In this study, a human ex vivo corneal
Fusarium keratitis model was established. The results show that there was a significant difference in the volume of invading fungus within the infected corneal tissue between the groups, with the infected corneas treated with PACK-CXL showing a significant decrease in the fungal volume compared to the other experimental group. This is a good indication that the PACK-CXL procedure is effective in suppressing the progression of fungal infection. The favorable outcomes obtained in this study are contradicted by some published studies, which report that the PACK-CXL procedure is not effective in managing fungal keratitis.
37,46,47 Nevertheless, these observations are supported by numerous in vivo studies that attempted to examine the influence of combined riboflavin/UV-A in the management of infectious fungal keratitis. It is suggested that this combination has positive antimicrobial effects, which assist in inhibiting the growth of pathogens and managing the infection.
30,48–50 The beneficial antimicrobial effect of the PACK-CXL procedure in suppressing infection by
Fusarium is likely to be attributable to several mechanisms. Inactivation of ribonucleic acids of organisms may occur by the combined UV-A/riboflavin–induced cross-linking that may have a cytotoxic effect on the pathogens.
23,51 In addition, it has been shown that the PACK-CXL technique brings about cross-linking in the corneal collagen fibers, thus increasing the strength and simultaneously reducing its penetrability by fungal hyphae. Furthermore, the cross-linked collagen is more resistant to enzymatic digestion by microbial pathogens, which in turn reduces corneal melting.
52,53 In this study, the direct antifungal activity of the UV-A/riboflavin combination is perhaps the greater contributory factor. It is also possible that cross-linking might entrap the fungal hyphae within the collagen matrix, thereby reducing the growth rate still further. The proposal that the direct antifungal effect of the UV-A/riboflavin treatment is the major component in reducing corneal infection is supported by the fact that the IX group showed a significant reduction in the number of
Fusarium spores present after the PACK-CXL treatment compared with the I group. This is indicative of the riboflavin/UV-A treatment having an inhibitory effect on
Fusarium sporulation. On the other hand, debriding the epithelial layer post inoculation and prior to the PACK-CXL treatment may have played a role in reducing the infection by removing some of the
Fusarium hyphae growing within the corneal epithelial tissue. Nevertheless, the reduction in spore numbers in the culture medium post PACK-CXL treatment lends some support to the hypothesis that the riboflavin/UV-A combination has an antifungal action.