What is the most common pathogen in contact lens-related microbial keratitis?

Pseudomonas aeruginosa is the most common and aggressive bacterial cause. This gram-negative organism thrives in moist environments like lens cases and can rapidly destroy corneal tissue within 24-48 hours through its proteolytic enzymes.

What is the difference between a corneal ulcer and a corneal infiltrate?

A corneal infiltrate is a collection of inflammatory cells in the stroma, which may have an intact epithelium. A corneal ulcer is an infiltrate with an overlying epithelial defect, creating an open wound. Microbial keratitis presents as a corneal ulcer because organisms breach the epithelial barrier.

Why should you never patch a suspected infected eye?

Patching creates a warm, dark, moist environment that promotes bacterial growth and accelerates tissue destruction. Infected eyes need exposure to topical antibiotics applied frequently (hourly) and air circulation. Patching could allow rapid progression of the infection.

Microbial Keratitis in Contact Lens Wearers: Pathogens, Risk Factors, and Emergency Management

Understanding Microbial Keratitis

Microbial keratitis (MK) is an infection of the cornea caused by bacteria, fungi, or parasites that invade the corneal tissue. It is the most serious complication of contact lens wear and represents a true ocular emergency. Without prompt, aggressive treatment, MK can lead to corneal scarring, perforation, and permanent vision loss.

Contact lens wear is the single most common risk factor for microbial keratitis in developed countries, making recognition and appropriate referral essential knowledge for every contact lens practitioner.

Key Pathogens

Pseudomonas aeruginosa

Pseudomonas aeruginosa is the most common and aggressive bacterial cause of contact lens-related keratitis. This gram-negative rod:

Thrives in moist environments, including lens cases and stagnant solutions
Can rapidly destroy corneal tissue within 24-48 hours
Produces proteolytic enzymes that digest the corneal stroma
Often presents with a large, rapidly expanding corneal ulcer with a dense, yellowish-white infiltrate
May produce a characteristic "ring" infiltrate

Acanthamoeba

Acanthamoeba is a free-living amoeba found in tap water, swimming pools, and soil. Acanthamoeba keratitis is:

Strongly associated with tap water exposure to contact lenses or cases
Extremely painful, often disproportionate to clinical signs
Characterized by a ring-shaped infiltrate (perineural infiltrate)
Notoriously difficult to treat, often requiring months of aggressive anti-amoebic therapy

Fungal Organisms

Fungal keratitis is less common but can occur, particularly in warm, humid climates or after organic matter exposure. Fusarium outbreaks have been linked to specific contact lens solution formulations.

Corneal Ulcer vs. Corneal Infiltrate

Understanding the distinction between these terms is essential:

Corneal infiltrate: A collection of inflammatory cells (white blood cells) in the corneal stroma. The overlying epithelium may be intact. Infiltrates can be sterile (as in CLARE) or infectious
Corneal ulcer: An infiltrate with an overlying epithelial defect. The surface is broken, creating an open wound. Microbial keratitis characteristically presents as a corneal ulcer because the organisms have breached the epithelial barrier

All corneal ulcers involve an epithelial defect over a stromal infiltrate. Not all infiltrates are ulcers. This distinction matters because ulcers with active infection require culture and aggressive antimicrobial therapy, while sterile infiltrates may resolve with lens removal alone.

Risk Factors

Major risk factors for microbial keratitis in contact lens wearers:

Overnight wear: Increases MK risk 4-5 times compared to daily wear
Poor hygiene: Inadequate handwashing, not replacing lens cases, topping off solutions
Tap water exposure: Swimming, showering, or rinsing lenses with tap water
Extended replacement schedules: Wearing lenses beyond their intended replacement period
Smoking: Increases corneal susceptibility to infection
Previous corneal injury: Compromised epithelial barrier allows easier microbial entry

Clinical Presentation

Severe pain: Often disproportionate to the visible clinical findings
Red eye: Intense conjunctival and ciliary injection
Photophobia: Significant light sensitivity
Mucopurulent discharge: Yellow or green discharge from active infection
Corneal opacity: Dense, white or yellow stromal infiltrate, often central
Epithelial defect: Visible with fluorescein staining overlying the infiltrate
Anterior chamber reaction: Cells and flare in the aqueous humor; possible hypopyon (pus layer) in severe cases

Emergency Management

Suspected microbial keratitis requires urgent action:

Remove the contact lens: Save it in a sterile container for possible culture
Culture the cornea: Obtain corneal scrapings for culture and sensitivity before starting antibiotics
Save the lens case: The case often harbors the causative organism
Initiate aggressive antibiotic therapy: Typically fortified broad-spectrum topical antibiotics (often a fluoroquinolone) every 30-60 minutes around the clock
Refer urgently: If you are not equipped to manage MK, refer to a cornea specialist immediately
No patching: Never patch an infected eye, as the warm, dark environment promotes bacterial growth

Treating a suspected microbial keratitis with a standard antibiotic drop four times daily. MK requires aggressive, hourly dosing with fortified antibiotics. Standard dosing allows the infection to progress and can result in corneal perforation.

When in doubt about whether an infiltrate is sterile or infectious, err on the side of treating as infectious. Culture first if possible, then begin aggressive antibiotic therapy. Undertreating an infection has far worse consequences than briefly overtreating a sterile infiltrate.

Key Takeaways

Microbial keratitis is the most serious contact lens complication, requiring emergency management
Pseudomonas aeruginosa is the most common bacterial pathogen and can destroy corneal tissue within 24-48 hours
A corneal ulcer is an infiltrate with an overlying epithelial defect; not all infiltrates are ulcers
Overnight wear, poor hygiene, and tap water exposure are the major risk factors
Treatment requires aggressive hourly topical antibiotics after obtaining cultures
Never patch a suspected infected eye