Corneal Infections and Ulcers

Corneal ulcers account for an estimated 930,000 clinic and emergency department visits per year in the United States, and contact lens wear remains the single largest risk factor in developed nations (AAO EyeWiki). Left untreated, a corneal ulcer can perforate within 24–48 hours, making this one of the genuine emergencies in ophthalmology — the kind that does not wait for a convenient appointment slot.

Anatomy of the Problem

The cornea is a transparent, avascular structure roughly 540 micrometers thick at its center. It functions as both a protective barrier and the eye's primary refractive surface, contributing about 43 diopters of the total 60-diopter optical power. When that barrier is breached — by a scratch, a drying contact lens, or a foreign body — microorganisms gain access to the stroma, and an infectious ulcer can establish itself with startling speed.

The term "corneal ulcer" technically describes any epithelial defect with underlying stromal inflammation and tissue loss. "Microbial keratitis" is the broader clinical term when an infectious agent is confirmed or strongly suspected. The distinction matters because not every corneal ulcer is infectious; sterile ulcers occur in autoimmune conditions like rheumatoid arthritis and in neurotrophic corneas with impaired sensation.

Causative Organisms

Bacterial Keratitis

Bacteria cause the majority of infectious corneal ulcers in temperate climates. Pseudomonas aeruginosa and Staphylococcus aureus together account for the largest share of culture-positive cases. Pseudomonas is especially notorious among contact lens wearers — it thrives in biofilm on lens cases and can produce a rapidly progressive, suppurative ulcer with a characteristic greenish mucopurulent discharge. The Steroids for Corneal Ulcers Trial (SCUT), a large randomized study funded by the National Eye Institute, confirmed that Pseudomonas ulcers tend to carry a worse visual prognosis than those caused by gram-positive cocci (NEI/NIH).

Fungal Keratitis

Fungal ulcers represent a disproportionate burden in tropical and agricultural regions. Fusarium and Aspergillus species are the most common filamentary fungi isolated, while Candida predominates in eyes with pre-existing ocular surface disease. A 2006 Fusarium keratitis outbreak linked to a specific contact lens solution (Bausch & Lomb ReNu with MoistureLoc) led to a voluntary product recall and heightened FDA scrutiny of multipurpose solution efficacy (FDA). Clinically, fungal ulcers tend to have feathery borders, satellite lesions, and an indolent course — clues that should trigger suspicion, since standard antibacterial drops will not help.

Acanthamoeba Keratitis

Acanthamoeba is a free-living amoeba found in tap water, hot tubs, and soil. The CDC estimates that roughly 85% of Acanthamoeba keratitis cases in the United States occur in contact lens wearers, often those who rinse or store lenses in tap water (CDC). The hallmark early finding is pain out of proportion to clinical signs, sometimes accompanied by a ring-shaped stromal infiltrate (Wessely ring). Treatment is prolonged — often months of hourly polyhexamethylene biguanide (PHMB) or chlorhexidine drops — and delayed diagnosis is the single most reliable predictor of a poor outcome.

Herpes Simplex Keratitis

Herpes simplex virus type 1 (HSV-1) is the leading infectious cause of corneal blindness in high-income countries. The classic dendritic epithelial ulcer, with its terminal bulbs visible on fluorescein staining, is almost pathognomonic. The Herpetic Eye Disease Study (HEDS), sponsored by the National Eye Institute, demonstrated that oral acyclovir at 400 mg twice daily reduced the recurrence rate of stromal keratitis by roughly 50% over 18 months (NEI/NIH).

Diagnosis

Slit-lamp biomicroscopy remains the cornerstone. Corneal scraping for Gram stain, culture on blood agar, chocolate agar, and Sabouraud dextrose agar is standard for ulcers that are central, large (greater than 2 mm), deep, or unresponsive to empiric therapy. Confocal microscopy can identify Acanthamoeba cysts in vivo, and polymerase chain reaction (PCR) testing has improved sensitivity for HSV and fungi, though availability outside academic centers varies.

Treatment Principles

Empiric therapy for suspected bacterial keratitis typically begins with fortified topical antibiotics — a combination of fortified tobramycin (14 mg/mL) and fortified cefazolin (50 mg/mL), or a fluoroquinolone monotherapy (moxifloxacin 0.5% or gatifloxacin 0.5%) for smaller, peripheral ulcers. Dosing frequency in severe cases starts at every 30–60 minutes around the clock, a regimen that tests any patient's resolve.

Topical corticosteroids remain controversial. The SCUT trial found no statistically significant overall benefit from adjunctive prednisolone phosphate 1% in bacterial keratitis, though a subgroup analysis suggested possible benefit in central ulcers with initially poor vision (SCUT — NEI Clinical Trials).

Surgical intervention — therapeutic penetrating keratoplasty — is reserved for cases with impending or actual perforation, or when medical therapy fails to control infection.

Prevention

Contact lens hygiene is the single most modifiable risk factor. The CDC's Healthy Contact Lens program recommends replacing lens cases at least every three months, never topping off old solution, and removing lenses before swimming or showering (CDC). Extended or overnight wear multiplies the risk of microbial keratitis by a factor of 5–10 compared with daily wear, a statistic worth remembering the next time fatigue tempts a lens wearer to skip the nightly removal routine.

Frequently Asked Questions

How quickly can a corneal ulcer cause permanent vision loss?

A Pseudomonas ulcer can perforate the cornea within 24–48 hours of symptom onset. Even less aggressive organisms may cause irreversible stromal scarring within days if untreated. Any contact lens wearer who develops a painful red eye with discharge should be evaluated the same day.

Is it safe to use over-the-counter eye drops for a suspected corneal ulcer?

Over-the-counter artificial tears and redness-relief drops do not contain antibiotics or antifungals. Applying vasoconstrictor drops can mask redness and delay diagnosis. A suspected corneal ulcer warrants prompt evaluation by an ophthalmologist or an emergency department equipped for slit-lamp examination.

Can corneal ulcers recur?

Herpes simplex keratitis recurs in approximately 27% of patients within two years of an initial episode, according to findings from the HEDS trial (NEI/NIH). Long-term oral antiviral prophylaxis significantly reduces that rate. Bacterial and fungal ulcers do not typically recur unless the underlying risk factor — contact lens misuse, chronic ocular surface disease — persists.

References

• AAO EyeWiki — Corneal Ulcer
• National Eye Institute (NEI) — Herpetic Eye Disease Studies
• FDA — Fusarium Keratitis 2006 Update
• CDC — Acanthamoeba Infection
• CDC — Healthy Contact Lens Wear and Care

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