New hope for keratoconus and corneal ectasia: PRK and simultaneous corneal cross-linking
Keratoconus and corneal ectasia following laser refractive surgery were until now conditions for which no specific treatment was available. These patients develop progressive corneal bulging/protrusion due to an inherent weakness of the corneal collagen. In keratoconus, the corneal distortion happens spontaneously, while in post laser ectasia, the distortion happens after a surgical procedure first weakens the cornea.
Collagen basically consists of many adjacent fibrils. The stiffness and strength of collagen is in turn related to the number of links between adjacent fibrils. These cross-links increase spontaneously as we age, and are also seen more in patients with diabetes. This fact correlates with the rarity of keratoconus in older patients and the fact that keratoconus tends to stabilise with age.
In the last few years a way to medically strengthen the cornea by increasing the number of collagen cross-links has become available. This method was introduced by Prof Theo Seiler of the Institut fr Refraktive und Ophthalmo-Chirurgie in Switzerland. After first instilling vitamin B2 (riboflavin) into the eye, ultraviolet light is shone onto the cornea for 30 minutes. The ultraviolet light causes the riboflavin to release free radicals which then induce cross-links between collagen fibrils. The original limitation of this technique was that while it strengthened the cornea, the cornea maintained its original, distorted shape. Therefore vision remained blurred although progression of the condition was halted.
There are 2 main ways to reshape the ectatic cornea and simultaneously cross-link it.
- The first uses intra-corneal rings, much like those previously used to correct myopia. However, these rings have the same inherent limitations as when used in myopic patients ie. lack of predictability, slightly higher risk of corneal complications and the development of intracorneal deposits in the long term.
- The development of topography guided photorefractive keratectomy (PRK) has opened up a whole new frontier for correction of these problems. Corneal topography essentially refers to mapping the height of the cornea at various positions, much like mapping hills and valleys on dry land. By incorporating topography measurements into the laser pattern, it is now possible to smoothen most irregularities on the cornea, the only limitation being the corneal stromal thickness. Hitherto, further laser treatment for a laser induced problem like ectasia seemed counter intuitive and anathema. However, when combined with a strengthening treatment like ultraviolet collagen cross-linking, this is possibly the most precise method available for treating corneal ectasia. The laser essentially flattens the protruding portions of the cornea, and steepens the flatter parts.
At Jerry Tan Eye Surgery, 25 keratoconus and corneal ectasia cases have undergone this procedure with good results. Essentially, patients have topography guided PRK with mitomycin, followed immediately by 30 minutes' instillation of riboflavin, and then 30 minutes of ultraviolet light exposure.
Mr D underwent right and left LASIK in 2004 for low myopia of -3.00 right and -3.75 left. Corneal topographies were normal in both eyes. Post-operatively unaided vision was 6/4.5 bilaterally. He was seeing well until he returned to our practice in November 2010 when he noticed blurring of vision in his left eye which had been getting worse for the last few months. Unaided vision was 6/6 right and 6/24 left. Refraction in the left eye now showed -2.50D of astigmatism, and corneal topographies showed irregular steepening of the left infero-temporal cornea consistent with left corneal ectasia.
Mr D underwent left photorefractive keratectomy (PRK) with mitomycin C and ultraviolet light cross-linking 2 days later. As the epithelium healed, vision improved substantially. One month later, he was seeing 6/6 in the left eye without glasses again and his cornea was 4 times stronger than it had been. The following figures show his corneal shape before and after the corneal procedure.Summary
- Corneal transplantation can be avoided in many patients with corneal ectatic diseases now that PRK and cross-linking is available.
- PRK combined with ultraviolet light/riboflavin collagen cross-linking is an effective way to treat keratoconus and corneal ectasia.
- PRK with cross-linking is safe and the effects are long lasting.
- The main limitation is corneal thickness and degree of ectasia. If the cornea is thin, PRK may not be possible and only cross linking may be done. If the cornea is thinner still, even cross-linking alone may not be safe.
- Therefore, patients with these conditions should be seen early and treated early, before the cornea becomes too thin.