SAEVA Congress 2018 Proceedings | 12-15 February 2018 | ATKV Goudini Spa
keratopathy including breed related endothelial dystrophy [ Band keratopathy in horses ], iris to cornea persistent pupillary membranes , trauma , uveitis , glaucoma and age related endothelial degeneration . The traditional treatment for bullous keratopathy is frequent applications of 5 % NaCl drops , this however does not lead to significant decrease in stromal oedema , corneal opacification or even the possibility of recurrent corneal bullae and erosions . Other more invasive treatments include 360 degree conjunctival grafts and thermokeratoplasty . Both of these treatments reduce corneal transparency even more . CXL decreases corneal oedema and increases visual acuity in humans . A case report by Pot et al describes 4 dogs suffering from persistent corneal erosions due to bullous keratopathy treated with CXL . The corneal erosions healed within 7 days in all 4 dogs after treatment , and remained so for the follow up period that was 17 months in 1 patient .
As mentioned , the riboflavin acts as a photosensitiser when exposed to UV – A light . Riboflavin absorbs the UV – A producing reactive oxygen species [ ROS ]. These free radicals introduce the new crosslinks between collagen fibres increasing the biomechanical stability of the cornea but also the free radicals produced directly damage and destroy microorganisms and lead to apoptosis of cells in the irradiated area . Thus this is also a form of treatment and sterilisation of the infection in the stroma .
There are however certain risks to the eye from the exposure to UV – A namely damage to corneal cells including endothelial cells as well as intraocular structures . Riboflavin limits the risks of direct damage , as it limits radiant transmission to deeper ocular structures by absorbing UV – A light . 4 In human , pig and rabbit corneas damage occurs to a depth of 300ųm with surface irradiation of 3mW / cm 2 . It is therefore recommended that CXL is only done in corneas thicker than 400ųm as this will prevent damage to the corneal endothelial cells and intraocular structures . The equine cornea is about 812-835 µ m thick .
Method
In horses the CXL procedure can be performed as a standing procedure under sedation . The ulcer is cleaned and the surrounding 2-3 mm of epithelial cells are removed with a scalpel or diamond polisher [ Algerbrush ]. This is known as the “ epithelium-off ” CXL procedure . 0.1 % Ribovlavin solution is applied , 1 drop every 2 minutes for 30 minutes . Riboflavin is a hydrophilic molecule which cannot easily pass the tight junctions of the intact epithelial barrier . Penetration of the Riboflavin is then confirmed by visualising the fluorescence of Riboflavin in the anterior chamber with slitlamp biomicroscopy using a cobalt blue light . The cornea is then rinsed and the cornea is irradiated . Various different protocols have been described varying from 3 mW / cm 2 for 30 minutes to 30 mW / cm 2 for 3 minutes . The current state of the art PESCHKE unit uses 45 mW / cm 2 for 2 minutes .
“ Epithelium – on ” techniques are in use and the absorbtion of riboflavin can be improved whilst maintaining the epithelium intact and this can be done by addition of EDTA , reducing the dextran but adding sodium chloride and benzylkonium chloride . In addition to changing chemical composition enhanced riboflavin absorption can be achieved by iontophoresis delivery . Riboflavin is a small negatively charged
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