Cross linking related infections
Crosslinking (CXL) is an effective procedure where the cornea is radiated with UV light to promote a strengthening of the collagen fibrils to stop the progression of keratoconus. As with all medical procedures, there are several risks associated with this technique, in this article the focus is going to be on infection.
Crosslinking is considered relatively safe due to the fact the procedure is performed under sterile conditions and by itself has shown to inhibit bacterial replication by oxidative damage to the DNA and RNA of bacteria, performing a tougher environment for bacteria to proliferate, reduction of inflammatory cells, and several other beneficial properties. CXL has actually been used in several cases of microbial keratitis that does not respond to medical treatment.
The risk for infection is not known because it is very infrequent; however there are several case reports of infection after crosslinking. It is probable that the epi-off technique may be associated with a slightly higher risk of infection because of the rupture of the natural external barrier of the eye, as well as the common use of a bandage contact lens in the postoperative period.
There are several bacterial agents that have been studied  because of their frequent isolation in infectious keratitis, being Staphylococcus aureus, Streptococcus pneumonie and Pseudomona aeruginosa the most common. The process of infection has been studied in animal models, specifically the bacterial proteins secreted for infection, since one can treat an infection with antibiotics but for a certain period of time, the inflammation process may persist because of this proteins (this is where steroids take place in the treatment of infectious keratitis).
- Pseudomona aeruginosa: these infections are related to the use of contact lenses. The bacteria is a gram negative rod, found in the environment, usually in moist places. P. aeruginosa is capable of secreting at leas seven proteases: elastase A, elastase B, modified elastase, alkaline protease, protease IV, pseudomona aeruginosa small protease, and large exoprotease. These enzymes, especially elastase B, can produce severe corneal damage. elastase A can cleave a host transmembrane protein, desiganted the protease-activated receptor, which, when cleaved, can start the production of cytokines. On the other hand the pseudomona aeruginosa small protease, is able to cleave collagens and this suggest that this enzyme could be important in the destruction of the cornea, besides this protease cleaves with lysine and arginine and it can convert poly-L-arginine or poly-L-lisine to small peptides and free amino acids.
- Streptococcus pneumoniae: these infections are related to ocular trauma and eye surgery. The outer capsule of the Pneumococcus formed of polysaccharides is the most studied virulence factor. Normally it is thought that for pneumonia, otitis media, meningitis and septicemia, the bacteria needs its capsule for infection, but this may not be true for keratitis, as non capsular strains can cause infection in the cornea of animal models. Besides its capsule, the pneumococcus has pneumolysin, a protein that is a toxin that is related to cell lysis and cell wall lisys (by pore formation, and leukocyte migration). Pneumococcus has other surface proteins A and C which anchor to choline in the cell wall; neuroaminidase A which cleaves N-acetylneuraminic acid from the host cells and degrades host cell mucin. Four zinc metalloproteinases have been found as well.
- Staphylococcus aureus: S. aureus is the most common cause of bacterial keratitis. S. aureus has been shown to bind to human corneal cells in culture due to a fibronectin-binding protein on the bacterial surface, but this process may be aided by a collagen-binding adhesin on the bacterial surface. despite this binding ability and the coccus, and the presence of the bacteria in the eyelid skin, infections rarely occurs. Perhaps the most important factor and protecting barrier is the phospolipase A2 enzyme found in the tear film. S. aureus has the several toxins, such as the alpha-toxin, which is a lytic cytotoxin that causes extensive sloughing of the corneal epithelium, corneal edema and severe iritis. The alpha-toxin enters the cytoplasmic membrane and moves laterally until seven subunits form a circular structure forming a pore. A gamma-toxin has been isolated as well that has been related to keratitis and to endophthalmitis.
Pollhammer et al  reported a case of a 42 year old patient in which crosslinking epi-off was performed due to a keratoconus. Three days after the procedure the patient had decreased vision (20/400), diffuse conjunctival injection, corneal edema, and multiple paracentral infiltrations in the corneal stroma with a moderate anterior chamber inflammation without epithelial defect. A corneal scrape and culture were performed where Escherichia coli was found. Treatment with toprabymicin 14 mg/mL and cephazolin 50 mg/mL was started. After 10 weeks an avascularized stromal scar was left with a BCVA of 20/63
Rana et al reported 2 cases of bacterial keratitis with subsequent corneal perforation after crosslinking. The first case was a 19 year-old female with a past medical history of atopy with allergic eye disease, and a history of keratoconus referred with a painful red eye 3 days after transepitelial CXL. In the findings a large central cornea abscess was found with a 3 mm hypopion, corneal scrapes were taken and Staphylococcus aureus was isolated. Treatment was given initially with cefuroxime 5% and gentamycin 1.5 % at half hour intervals, as ciprofloxacin 750 mg PO. Seven days after CXL treatment a corneal perforation was found and cyanocrylate glue was needed. The infection could be controlled but left a large central vascularized scar, with a counting fingers visual acuity. The second case was an 18 year old male referred with a corneal abscess in his right eye 5 days after CXL performed for keratoconus. Corneal scrapes were taken and methicillin resistant Staphylococcus aureus was isolated. At 48 hrs of presentation the cornea perforated and cyanocrylate glue was needed as well. The infection could be controlled but vision is severely limited.
Perez-Santoja et al also reported a case of microbial keratitis after CXL. The case is a 29 year-old woman with no systemic disease, and a diagnosis of progressive keratoconus. The patient was scheduled for intracorneal ring segment implatantion in the left eye, which was performed without complications. Later on the patient was scheduled for CXL in the right eye, which was performed with the epi-off technique. In the first 2 follow-up visits, the patient had photophobia and blurred vision; in the biomicroscopy exam 4 well defined white nodules surrounded by small satellite lesions and hazy stromal infiltrate in the upper midperipheral cornea were found. Cultures were taken from the nodules and Staphylococcus epidermidis was isolated. Treatment was started with cefazolin 50 mg /mL and tobramycin 15 mg/mL, but then changed to ofloxacin 0.3% and tobramycin 15 mg/mL as the results of the antibiogram were released. After control of the infection topical fluorometholone 0.10% tid was added. Five months after treatment BSCVA is 20/22
Sharma et al reported a case of a Pseudomona keratitis after a CXL. This case presents a 19 year old woman with a 3 day history of pain, redness and decreased vision after one day after epi-off CXL for keratoconus. Of note, the author mentions that the bandage contact lens that was inserted after the procedure had fallen off 1 day prior to office visit due to excessive eye rubbing with unclear fingers. At presentation the CDVA was hand motion, with a biomicroscopy revealing severe conjunctival injection along with a corneal ulcer with a central 7 x 6 mm yellow infiltrate with overlying epithelial defect involving 90% of corneal depth. There were mild anterior chamber reaction and a 1.5 mm hypopyon. The cornea was scraped and Pseudomona aeruginosa was isolated. Treatment was started with fortified cefazolin 5% q1h, tobramycin sulfate 1.5% q1h, and homoatropine 2% qid. Following treatment, the infiltrates decreased in size and the hypopyon gradually disappeared, leaving a leucomatous corneal opacification. At 2 months the CDVA was 20/200.
Rama et al reported a case of a bilateral keratoconus in a 32 year-old man with no systemic disease. He underwent an epi-off CXL and was treated postoperatively with a bandage contact lens, topic ofloxacin tid, topic lurbiprofen qid and artificial tears tid. On the following day, the patient noticed a conjunctival discharge and eye redness, so he instilled the medications and rinse his eyelids with tap water. Later that day he was referred to an ophthalmologist who diagnosed acute catarrhal conjunctivitis and added levofloxacin drops qid. On the third day the ophthalmologist noticed a progressive corneal involvement with corneal opacification and initial descemetocele in the inferior sectors. Treatment was augmented to oflaxacin 5 times daily and levofloxacin drops q1h, flurbiprofen tid, and chloramphenicol plus betamethasone ointment qid; as well asceftriaxon 4 g IM qd, and predinsone 25 mg PO qd. Despite the therapy the patient got worst and on the fifth day the patient was programmed for conjunctival flap because of the descemetocele. The next day the patient presented with severe inflammation, corneal ectasia, opacificaction , subtotal de-epithelization and corneal melting. Cultures were taken and the smears were positive for Acanthamoeba. Despite a change in therapy on the 11th day the cornea perforated and a therapeutic penetrating keratoplasty was performed. The postoperative follow-up was uneventful reaching a 20/40 with pinhole.
Kymionis et al reported a case of a 21 year-old woman who underwent CXL for keratoconus, with no prior history of herpetic keratitis, autoimmune disease or systemic connective tissue disease. The UCVA was 20/25 and hand motions respectively, with an advanced corneal ectasia in the left eye. The patient was proposed to perform a CXL in the right eye and a PKP in the left eye. The CXL was performed with the epi-off technique. After the treatment, a bandage contact lens was placed and tobramycin-dexamethasona was given bid. On the fifth day postoperatively the patient presented with a geographic epithelial defect, stromal edema and 2+ cells in the anterior chamber. PCR analysis was positive for HSV DNA detection. Topical coritocsteroids were discontinued and oral aciclovir 400 mg qid, methylprednisolone 32 mg qd and topical cyclopentolate tid was started. After 2 days the geographic ulcer had reepithelized and topical steroids were added. Two months later, a mild central corneal opacity remained, but the UCVA recovered to 20/25
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