14.3 Microbial Keratitis, Acanthamoeba, Dendritic Ulcer & Infiltrates

Infectious & Inflammatory Corneal Complications

Quick Answer: Suspect infection when there is pain, redness, discharge, photophobia, and a white infiltrate/ulcer. Bacterial keratitis is most often Pseudomonas in extended wear (rapid, greenish discharge) and Staph/Strep in daily wear. Acanthamoeba is pain out of proportion to findings with a ring infiltrate, linked to tap water/swimming. HSV dendritic ulcer shows branching staining with fluorescein + rose bengal. Sterile vs infectious: small/peripheral/quiet = sterile; large/central/inflamed = infectious.

Sterile vs Infectious Infiltrates

The single most important judgment at the slit lamp is whether an infiltrate is sterile or infectious. Misjudging an infectious ulcer as sterile and treating with steroid alone can destroy the eye.

When in doubt, treat as infectious and refer urgently.

Microbial Keratitis (Bacterial)

Presentation: Acute pain, redness, mucopurulent discharge, photophobia, lid edema, and a white infiltrate with an overlying epithelial ulcer. Severe cases develop a hypopyon (layered white cells in the anterior chamber).

Organisms:

• Pseudomonas aeruginosa — most common in extended wear. Rapidly progressive, greenish-yellow discharge, can perforate the cornea within 24–48 hours.
• Staphylococcus / Streptococcus — more common in daily wear. Slower, less aggressive.
• Atypical mycobacteria — less common, often after trauma.

Management:

1. Discontinue lens wear immediately.
2. Culture the ulcer (corneal scraping), lens, and case before starting antibiotics if possible.
3. Refer urgently to ophthalmology.
4. Intensive topical antibiotics — frequently fortified tobramycin + cefazoline, or a fluoroquinolone (moxifloxacin) as monotherapy for less severe cases.
5. No steroid until infection is controlled and epithelium healed (steroid can worsen infection, especially Acanthamoeba and fungal).
6. Counsel patient on risk factors: extended wear, poor hygiene, tap water exposure.

Acanthamoeba Keratitis

Presentation: A protozoan infection — classically excruciating pain out of proportion to slit-lamp findings, especially in early disease. Later a ring infiltrate forms. Often bilateral in contact lens wearers. Delayed diagnosis is the rule because early findings mimic other conditions.

Risk factors:

• Tap water exposure (rinsing lenses or case in tap water).
• Swimming (pools, lakes, hot tubs) in lenses.
• Poor hygiene; contaminated storage cases.

Diagnosis:

• Confocal microscopy (in vivo visualization of cysts).
• Culture on Escherichia coli-seeded non-nutrient agar (Acanthamoeba feeds on E. coli).
• PCR and corneal biopsy in atypical cases.

Treatment (difficult, often months-long):

• Polyhexamethylene biguanide (PHMB) 0.02% — cornerstone biguanide.
• Propamidine isethionate (Brolene) — diamidine.
• Neomycin drops.
• Combination therapy for many months; debridement may help. Steroids are contraindicated (encystment).

Prevention: Never expose lenses or storage case to tap water; use only sterile solution; avoid swimming in lenses; consider daily disposables.

Dendritic Ulcer (HSV Keratitis)

Presentation: Herpes simplex virus (HSV-1) keratitis — branching, dendritic epithelial staining pattern. Best seen with fluorescein + rose bengal (rose bengal stains the dead cells at the dendrite terminals). The classic dendrite has terminal bulbs.

Relation to contact lens wear: HSV is not contact-lens-specific, but lens trauma can trigger a recurrence in a patient with latent HSV. A contact lens wearer with a dendritic ulcer must be evaluated for HSV, not assumed to have a sterile infiltrate.

Management:

• Discontinue lens wear.
• Topical antiviral (trifluridine 1%) or oral antiviral (acyclovir, valacyclovir) per ophthalmology.
• Avoid topical steroid — steroid can convert epithelial HSV into geographic or stromal keratitis and worsen disease.
• Refit lens only after complete healing and ophthalmology clearance.

Infiltrates — General Approach

Any new corneal infiltrate in a contact lens wearer requires:

1. Slit-lamp evaluation with measurement of size, depth, and location; assess anterior chamber reaction.
2. Seidel test if perforation suspected.
3. Decision: sterile vs infectious — using the table above.
4. Sterile workup: discontinue lens, topical antibiotic prophylaxis, close follow-up in 24 hours.
5. Infectious workup: discontinue, culture, urgent ophthalmology referral, intensive antibiotics.

Red-Flag Signs Mandating Same-Day Ophthalmology Referral

• Large (>1–2 mm) or central infiltrate.
• Hypopyon.
• Severe pain disproportionate to findings (Acanthamoeba).
• Rapid progression.
• Greenish discharge (Pseudomonas).
• Ring infiltrate (Acanthamoeba or fungal).
• Dendritic staining (HSV).

Key Takeaways

• Bacterial MK = pain + redness + discharge + ulcer + white infiltrate; Pseudomonas in EW, Staph/Strep in DW.
• Acanthamoeba = pain out of proportion, ring infiltrate, tap water link; diagnose with confocal or E. coli agar; treat with PHMB, propamidine, neomycin; no steroid.
• HSV dendritic ulcer = branching staining with terminal bulbs; treat with antivirals; no steroid.
• Sterile infiltrates are small/peripheral/quiet; infectious are large/central/inflamed.
• When in doubt, treat as infectious and refer urgently.