Q: In what cases of corneal infection should regular debridement be used for better delivery of topical medication or other therapeutic benefits? That is, when should debridement be performed, and when should it not be?
A: “Drug delivery to a corneal lesion is dependent on several factors,” explains James V. Aquavella, MD, of the University of Rochester Medical Center. “Lipid solubility is necessary to traverse the tight epithelial junction while hydrophilic drugs more readily traverse stromal collagen.”
For severe bacterial lesions, frequent instillation of fortified antibiotics is the technique of choice, Dr. Aquavella says, adding that general drug delivery can be accelerated by application of a collagen shield pre-soaked in the appropriate antibiotic solution. As the collagen bonds in the shield dissolve, the drug is delivered to the corneal surface over time. Dr. Aquavella cautions that a toxicity reaction to the fortified antibiotics or related preservative substances can disrupt the epithelial junction bonds to increase the permeability of the antibiotics, even if it is hydrophilic in nature.
|Fungal keratitis is one case in which debridement may be indicated. Photo: Natalie C. Cheung, MD and Kristin M. Hammersmith, MD.
Aaron Bronner, OD, of the Pacific Cataract and Laser Institute, points out there are two primary recognized conditions in which scraping the epithelium to allow drug penetration may be suitable. “The first, most cited indication is in cases of fungal keratitis, primarily due to poor penetrance of topical antifungals,” he says. “Stromal concentrations of natamycin 5% and compounded amphotericin B are reduced to ineffective levels by an intact epithelium. The newer antifungal voriconazole is not as significantly limited as others; however, stromal concentrations of this drug are typically reduced by 50% in the case of an intact epithelium.”
Dr. Bronner points out that exacerbating the issue of poor drug penetration is the fact that fungal pathogens often form deep keratitis that worsens despite an intact epithelium, which can lead to the unique possibility of penetration into the anterior chamber even without perforation. He recommends debriding cases of fungal keratitis during treatment to enhance drug delivery, but adds that this practice should only be completed in confirmed cases of fungal disease rather than presumed cases, as removing epithelial tissue unnecessarily in the setting of an infiltrate can perpetuate inflammation and even increase risk of corneal melt in other forms of both infectious and inflammatory keratitis. In effect, in most other cases of keratitis, “we are trying to encourage epithelial healing as much as possible.”Epithelial debridement may also be helpful in cases of herpes simplex keratitis, as it can shorten the course of infection by removing the epithelial cells that surround the ulceration. This “edge” is a reservoir of active virus that can proliferate to adjacent cells and gradually increase the size of the ulceration. Dr. Bronner cautions that this approach, however, should only be applied in certain specific infectious corneal manifestations of HSV keratitis (i.e., dendritic and geographic ulcerations), not stromal, endothelial or neurotrophic keratitis. This technique should also only be used in initial episodes; more caution is necessary in recurrent cases.