Corneal cross-linking may have application in infective keratitis

Issue: August 10, 2015

ByDhivya Ashok Kumar, MD, FRCS, FICO, FAICO

ByAmar Agarwal, MS, FRCS, FRCOphth

Corneal cross-linking has been popularly used to prevent progression of keratoconus and corneal ectasia. The procedure is based on using riboflavin as a photosensitizer, which generates reactive oxygen species when activated by ultraviolet A at 365 nm. It induces a photochemical reaction that forms covalent bonds or cross-links in the corneal stroma. In the last decade, it has been introduced in keratitis and infective corneal ulcers that are resistant to drugs. With the rising incidence of multidrug resistance, corneal infections are increasing and devastating consequences are faced in aggressive microbial infections. Corneal cross-linking has provided a significant contribution in controlling the pathogenesis of the infections.

History

In 1965, Tsugita and colleagues noted that riboflavin, when subjected to either visible or UV light, could inactivate the RNA of tobacco mosaic virus. Spoerl and colleagues proved by in vitro study that a cross-linked cornea is more resistant to collagenases enzymatic activity than the normal or diseased cornea. Although there have been studies showing the antibacterial activity of riboflavin with UV light in vitro, the pilot study on human eyes was performed by Makdoumi and colleagues. They performed a pilot study to investigate the photochemical interaction used in corneal cross-linking as the primary therapy for bacterial keratitis. For microbial keratitis, Hafezi and colleagues named the therapy photoactivated chromophore for keratitis corneal cross-linking, or PACK-CXL.

Role of cross-linking in infection

Microbes are known to develop resistance to drugs due to chromosomal mutation or exchange of genetic material by transformation, conjugation or bacteriophage transduction. As drug-resistant infections peak day by day, newer modalities of treatment are inevitable. UV light-based corneal cross-linking with riboflavin can function in the following ways:

1. Riboflavin interacts with the DNA of the microbes and inhibits replication. Riboflavin has a planar ring that intercalates between bases of DNA and RNA, which results in the oxidation of nucleic acids on exposure to UV light.

2. Oxidative damage to pathogens is also mediated by nonspecific photochemical reactions. Injury or damage to the microbial cell wall releases active free oxygen species.

Figure 1. Intraoperative picture of corneal ulcer with epithelium debrided (a). 0.1% isotonic riboflavin solution instilled on the cornea (b). UVA 365 nm irradiance initiated, and riboflavin instilled intermittently (c and d).

Figure 2. Preoperative non-healing bacterial corneal ulcer (a). Two weeks postoperative after corneal cross-linking (b).

3. Ultraviolet irradiation itself has a dose-dependent ability to directly destroy micro-organisms. This property has been used in sterilization processes.

Bacterial infections: Cross-linking also induced structural changes in the stromal collagen, which prevents the further cleavage action of collagenases. In vitro studies have demonstrated its bactericidal effect against Staphylococcus aureus, S epidermidis, Pseudomonas aeruginosa, Streptococcus pneumoniae and MRSA.

Fungal infections: Corneal cross-linking does not eradicate Candida albicans, Fusarium species or Aspergillus fumigatus in vitro, although it may enhance the effect of antifungal medications.

Acanthamoeba infections: Acanthamoeba species are also difficult to eradicate due to their prolonged and aggressive nature and structural difference. However, healing can be enhanced and the effect of enzymatic digestion on the cornea can be prevented after cross-linking.

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Technique

Initial corneal scraping for staining and culture is routine in all preoperative cases. Clinical photography and slit lamp examination are performed. The corneal stroma should be instilled with frequent antibiotics 1 hour before cross-linking. In our center, we give antibiotics every 10 minutes before cross-linking starts. Under topical anesthesia of 0.5% proparacaine, the infective region is marked and localized epithelial debridement is done. After epithelial removal, 0.1% isotonic riboflavin solution in 20% dextran is instilled for 30 minutes (Figure 1). By the end of 30 minutes, the cornea is visualized in the microscope and UV exposure is started. The setting is kept at 3 mw/cm2 for 30 minutes. The surrounding normal cornea is covered by a shield with a central aperture to prevent UV exposure outside the infiltrate zone. Intermittently, 0.1% riboflavin is administered as in conventional cross-linking. By the end of 30 minutes of UV exposure, the cornea is washed with balanced salt solution and topical antibiotic is administered.

In the postoperative period, the cornea is watched for epithelial defect size, corneal infiltration and thinning. All preoperative antimicrobial medicines can be continued postoperatively. Additional lubricants can be included in the existing regimen. Frequent follow-up visits in the initial few days are mandatory (Figure 2). Anterior segment OCT may be used for prognosis and documentation.

Figure 3. Microbial keratitis not responding to medical treatment (a). Two months after cross-linking. Note the scar formation (b).

Figure 4. Preoperative non-healing bacterial corneal ulcer (a). Two weeks postoperative after corneal cross-linking (b).

Advantages

Photoactivation of a chromophore (riboflavin) acts like a disinfectant, reducing the organism concentration in the corneal surface and stroma. Infective keratitis, irrespective of the nature of the microbe (bacterial, fungal or Acanthamoeba), requires aggressive management to arrest the pathogenesis. Collagen deposition and granulation tissue remodeling lead to corneal scarring after infective keratitis, which leads to loss of vision in its natural course. By doing cross-linking in the active phase of infection, we prepare the cornea to form more cross-links, which will make it stronger against the action of enzymatic digestion and also prevent the excess scarring that occurs after keratitis (Figure 3). UV irradiation and free oxygen radicals interfere with cell membrane integrity, leading to direct destruction of bacteria. However, once melting starts in corneal infection, the surgical intervention is initiated. Thus, by preventing the stage of melting from happening, one can decrease the need for surgical intervention (Figure 4). Nevertheless, Iseli and colleagues have done cross-linking even in melting cornea due to infection. Infective keratitis-induced corneal blindness has become one of the leading causes of monocular vision loss in developing countries. With the rise in multidrug resistance organisms and a more destructive nature of the microbes, it is necessary to focus on alternative treatment options. We hope UV light-based corneal cross-linking has potential to provide promising results in such situations.

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For more information:
Amar Agarwal, MS, FRCS, FRCOphth, is director of Dr. Agarwal’s Eye Hospital and Eye Research Centre. Agarwal is the author of several books published by SLACK Incorporated, publisher of Ocular Surgery News, including Phaco Nightmares: Conquering Cataract Catastrophes, Bimanual Phaco: Mastering the Phakonit/MICS Technique, Dry Eye: A Practical Guide to Ocular Surface Disorders and Stem Cell Surgery and Presbyopia: A Surgical Textbook. He can be reached at 19 Cathedral Road, Chennai 600 086, India; email: dragarwal@vsnl.com; website: www.dragarwal.com.

Disclosure: Agarwal and Kumar report no relevant financial disclosures.