CA-CXL expands candidates to eyes with thin corneas

By Cheryl Guttman Krader; Reviewed by Soosan Jacob, MD

Chennai, India-Contact lens-assisted corneal crosslinking (CA-CXL) is a novel technique that allows eyes with thin corneas to undergo epi-off CXL, according to Soosan Jacob, MD.

Outcomes so far from a series of 90 eyes support the efficacy and safety of the simple and efficient procedure as an intervention for progressive keratectasia.

As reported in a published paper [Jacob S, et al. J Refract Surg. 2014;30:366-372] analyzing data from a consecutive cohort of 14 eyes, the average depth of the stromal demarcation line was 252.9 µm (range 208 to 260 µm), while topographic evaluation showed absence of keratometric steepening during a mean follow-up of 6.1 months.

In addition, there was no change in average endothelial cell density, the corneas remained clear, and there were no other complications.

“CA-CXL allows treatment of many eyes that would otherwise be excluded from CXL because they do not meet the minimum pachymetry requirement of 400 µm,” said Dr. Jacob, director and head, Dr. Agarwal’s Refractive and Cornea Foundation, Dr. Agarwal’s Eye Hospital, Chennai, India. “We also believe this approach has benefits compared with alternative techniques for applying CXL in thinner corneas, and it is further enhanced by use of the accelerated CXL protocol.”

However, surgeons should consider that any CXL technique may not be effective in extremely thin corneas where there may not be sufficient stroma that can be crosslinked to prevent ectatic progression. Therefore, deep anterior lamellar keratoplasty is preferred over CXL in extremely thin or very steep corneas, Dr. Jacob noted.

In CA-CXL, a riboflavin-soaked ultraviolet (UV) barrier-free soft contact lens is placed over the riboflavin-soaked denuded cornea to protect against endothelial damage.

“The idea for CA-CXL is based on the Beer-Lambert law, which states that each unit layer of a solution absorbs an equal fraction of light passing through it,” Dr. Jacob said.

Surgeons can identify eyes that would be candidates for the procedure by subtracting roughly 50 µm from the preoperative measured corneal thickness.

Corneal thickness is re-measured after epithelial removal using either ultrasound pachymetry or anterior segment OCT. If below 400 µm at the thinnest point, both the denuded cornea and a UV barrier free soft contact lens are soaked with iso-osmolar 0.1% riboflavin for 30 minutes.

Then, the contact lens is applied to the eye and the corneal thickness measured to confirm it is at least 400 µm prior to beginning UVA irradiation.

“If the pachymetry is still less than 400 µm, the thickness can be increased by instilling 2 drops of distilled water under the contact lens,” Dr. Jacob said. “This technique will minimally swell the cornea without causing the disadvantages that accompany large amounts of corneal swelling.”

Dr. Jacob and colleagues perform the procedure using a daily disposable hydrophilic contact lens made of Hilafilcon B (Soflens, Bausch + Lomb) and of negligible power. It has a center thickness of 90 µm and a diameter of 14 mm that provides good surface coverage. Bench testing demonstrated there was 60% to 70% UVA transmission through the riboflavin-soaked device. After riboflavin soaking, eyes are treated with the accelerated CXL protocol that exposes the cornea to 5.4 J/cm2 UVA energy by delivering 10 mW/cm2 over 9 minutes.

“The reduced procedure time of the accelerated technique is a benefit for patients and surgeons, and it is associated with reduced intraoperative dehydration, which is significant in these already thin corneas,” Dr. Jacob said. “The use of HPMC-based riboflavin solutions instead of riboflavin in dextran may also be beneficial to prevent intra-operative dehydration.”

Other options implemented for enabling CXL in thin corneas include hypoosmolar CXL, an epi-on approach, and SMILE lenticule-assisted CXL. However, CA-CXL has advantages over all of those methods, she noted.

CA-CXL avoids increased spacing of collagen fibrils secondary to artificial hydration, Dr. Jacob explained.

The technique also allows the greater efficacy of an epi-off versus epi-on technique, and it is much simpler and widely accessible than the recently described SMILE lenticule-assisted CXL method, she noted. 

Soosan Jacob, MD

e: dr_soosanj@hotmail.com

This article is adapted from Dr. Jacob’s presentation at the 2015 meeting of the American Society of Cataract and Refractive Surgery. Dr. Jacob has no relevant financial interests to disclose.