Texas Equine Veterinary Association

(Curr Eye Res. 2004). Riboflavin and UVA are also antimicrobial and have demonstrated antimicrobial effects in vivo and in vitro (Exp Ophthalmol. 2010, Cornea. 2008). Because infectious keratitis in horse eyes often leads to keratomalacia from collagen degrading enzymes, the application of CXL to stop melting, stiffen the cornea, reduce enzyme activity and sterilize the cornea seems like an exciting tool to bring to the equine opthalmology tool bag. A typical protocol involves slowly bathing the cornea (two drops every two minutes for 30 min) with riboflavin and then applying a focused UVA light. The length of time for the light exposure depends on the intensity, but most total energy protocols are 6 J/cm2. A hand-held meter can confirm the energy intensity and help determine the length of treatment. There are limitations to the use of this procedure. The subsequent production of ROS can be damaging to the stroma and anterior occular structures, especially when the cornea is thin. A typical equine cornea is 800 to 1500 um thick.The depth of the CXL effect in the equine cornea ranges from 149 to 173 um (Vet Ophthalmol. 2016). Descemet's membrane is ~21 um thick. Because of the potential for UV damage to the lens or further damage to a thin membrane, CXL should be used with caution on descemetoceles or ulcers >75% depth. For the last year, we have been using CXL on a variety of ocular cases including eight cases of keratomalacia (including two descemetoceles). Of the eight, we identified pathogens on six, including four with fungal hyphae on cytology. Within 24 hours, all eight cases had stopped melting and all corneas subsequently healed with continued topical treatment using antimicrobials and serum. All horses tolerated the treatment well. Subjectively, the level of scarring in CXL treated horse is no different from horses managed medically and better than cases where we debrided the cornea under general anesthesia and applied a conjuctival graft surgically. A normal finding post-CXL treatment is a yellow plaque of necrotic cornea that appears over the ulcer bed roughly four to seven days post treatment. In some cases it resembled a fungal plaque. Attempts to treat a deep stromal abscess with CXL did not appear to be successful. The lack of effect was likely attributed to the shallow penetration of the CXL therapy. Because the depth of effect is only 173 um, it makes sense that CXL would not penetrate to descemet's memberane in an intact cornea. The protocol we have been using at BVEH is different from the published literature on the application of this threatment in horses. We are using shorter treatment times with the UVA at a higher mW to achieve the same amount of energy ~6 J applied to the cornea after bathing with riboflavin. The shorter treatment times makes standing treatment easier. Corneal collagen cross- linking is a new technology that is showing promise to treat melting ulcers in horses with limited risk.

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