Acanthamoeba Keratitis

Overview

Acanthamoeba keratitis (AK) is a severe corneal infection caused by Acanthamoeba, a free-living protozoan ubiquitous in soil, dust, and virtually all water sources (tap water, swimming pools, hot tubs, contact lens cases).³ It disproportionately affects contact lens wearers and is notorious for diagnostic delay, since early presentation can closely mimic herpes simplex or bacterial keratitis, and this delay is strongly linked to poorer outcomes.²

Epidemiology & Risk Factors

Contact lens wear is the dominant risk factor for AK in developed countries, particularly with poor lens hygiene, use of non-sterile (e.g., tap) water for lens cleaning or rinsing, swimming or showering while wearing lenses, or use of homemade/inadequate disinfection solutions.³ Non-contact-lens-wearers can develop AK following corneal trauma with soil or contaminated water exposure. The organism exists in two forms — a motile, replicating trophozoite and a highly resistant, dormant double-walled cyst — with the cyst form conferring substantial resistance to antiamoebic drugs and to environmental extremes.³

Pathophysiology

Acanthamoeba trophozoites adhere to and invade the corneal epithelium (facilitated in contact lens wearers by lens-induced epithelial microtrauma), then migrate into the stroma, often tracking along corneal nerves — producing the characteristic radial perineural infiltrates and severe pain out of proportion to visible findings, thought to result from perineural inflammation. The organism's ability to encyst under adverse conditions (including exposure to antiamoebic drugs or host immune attack) allows it to persist and later re-emerge as active trophozoites, explaining relapses during treatment and the need for prolonged therapy.³

Clinical Presentation

Early AK often mimics herpes simplex or bacterial keratitis, contributing to diagnostic delay. Classic features include:

• Severe pain, often strikingly disproportionate to the modest early clinical findings
• Photophobia and foreign-body sensation
• Progressive, often waxing-and-waning course despite standard antibacterial/antiviral therapy
• Later stages: ring-shaped stromal infiltrate (a relatively specific but late sign), radial perineural infiltrates (Fanselow: a suggestive early sign), and epithelial or subepithelial irregularity

Exam Findings

• Early: punctate epithelial keratopathy, epithelial or subepithelial infiltrates, pseudodendrites (can be mistaken for herpes simplex)
• Radial perineural infiltrates (neurokeratitis) — a relatively specific finding
• Ring infiltrate — classic but typically a later-stage finding
• Stromal haze, edema
• Scleritis in more advanced/severe disease

Differential Diagnosis

• Herpes simplex keratitis (most commonly confused early on, given pseudodendrite formation)
• Bacterial keratitis
• Fungal keratitis
• Sterile inflammatory keratitis/neurotrophic keratopathy

Management

Diagnosis relies on a combination of clinical suspicion (contact lens wearer or water/soil exposure with disproportionate pain and a course refractory to standard antibacterial/antiviral treatment), corneal scraping for culture on non-nutrient agar with E. coli overlay, and increasingly, in vivo confocal microscopy, which can directly visualize cysts non-invasively and has become an important diagnostic tool, along with PCR-based testing and impression cytology.³ Diagnostic delay remains common and is strongly associated with worse outcomes, since antiamoebic treatment is frequently started only in a later treatment phase after initial (ineffective) antibacterial or antiviral therapy has failed.²

Treatment is prolonged (often many months) and centers on topical biguanides (polyhexamethylene biguonide or chlorhexidine) combined with diamidines (propamidine or hexamidine) — combination therapy has traditionally been the mainstay, though some evidence suggests biguanide monotherapy at higher concentration may achieve comparable outcomes, an area of ongoing study.¹ Topical corticosteroids are controversial: they can reduce inflammatory symptoms but may worsen the infection if introduced too early, before adequate antiamoebic control, since they can promote trophozoite proliferation and impair host defenses.¹ For advanced or medically refractory disease, therapeutic penetrating keratoplasty may be required, though surgery during active inflammation carries a high risk of recurrence in the graft.¹

Complications

• Corneal scarring and perforation
• Scleritis, a marker of more severe/advanced disease
• Recurrence of infection within a corneal graft if keratoplasty is performed during active disease
• Permanent visual impairment, often compounded by diagnostic delay

Clinical Pearls

• Pain disproportionate to clinical findings in a contact lens wearer should raise suspicion for AK, especially if the course fails to improve with standard antibacterial/antiviral therapy
• Radial perineural infiltrates are a suggestive early finding; a ring infiltrate is classic but typically appears later
• Diagnostic delay — often from initially treating for bacterial or herpetic keratitis — is strongly linked to worse outcomes²
• In vivo confocal microscopy can directly visualize Acanthamoeba cysts and has become an important non-invasive diagnostic tool³
• Introducing topical steroids too early, before adequate antiamoebic control, can worsen the infection

Common Pitfalls

• Mistaking early AK pseudodendrites for herpes simplex keratitis and treating with antivirals alone
• Delaying antiamoebic therapy while cycling through ineffective antibacterial or antiviral treatments
• Starting topical corticosteroids before adequate antiamoebic treatment has been established
• Underestimating the risk in contact lens wearers who use tap water for lens care or who swim/shower in lenses
• Performing keratoplasty during active inflammation without recognizing the high risk of recurrence in the graft