Corneal crosslinking (CXL) with a photoactivated chromophore for keratitis is a successful option for the treatment of infective keratitis of different microbial etiologies. It has been recently suggested as a first-line effective treatment in infectious keratitis not responding to topical therapy.1–3 Corneal crosslinking increases the tissue's resistance to enzymatic digestion and reduces the penetration of pathogens. Furthermore, it causes keratocyte apoptosis and direct damage to pathogens as a result of free radical production.4
A 44-year-old man who had hyperopic laser in situ keratomileusis (LASIK) in 2000 was referred to the emergency room in May 2015. He reported redness, pain, photophobia, and tearing in the right eye, which was eye injured while the patient was gardening a few days earlier. The disposable contact lenses he was wearing for residual hyperopia were immediately removed after the trauma. At admission, a corneal infiltrate, corneal edema, and an anterior chamber inflammatory reaction were reported, and topical therapy with tobramycin and levofloxacin eyedrops every 2 hours was started. Despite an initial clinical improvement, the symptoms worsened 3 weeks after the treatment was initiated.
The caregiver referred the patient to our eye clinic. He was healthy with an unremarkable medical history. At the time, he was taking topical terbinafine hydrochloride for chronic onychomycosis on the right foot.
Slitlamp examination showed a white ring infiltrate under the LASIK flap (Figure 1, a), which was causing pain, and an anterior chamber inflammatory reaction without vitreous or retinal involvement. Confocal microscopy examination (HRT3-RCM, Heidelberg Engineering GmbH) showed trophozoites and cysts of Acanthamoeba species (Figure 1, b) and mold (Figure 1, c), which was identified as Fusarium solani on culture of corneal scrapings. Pachymetry showed a thinnest flap-free corneal thickness of 427 μm and that the depth of the infection reached 293 μm under the flap. Eight days after culture, the antifungal susceptibility test confirmed the sensitivity to voriconazole. After confocal microscopy imaging, the current topical therapy was changed to polyhexamethylene biguanide 0.02%, hexamidine 0.1%, voriconazole 2%, and tobramycin eyedrops every hour.
The clinical situation worsened significantly in the following week, with the onset of hypopyon, a stromal deep infiltrate, and emerging corneal melting involving the entire LASIK flap. After Fusarium was identified, intravenous voriconazole (4 mg/kg daily) was added to reduce the dissemination of the infection.
A few days later, accelerated CXL using the photoactivated chromophore for keratitis was performed. Lifting of the LASIK flap was followed by a 15-minute application of isotonic riboflavin 0.1% solution and 9 minutes of ultraviolet-A (UVA) irradiation (365 nm, 9 mW/cm2) (UV-X, IROC Innocross AG). Over the following 3 days, rapid, increasing corneal melting occurred, with growth of the infiltrate, worsening of the hypopyon, and finally, corneal perforation.
Amniotic membrane transplantation was performed and after 7 days, the hypopyon disappeared; however, the cornea was almost perforated again. Reconstructive penetrating keratoplasty (PKP) was performed, and topical and systemic therapy was continued. After 6 months, the graft appeared to have no signs of infection. The patient's corrected distance visual acuity (CDVA) was light perception because of cataract with posterior synechiae and mild corneal opacity (Figure 1, d). In October 2016, the patient had cataract extraction, after which the CDVA was 20/40.
The efficacy of traditional CXL (Dresden protocol: riboflavin 0.1%, UVA 370 nm at 3 mW/cm2 for 30 minutes) in treating several microbial etiologies has been reported, although it has not shown it to be better than topical therapy in cases of fungal keratitis.3,5
Regarding the differences between the accelerated CXL and traditional CXL protocols, they are equally effective for ectatic disorders. Their efficacy in the treatment of corneal infections has not been shown. According to the Bunsen-Roscoe law of reciprocity, to maintain the same efficacy, when the duration of UVA light exposure is reduced, the intensity of the treatment should be increased.6
Although the traditional and accelerated CXL using the photoactivated chromophore for keratitis follow this law of reciprocity, they do not achieve the same results. In cases of superficial infectious keratitis, accelerated CXL using the photoactivated chromophore for keratitis might be more effective than the traditional approach.7 On the other hand, in cases of deep fungal keratitis, the efficacy of the traditional protocol is better than the accelerated one; however, it is associated with an increased risk for perforation.3,4
Ideally, accelerated CXL using the photoactivated chromophore for keratitis could be more effective if the infection does not involve corneal layers below 300 μm because the absorption rate of riboflavin 0.1% shows a logarithmic decay and most of the energy of the UVA radiation is absorbed within the first 100 μm. Therefore, when the cornea is not transparent, as in eyes with a deep fungal infection, the intensity of the UVA light is not be sufficient to treat a deep infection.8
We were unable to find previous studies of accelerated CXL using the photoactivated chromophore for keratitis to treat a coinfection of Acanthamoeba and Fusarium involving the LASIK flap; thus, we used this treatment to counteract the worsening evolution of unresponsive keratitis by compacting the corneal stroma. Although we expected a weak effect on the cystic form of the Acanthamoeba, we decided to treat it with prolonged topical eyedrops.9,10
In our case, significant corneal melting occurred and we could not determine whether accelerated CXL using the photoactivated chromophore for keratitis had any effect on the evolution of the infection. Several possible reasons could explain the failure of our approach. First, we treated nonresponding keratitis only after prolonged medical treatment, when the infection was too deep and corneal melting was minimal but present. Moreover, the topical treatment would have been more effective had the LASIK flap been removed at the first medical examination.11 Second, the photoactivated chromophore for could have enabled microbial penetration by further depleting the immunocompetent sites in the anterior part of the cornea that were already severely damaged by the deep infection.12 Third, the UV radiation was probably too powerful for a cornea that showed melting, an imminent perforation, and reduced thickness.
In conclusion, we believe that accelerated CXL using the photoactivated chromophore for keratitis remains a possible option to treat infectious keratitis not responding to topical therapy, even though our experience was disappointing. The delay in its application and the lack of standardized protocols for infectious keratitis could explain why our treatment failed. In the future, a specific procedure for infectious keratitis should be developed defined with inclusion and exclusion criteria to improve management of this condition. This procedure could help improve the prognosis of severe corneal conditions without the need for therapeutic keratoplasty.
None of the authors has a financial or proprietary interest in any material or method mentioned.
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