Corneal Cross Linking is a new method to treat Keratoconus or bulging eye disorder. CXL in children is now an option.
We know that in keratoconus the corneal collagen fibers are decreased in number and volume. This decreases the tensile strength of the cornea to hold back the internal pressure of the eye. We can increase the young’s modulus or the rigidity of the cornea with corneal cross linking. This intervention is more challenging in extremely thin corneas. We need to protect the inner cornea and the inside of the eye from the UV radiation we apply. It has been determined that 400 microns is a safe limit. If the cornea is thinner than that we need to strategize. Also in epi off Corneal Cross Linking we remove the top 5o microns when we discard the epithelium. Hence the cornea has to be more than 450 microns to start safely. Wait there is one more factor we need to be aware of. When we apply Photrexa with Dextran for 30 minutes it causes corneal to become compact or thinner.
Corneal Cross Linking in Thin Cornea
- Use Photrexa without Dextran This is a an FDA approved riboflavin product manufactured with the highest standards by Avedro. The protocol states that if the cornea after soaking with Photrexa with dextran for 30 minutes is thinner than 400 microns Photrexa should be used as follows: Put 2 drops of PHOTREXA every 5 to 10 seconds till 400 microns thickness of the cornea is measured. In thinner corneas some keratoconus experts may bypass the dextran version and use only the Photrexa to avoid compaction with dextran.
- Swell the cornea with sterile water : If the above strategy does not increase the corneal thickness to 400 soaking the cornea with distilled water can work. Water is dripped and a the eye is covered with a patch for 5 minutes. Intuitively you may think the water will push the corneal fibres away preventing effective corneal cross linking. Clinically it has been shown to work well in studies published in peer reviewed journal.
- Switch to epi on or transepithelial procedure It is common knowledge amongst keratoconus surgeons that epi off cxl is more effective than transepi corneal cross linking also know as epi on corneal cross linking. In thin conreas it may used as the 50 micron of epithelium adds a barrier to the UV light.
- Contact lens assisted C3R What about extremely thin corneas where even the epithelium protection is not enough a novel strategy has been devised. A 300 micron contact lens is soaked in riboflavin and than placed over the bare cornea. It helps swell up the cornea as well as prevents UVA radiation from penetrating deeper.