Corneal Cross Linking for Keratoconus
What is Keratoconus?
Keratoconus is a non-inflammatory degenerative disease of the cornea characterised by progressive thinning and stretching of the cornea into a cone shape. It occurs in 1 out of 2000 patients between the ages of 10-25 years. 90% of cases are bilateral(occurring in both eyes).
Progression of Keratoconus
If the cornea becomes too thin, the corneal apex ulcerates and eventually perforates. This leads to pain, lachrymation(excessive tearing) and blepharospasm. Alteration in the arrangement of corneal proteins, causes micro scars which distort images. The astigmatism(rugby ball shaped cornea) becomes so irregular that spectacles don’t ensure good sight. Hard contact lenses are then used in an attempt to push the cone back.
When a scar has formed on the cornea, spectacles and contact lenses cease to help. This is when a corneal transplant/graft is indicated. The damaged cornea is removed and replaced by donor cornea.
A corneal graft has its limitations. Firstly, donor corneas are difficult to obtain locally. Patients can be on a waiting list for years. Overseas corneas are available sooner at a higher cost. It is an expensive procedure that may not be covered fully by the patient’s medical aid. Postoperatively, there may be pain and discomfort that can last a while. The patient will still require spectacles or hard contact lenses to get acceptable vision.
Corneal cross linking is not a cure for keratoconus. It is a procedure invented by ophthalmologist Dr Theo Siler, to preserve the corneas in their current state, hence halting the progression of the keratoconus. If the keratoconus is detected early enough and treated by cross linking the corneas, a corneal transplant can be prevented.
How does Corneal Cross Linking Work?
It is a day procedure(no overnight stay in hospital) performed in the doctors rooms under local anaesthetic. A combination of UVA light and Vitamin B2(Riboflavin) eye drops enables the formation of new links between the corneal collagen fibres(cross-links). This increases the biochemical rigidity of the cornea, reduces the astigmatism and thereby arrests the progression of the keratoconus.
The Chemistry Behind Corneal Cross Linking
Irradiation of the riboflavin molecules by UVA causes them to lose their internal chemical balance, producing oxygen free radicals. At this point, the riboflavin molecule is unstable and becomes stable only when it is linked to two collagen fibrils. This creates a crossed bridge between the collagen fibrils leading to the general strengthening of the cornea.
The treatment procedure
The area around the eye is cleaned with an antiseptic solution. Anaesthetic eye drops is applied to numb the eye. The surgeon then removes the corneal epithelium with a blunt instrument. No pain is felt during the procedure. Riboflavin is instilled into the eye every 1-2 minutes for approximately 30minutes. This is followed by irradiation with 370nm UVA rays for 30 minutes by a special device. Lastly, antibiotic drops are instilled and a therapeutic contact lens is inserted. Post operative medication(antibiotic drops, anti inflammatory drops, anaesthetic drops,lubricating drops, oral analgesics and sleeping pills) are prescribed.
The patient is seen 4-5 days after the procedure to remove the contact lens. They are followed up approximately 1 week, 1 month and 3 months later. It is important to remember that this is not a refractive procedure. The patient will still require visual correction. Soft contact lenses can be resumed 2 months later and hard contact lenses 3 months later.
What to expect after the Procedure
There is a fair amount of pain and discomfort due to the removal of the corneal epithelium. After removal of the bandage contact lens 4-5 days later, the cornea should have re-epithelialised and the pain and discomfort is almost negligible. There is also a foreign body sensation, photophobia(sensitivity to light) and excessive tearing. Wearing sunglasses even indoors for a week helps relieve this. Visual recovery is slow and it can take 3 – 4 weeks, sometimes longer, for the vision to come back to how it was before the procedure. For this reason, the second eye is only cross linked approximately 1 month later .
Transepithelial/Epi On Cross Linking
This method of cross linking is non invasive and as the name suggests, the corneal epithelium is left on or intact. This is indicated when the cornea is very thin and the surgeon wants to prevent making the cornea thinner by removing the epithelium. A thinner constituency of Riboflavin is used to ensure that it penetrates the intact cornea.
No postoperative pain or deterioration of vision is experienced with this procedure and there is no need for so many postop medicines. However, not all keratoconic patients are a candidate for this procedure. The patient needs to be assessed by an ophthalmologist to determine which procedure will work best for them.