A BETTER CHOICE

Traditional 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 cross-linking, 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 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 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

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 cross-linking 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 according to thickness. CACXL is started and proceeded with when pachymetry is confirmed to be above 400 microns after applying the soft contact lens. However, if thickness is found to be less than 400 microns, a few drops of distilled water is used to swell the cornea just enough to top up the deficient thickness. As the contact lens technique adds approximately 100 microns to thickness, the amount of additional swelling needed to take pachymetry above 400 microns is generally low and can be quickly attained by this means. Thus the principles of both CACXL as well as hypotonic CXL may be combined effectively without the need for large amounts of swelling or special hypotonic solutions. In any event, cross-linking should not be carried out without attaining a functional corneal thickness
of 400 microns.

Severe keratoconus

In very thin and steep corneas, DALK is preferable for the anatomic and visual rehabilitation that it provides. Such patients usually have a decreased best spectacle corrected visual acuity (BSCVA). However, they generally show very good visual potential on examination with a rigid gas permeable contact lens. Performing a DALK in such patients gives very good visual results because of the excellent flattening of the anterior chamber and a decrease in myopia that can be obtained by taking a same-sized graft.

Good astigmatism control may be obtained by intra-operative keratometry and if required by postoperative suture adjustment. A DALK also provides better anatomic rehabilitation in these extremely thin corneas. In contrast, the amount of visual rehabilitation that CXL would provide in these patients would not be sufficient and there would be poor improvement in both BSCVA and myopic astigmatism postoperatively. With CXL, the strengthening of extremely thinned out cornea is also likely to be insufficient and the disease may continue to progress. Such advanced corneas are also at greater risk for haze formation. Hence for the above-mentioned reasons, in advanced keratoconus with high keratometric values, I prefer to do a DALK.

* Dr Soosan Jacob is
a senior consultant ophthalmologist at Dr Agarwal's Eye Hospital, Chennai, India and can be reached at: dr_soosanj@hotmail.com