Corneal Cross Linking (CXL) utilises principles from dentistry where light can be used to strengthen polymers used for fillings and tooth restoration. In the treatment of keratoconus or Pellucid Marginal Degeneration (PMD) the aim is to strengthen the corneal collagen by exposing it to UV light. This is achieved by soaking the corneal collagen with a green dye in the form of Riboflavin. Riboflavin specifically absorbs UV light releasing a reactive oxygen particle. This particle binds together the side chains of the collagen leading to an immediate and generally permanent increase in the structural strength of the cornea. This increase is approximately 3 times that of the untreated cornea enabling it to withstand the deformation forces in the eye that lead to the cone shape distortion seen in keratoconus.
In mild to moderate keratoconus, CXL can stop disease progression in virtually all eyes. It is therefore preferable to treat eyes before the disease produces marked difficulties with vision. When to treat remains a matter of judgement but can be assisted by the use of diagnostic devices such as the Pentacam which can document progressive steepening of the cornea and therefore of the disease. As keratoconus frequently runs in families, siblings and children of keratoconus sufferers can be screened for the disease early and monitored for progression. In the future, there exists the potential to intervene early in the disease and prevent many of its consequences.
In order to deliver sufficient riboflavin into the cornea the epithelium must be bypassed since riboflavin is too large a molecule to passively pass through it. Classically this means removing a large central area of epithelium, about 9mm round, under topical anaesthetic drops and then applying the riboflavin for 20-30 minutes. Because epithelial removal results in more discomfort and slower recovery, attempts are being made to deliver the riboflavin with the epithelium intact. Thus far this approach is controversial as it may not provide full strengthening of the cornea. Currently Adelaide Eye & Laser Centre employs an epithelium off approach to guarantee the full effect but this may change in the future.
Once there is sufficient riboflavin in the cornea, 10 minutes of UV light is delivered to the cornea. This requires the eye to be open for the duration of the treatment and an eye speculum is used for this purpose. Sensitive cells at the edge of the cornea are protected and after completion of the treatment drops are instilled and a bandage contact lens is inserted to protect the regenerating epithelium.
In some eyes that have mild keratoconus and sufficient corneal thickness it is possible to perform a therapeutic laser treatment to treat the irregular astigmatism induced by the condition. This is designed to improve the best corrected vision of the eye and may even eliminate the need for spectacles or contact lenses. This treatment is performed using an excimer laser and based on the pre-operative Pentacam and customized topography maps which are used to direct the laser pulses onto the cornea. Cross linking is then performed in the routine manner immediately after.
It takes 4-5 days for the epithelium to regenerate and a further 2-4 weeks before the vision in the eye approaches pre-operative levels. Thereafter, stabilisation and in many cases improvement is seen. Drops are required for 6-8 weeks and appointments with our clinical optometrists are necessary at one, three, six and twelve months.
It is normal to see a reaction within the corneal collagen over the first few months characterised by a hazy appearance to the cornea of a mild degree. This is not normally noted by the patient but slight loss of contrast to lights may be noted for up to three months.
It is usually necessary to be away from work for 7-10 days. Once the epithelium has healed, normal activities can commence only being limited by the rate of visual recovery. Contact lenses can be worn again after two weeks but it may be four weeks before this can be done comfortably.
Fortunately complications are rare with this procedure. Infection has been reported but since the UV light sterilises the cornea it is very unlikely. If there is insufficient shielding of the eye by the riboflavin, damage to the corneal endothelium may arise. With current protocols this is very rare. More pronounced haze may be seen in a small number of eyes and this can affect visual contrast under low light conditions for a longer period than anticipated. It is rare for this to persist. Whilst it generally takes 4-5 days to heal, it is possible for the epithelium to take up to 10 days to recover.