Many conditions masquerade as bacterial keratitis

São Paulo, Brazil-Establishing the correct diagnosis and modifying therapy appropriately based on response are fundamental elements in the successful management of bacterial keratitis, said Richard L. Abbott, MD, at the World Congress of Ophthalmology.

"Left untreated, microbial keratitis leads to progressive tissue destruction with potential corneal perforation and extension to adjacent structures. Determining etiology is critical for effective management," said Dr. Abbott, Thomas W. Boyden professor of ophthalmology, Beckman Vision Center, University of California at San Francisco.

In their clinical evaluation, ophthalmologists need to recognize there are many conditions that are masqueraders of bacterial keratitis. To illustrate the point, Dr. Abbott presented examples of eyes infected with Fusarium, herpes simplex, Acanthamoeba, and combined beta-Streptococcus/Candida, as well as a case of hypopyon with no infiltrate or infection. Therefore, obtaining a culture with use of proper sampling and plating technique is critical in all suspect cases.

For culturing, the specimen should be obtained using a calcium alginate swab moistened with carrier media and streaked directly onto the agar plate using a C streak technique. Placing the specimen in a holding agent will result in a relatively low yield. Organisms growing off the C streaks are most likely contaminants, he noted.

Monitor closely for response

Once treatment is initiated, patients need to be closely observed to determine if the infection is responding, worsening, or if the changing clinical picture is indicative of medication-related toxicity. To guide that assessment, Dr. Abbott recommended noting and creating a carefully drawn diagram in the chart to delineate:

Those features should then be followed in the evaluation of the treatment response.

In the current era, treatment is usually initiated with a later-generation fluoroquinolone, which is administered in an intensive regimen for the first several days. However, based on the culture and sensitivity results, a fortified, third-generation cephalosporin may be added.

Once there is evidence of therapeutic response, the treatment should be tapered to avoid toxicity. Fluoroquinolone administration, however, should never be reduced to below four times a day since this practice may help induce resistant organisms to grow. Further tapering can be done by decreasing concentration with fortified antibiotics.

Key signs that the infection is resolving include the blunting of the edges of the infiltrate, reduction in epithelial defect size, and decrease in the density of white blood cell infiltration. Measurement of the defect is done with fluorescein staining and use of the built-in micrometer on the Haag-Streit 900 slit-lamp biomicroscope. Retroillumination is used to assess the density of the white blood cell infiltrate.

When fluoroquinolone discontinuation is deemed appropriate, clinicians who wish to maintain antibiotic coverage may prescribe bacitracin (Upjohn), because the agent offers broad-spectrum activity and is well-tolerated, said Dr. Abbott.