Laboratory Studies
Scrapings of the corneal ulcer, including the edges, should be obtained using a sterile spatula, blade, or sterile Mini-tip Calcium Alginate Swab, and they should be plated in chocolate, blood, and Sabouraud agar plates.
Microscope slides are used for stained smears with Gram, Giemsa, and acid-fast stain or acridine orange/calcofluor white (if fungi or Acanthamoeba are suspected).
Samples of the eyelids/conjunctiva, topical ocular medications, contact lens cases, and solutions also may be cultured.
In the past, some practitioners considered suspending antibiotic therapy for 12 hours before performing corneal scraping to increase the yield of a positive culture; however, most practitioners now would not recommend this technique for initial management.
In cases of mild keratitis (eg, a peripheral infiltrate < 1-2 mm), some practitioners defer corneal scrapings and treat empirically. This is especially common in community settings where a laboratory and specialized culture plates may not be readily available.
Cotton swabs contain fatty acids, which have an inhibitory effect on bacterial growth. On the other hand, calcium alginate moistened with trypticase soy broth can be used to obtain culture material to inoculate directly onto the culture media.
Topical anesthetic (proparacaine hydrochloride 0.5%) should be used to anesthetize the patient prior to culture scraping because it has the least inhibitory effect. In contrast, tetracaine and cocaine have bacteriostatic effects.
Repeat cultures can be obtained if the original cultures were negative and the ulcer is not improving clinically.
Corneal biopsy using a small trephine or a corneal blade should be considered in cases of deep stromal infiltrates, particularly if cultures are negative and the eye is not improving clinically.
Imaging Studies
Slit lamp photography can be useful to document the progression of the keratitis, and, in cases where the specific etiology is in doubt, it is used to obtain additional opinions, particularly in indolent and chronic cases not responding to antimicrobial therapy.
A B-scan ultrasound can be obtained in eyes with severe corneal ulcers with no view of the posterior segment where endophthalmitis is being considered.
Procedures
Corneal biopsy
A deep lamellar excision can be made using a disposable skin punch or a small Elliott corneal trephine. The superficial cornea is incised and deepened with a surgical blade to approximately 200 microns. Then, a lamellar dissection is performed, and the material is plated directly onto culture media. A portion also can be sent for histopathologic evaluation.
Histologic Findings
During the initial stages, the epithelium and the stroma in the area of injury and infection swell and undergo necrosis. Acute inflammatory cells (mainly neutrophils) surround the beginning ulcer and cause necrosis of the stromal lamellae. In cases of severe inflammation, a deep ulcer and a deep stromal abscess may coalesce, resulting in thinning of the cornea and sloughing of the infected stroma.
As the natural host defense mechanisms overcome the infection, humoral and cellular immune defenses combine with antibacterial therapy to retard bacterial replication. Following this process, phagocytosis of the organism and cellular debris take place, without further destruction of stromal collagen. During this stage, a distinct demarcation line may appear as the epithelial ulceration and stromal infiltration consolidate and the edges become rounded.
Vascularization of the cornea may follow if the keratitis becomes chronic. In the healing stage, the epithelium resurfaces the central area of ulceration and the necrotic stroma is replaced by scar tissue produced by fibroblasts. The reparative fibroblasts are derived from histiocytes and keratocytes that have undergone transformation. Areas of stromal thinning may be replaced partially by fibrous tissue. New blood vessel growth directed toward the area of ulceration occurs with delivery of humoral and cellular components to promote further healing. The Bowman layer does not regenerate but is replaced with fibrous tissue.
New epithelium slowly resurfaces the irregular base, and vascularization gradually disappears. With severe bacterial keratitis, the progressive stage advances beyond the point in which the regressive stage can lead to the healing stage. In such severe ulcerations, stromal keratolysis may progress to corneal perforation. Uveal blood vessels may participate in sealing the perforation, resulting in an adherent vascularized leukoma.
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Human eye anatomy.
