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EYE ON TECHNOLOGY
A BETTER CHOICE Contact lens-assisted collagen cross-linking aims to overcome limitations of traditional cross-linking. Soosan Jacob reports
C: 1.5 months’ postoperative slit lamp image of cornea following CACXL
Figure A: Preoperative ASOCT pachymetric map showing thinnest pachymetry of 421 microns
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raditional corneal collagen cross-linking (CXL) is limited to corneas with stromal thickness of greater than 400 microns in order to shield the endothelium from damaging UV rays. In epithelium off crosslinking, 400 microns of stromal thickness above the endothelium should be available after epithelial removal. Thin corneas with pachymetry less than 400 microns after epithelial removal are therefore a challenge to cross-link. However, there are many patients with keratoconus who have thinning that goes beyond this 400 micron limit for safe cross-linking. Within this sub-group of thin corneas, any amount of cross-linking may not provide sufficient strengthening to prevent progression in patients having advanced disease presenting with severe ectasia and thinning. A deep anterior lamellar keratoplasty (DALK) is a better choice in this group of patients. Within the same sub-group, however, there are also many patients who are not yet thin enough to indicate a DALK, yet thinner than the 400 micron cut-off. The treatment options for this group of patients is limited.
CONTACT LENS-ASSISTED COLLAGEN CROSS-LINKING (CACXL) This technique (which I started in 2013) may be utilised in such patients. CACXL acts by utilising the Beer Lambert law EUROTIMES | OCTOBER 2014
B: Postoperative ASOCT pachymetric map of same patient
nine minutes. At the end of treatment, the contact lens is removed, riboflavin is washed off and a fresh soft contact lens is applied until complete epithelial healing.
SOLUTION AND PROTOCOL which states that each unit layer of a solution absorbs an equal fraction of light passing through it. A UV barrier-free riboflavin-soaked soft contact lens is used to increase functional corneal thickness (stroma with contact lens). The contact lens used should be thin and should not have an in-built UV-barrier to avoid negating the effect. This may be checked in the product literature or by checking the UV irradiance that passes through the contact lens using a digital UV meter. We use the Bausch + Lomb daily disposable soft contact lens made of Hilafilcon B (Soflens™) for this purpose. The minimum pachymetry is confirmed after epithelial removal and the point of minimum pachymetry is marked. The contact lens is then soaked in 0.1 per cent riboflavin solution for the same half hour that the de-epithelialised cornea is soaked. The soaked lens is then placed on the surface of the cornea and the pachymetry is remeasured to confirm that thickness has gone above 400 microns with the lens on. The absolute thickness of the soaked lens is 90 microns. An additional thin film of riboflavin under the contact lens adds slightly more to the thickness. In our study, we found an average additional thickness of 107.9 ±9.4 microns attained by this technique. Once pachymetry is confirmed to be above 400 microns, UV-A application is done either following the classical Dresden protocol of 3mW/cm2 or as accelerated CXL. In case of accelerated CXL, our preference is to use an irradiance of 10 mW/cm2 for
Riboflavin in Dextran T500 is a dehydrating agent and can lead to increased thinning of the cornea. Hence, it may be ideal to decrease the time of exposure to dextran in such patients. This may be done by utilising accelerated crosslinking or by using riboflavin 0.1 per cent in HPMC as a soaking solution.
OTHER TECHNIQUES Hypotonic CXL, epithelium on and epithelial island techniques are other techniques that have been described to perform CXL in this group of patients. Though these are excellent techniques, limitations include intra and inter individual variations in the level of swelling that may be obtained as well as limited penetration of riboflavin through intact epithelium.
COMBINATION TREATMENTS The current treatment strategy should ideally focus on combination treatments
D: Slit view of the cornea showing well-defined demarcation line