What is the appropriate emergency management for a central corneal infiltrate identified on a dark‑room slit‑lamp examination?

Emergency Management of Central Corneal Infiltrate

When you identify a central corneal infiltrate on slit-lamp examination, immediately obtain corneal cultures and smears before initiating broad-spectrum topical antibiotic therapy, as central location is a mandatory indication for microbiological workup regardless of size. 1, 2

Immediate Assessment and Risk Stratification

Document Critical Features

• Measure infiltrate size and location precisely – central infiltrates (within 3 mm of corneal center) require mandatory culturing even if <2 mm, though ≥2 mm size further mandates cultures 1, 2
• Assess depth of involvement – evaluate for stromal melting, significant stromal involvement, or deep stromal abscess, all of which indicate high-risk infection 3, 1
• Check for anterior chamber reaction – presence of ≥1+ cells, hypopyon, or fibrin indicates severe infection requiring aggressive management 1, 4
• Evaluate epithelial integrity – apply fluorescein to identify epithelial defects and distinguish true ulceration from stain pooling in areas of thinning 1

Identify High-Risk Features Requiring Cultures

The American Academy of Ophthalmology mandates cultures for central infiltrates with any of these features 1, 2:

• Central location within 3 mm of corneal center AND ≥2 mm size
• Stromal melting or significant stromal involvement
• Multiple infiltrates or ≥2 adjacent lesions
• History of corneal surgery (refractive, transplant, or other)
• Atypical features suggesting fungal (feathery margins), amoebic (ring infiltrate), or mycobacterial infection

Microbiological Workup Protocol

Culture Technique

• Use proparacaine 0.5% only – avoid tetracaine and preservative-containing anesthetics as they have antimicrobial effects that reduce culture yield 2
• Scrape from the infiltrate edge, not the center – obtain material from the base and periphery using sterile blade, platinum spatula, or jeweler's forceps to maximize viable pathogen recovery 3, 2
• Inoculate directly onto culture media at bedside when possible to maximize yield; if unavailable, use nylon-tipped flocked swab in modified Amies medium 3, 2
• Include liquid culture media in addition to solid media, as this increases bacterial isolation rates 3, 2

Staining Requirements

For central infiltrates, obtain 1, 2:

• Gram stain and Giemsa stain for all cases
• Add potassium hydroxide (KOH), Calcofluor white, and acid-fast stains if post-surgical or atypical features present
• Recognize that clinical appearance alone differentiates bacterial from fungal keratitis in <70% of cases, even by cornea specialists 1, 5

Empiric Antibiotic Therapy

Initiate broad-spectrum fortified topical antibiotics immediately after obtaining cultures – do not delay treatment waiting for culture results, as untreated bacterial keratitis can progress rapidly to perforation 1

Treatment Considerations

• Start intensive dosing (every 30-60 minutes initially) with fortified antibiotics or fourth-generation fluoroquinolones for central infiltrates 3
• Tailor antibiotic therapy based on culture results once available 3
• If cultures remain negative after 48-72 hours and clinical response is poor, consider stopping antibiotics for 12-24 hours and reculturing 3

Special Situations Requiring Modified Approach

Deep Stromal Infiltrates with Clear Overlying Tissue

• Consider corneal biopsy if the infiltrate is in mid or deep stroma with uninvolved overlying tissue, especially if cultures are repeatedly negative but infection is strongly suspected 3
• Take biopsy from the edge of the infiltrate (as far from corneal center as possible) using 2-3 mm trephine to avoid central refractive error 3
• Biopsy yields organisms in 42% by culture and 40% by histopathology when traditional cultures fail 3

Infiltrate with Feathery Margins

• Suspect fungal keratitis and ensure fungal-specific stains (Calcofluor-KOH) and culture media are included, as feathery finger-like infiltration is pathognomonic for filamentous fungi 5
• Consider confocal microscopy if available for rapid in vivo identification 3, 5

Ring-Shaped Infiltrate

• Consider Acanthamoeba keratitis (one-third of cases present with ring infiltrate), but recognize that bacterial keratitis, fungal infections, and even sterile inflammatory conditions can produce ring infiltrates 6
• Obtain special culture media (non-nutrient agar seeded with E. coli) for Acanthamoeba if suspected 3

Critical Pitfalls to Avoid

• Never perform aqueous or vitreous tap for hypopyon in bacterial keratitis unless there is high suspicion of endophthalmitis, as hypopyon is usually sterile 1, 2
• Do not collect only purulent material – this yields inadequate results; scrape the infiltrate base and edges 2
• Do not rely on clinical appearance alone to differentiate bacterial from fungal keratitis, as accuracy is <70% even among specialists 1, 5
• Avoid empiric corticosteroids until infection is ruled out or adequately treated, as steroids can worsen fungal keratitis outcomes 5
• Do not delay cultures based on perceived "sterile" appearance – studies show significant numbers of peripheral and central infiltrates are culture-positive even when appearing clinically benign 7, 4