performed as a day only procedure under topical anaesthesia. The technique improves both unaided and best corrected visual acuity in greater than 90% suitable candidates. It also allows many patients to use an easier form of vision correction eg. moving from hard contact lenses to soft contact lenses. It is reversible, and does not affect the central cornea.
The combination of collagen cross linking and INTACS represent a potentially exciting combination. The cross linking will strengthen the cornea and prevent or slow disease progression, while INTACS helps to improve visual acuity.
Corneal Transplantation [ top ]
Some diseases of the cornea require corneal transplantation to restore sight. Corneal transplantation is the most common form of organ transplantation performed. Like all organ transplants, corneal transplantation occurs after a tragic event for the donor results in the hope for better vision in another. Corneal transplantation involves removing most of the diseased cornea with a circular trephine, and replacing this with the donated cornea. The surgery is performed under both local and general anaesthesia depending on the exact circumstances. The corneal graft is stitched into place with sixteen interrupted stitches or with one or two continuous sutures. Following the surgery the use of antibiotic and anti rejection drops is necessary. The anti rejection (steroid) drops may need to be continued indefinitely. Fortunately most patients do not require powerful tablet anti - rejection medications as with other organ transplants.
Because of the complexity and nature of the surgery, it normally takes months to achieve the best possible vision after a corneal transplant. Spectacles, contact lenses and occasionally laser surgery are required to obtain the best vision after corneal transplant surgery. The stitches are eventually removed, and this can be performed in the rooms with a drop of anaesthetic.
Traditionally, most corneal transplants performed were transplanted 100% of the cornea. For some conditions, such as keratoconus, it is sometimes possible to perform a transplant of 95% of the cornea, leaving the back two layers (the endothelium and Descemet's membrane) intact. This procedure, known as Deep Anterior Lamellar Keratoplasty (DALK) has the advantages of quicker recovery of vision and a reduced chance of corneal graft rejection. This surgery is not appropriate for all patients and conditions and the options need to be discussed on an individual basis.