A significant number of patients have a minimum stromal thickness of less than 400 μm after epithelial removal. This is a contraindication for conventional corneal collagen crosslinking (CXL), as it can lead to increased ultraviolet A (UVA) light irradiance at the level of the endothelium. At Dr. Agarwal’s Eye Hospital and Eye Research Centre, we are now using a new technique that I call contact lens-assisted CXL, or CACXL, for crosslinking thin corneas.
CACXL increases the functional thickness of the cornea by utilizing a soft contact lens soaked in riboflavin solution. When placed on the cornea, this contact lens attenuates the UV irradiance to safe amounts at the level of the endothelium. CACXL follows the Beer-Lambert principle—that is, each unit layer of a solution absorbs an equal fraction of light passing through it.
It is important that the contact lens used for CACXL does not have a built-in UV filter. We use the UV–barrier-free SofLens daily disposable contact lens (Bausch + Lomb) made of hilafilcon B. The soft lens design follows the shape of the cornea and adds a thickness of 90 μm to the functional corneal thickness. This helps in extending the benefit of CXL to thinner corneas.
An important issue to keep in mind while performing CACXL is the intraoperative corneal dehydration that can occur due to the use of riboflavin in dextran; this can be avoided by the use of riboflavin in hydroxypropyl methylcellulose (Vibex Rapid; Avedro) and is especially important in thin corneas. Another method we have used to tackle this problem is to perform accelerated crosslinking (CL-UVR Rapid; Appasamy Associates). This approach can reduce further intraoperative dehydration by decreasing the duration of contact with the dextran-containing solution.
To sum up, it is important to take extra precautions for crosslinking in patients with thin corneas. An existing effective technique is hypotonic CXL. However, with all techniques, it is important to always verify that the patient’s functional corneal thickness exceeds 400 μm. With CACXL, this is done by rechecking the pachymetry after placing the contact lens. It is also important to remember that very thin corneas may continue to show progression despite crosslinking, and, in such corneas, it may be preferable to perform deep anterior lamellar keratoplasty instead. Hence, the urge to crosslink every cornea should be resisted, and the decision to perform crosslinking with any technique should be made prudently.