Acanthamoeba Keratitis: Symptoms, Types, Causes and Treatment

Acanthamoeba keratitis is a rare but potentially devastating infection of the cornea, the clear dome at the front of the eye. Best known among contact lens users, this sight-threatening condition has been increasing in prevalence, and its diagnosis can be tricky due to symptom overlap with other types of eye infections. Whether you’re a contact lens wearer, an eye care professional, or simply want to learn about this unusual infection, understanding its symptoms, causes, types, and treatment is key to early intervention and vision preservation.

Symptoms of Acanthamoeba Keratitis

Acanthamoeba keratitis (AK) doesn’t have a single, clear-cut presentation. Instead, its symptoms can mimic other forms of keratitis, often leading to delays in diagnosis. However, some features are more suggestive of AK and can help distinguish it from other corneal infections. Recognizing these early is crucial for effective management.

Symptom	Description	Frequency/Notes	Sources
Pain	Severe, often out of proportion	Most common initial symptom	4 5
Photophobia	Light sensitivity	Common, but not universal	4
Redness	Inflammation of the eye	Typical of keratitis	1 4
Tearing	Excessive watering	May accompany other symptoms	1 4
Visual loss	Blurred or decreased vision	May develop as disease progresses	3 4
Epithelial defect	Damage to corneal surface	Present in all AK cases in some series	5
Ring infiltrate	Ring-shaped corneal lesion	Highly suggestive, but not always seen	1 2 5 6
Radial keratoneuritis	Inflammation along nerves	Unique, helps distinguish AK	2 5
Stromal infiltrates	White spots in cornea	Most frequent morphological finding	4 5 6
Endothelial plaques	Deposits on inner cornea	Seen in some cases	5

Table 1: Key Symptoms

Pain—The Hallmark Symptom

One of the most striking features of AK is severe eye pain, often described as “out of proportion” to the visible findings. This pain results from the organism’s infiltration along the corneal nerves (radial keratoneuritis), making it particularly intense compared to other types of keratitis 4 5 6.

Visual Changes and Redness

As the infection progresses, patients may notice blurred vision, redness, and tearing. Vision loss can occur if the infection involves deeper layers of the cornea, especially if diagnosis and treatment are delayed 3 4.

Unique Signs: Ring Infiltrate and Radial Keratoneuritis

A classic but not universal sign is the “ring infiltrate”—a circular white opacity in the cornea. While this is highly suggestive of AK, it may not appear in all cases, especially early on. Another distinguishing feature is radial keratoneuritis, where the corneal nerves become inflamed and visible as lines radiating from the center, a finding less common in other infections 2 5 6.

Advanced and Atypical Presentations

If left untreated, AK can lead to severe complications such as scleritis (inflammation of the white part of the eye), iris atrophy, secondary glaucoma, mature cataract, and even chorioretinitis (inflammation within the eye) 2.

Summary

• Early symptoms: Severe pain, photophobia, tearing, redness.
• Progressive findings: Epithelial defect, stromal infiltrates, ring infiltrate, radial keratoneuritis.
• Advanced complications: Vision loss, scleritis, secondary glaucoma, intraocular inflammation 1 2 3 4 5 6.

Types of Acanthamoeba Keratitis

While the classic image of AK is that of a contact lens-related infection, the disease is more nuanced. It can present in several forms, depending on the Acanthamoeba species and genotype involved, as well as the clinical stage and depth of corneal involvement.

Type/Genotype	Description	Relevance/Features	Sources
T4	Most common genotype worldwide	Associated with most AK cases	7 8 9 10
T3, T11, T12, T15, T5	Other identified genotypes	Less common, variable pathogenicity	7 8 9 10
Epithelial AK	Disease mainly in corneal surface	Early disease, better prognosis	6
Stromal AK	Involvement of deeper corneal layers	More severe, risk of scarring	2 4 5 6
Advanced AK	Scleritis or intraocular spread	Severe, vision-threatening	2 11

Table 2: Types and Genotypes

Genotypic Diversity

There are at least 23 genotypes of Acanthamoeba, but the T4 genotype is the most frequently implicated in keratitis globally, followed by T3, T11, T12, T15, and T5. The predominance of T4 is likely due to its enhanced pathogenicity and adaptability to the human cornea 7 8 9 10. T4 also encompasses several species, including A. castellanii, A. polyphaga, and A. culbertsoni 9 10.

Although T4 accounts for the majority of cases, rare genotypes such as T9 and T12 have been reported to cause AK, occasionally associated with unusual or severe presentations 7 9 10.

Clinical Subtypes by Disease Stage

Epithelial AK

• Disease confined to the corneal surface.
• Presents with punctate epithelial erosions or pseudodendrites (branching lesions).
• Better prognosis if treated early 6.

Stromal AK

• Involvement of deeper corneal layers, leading to stromal infiltrates and ring-shaped lesions.
• More severe, often requiring prolonged therapy 2 4 5.

Advanced AK

• Complications such as scleritis, iris atrophy, or intraocular spread.
• Associated with poor prognosis and higher risk of vision loss 2 11.

Impact of Genotype on Disease and Treatment

Different genotypes may vary in their pathogenicity, clinical severity, and response to treatment. Thus, genotyping can help tailor management and predict outcomes 9 10.

Causes of Acanthamoeba Keratitis

Understanding what causes AK is essential for both prevention and early detection. While contact lens use is the leading risk factor, the infection can affect anyone exposed to contaminated environments, especially after minor eye injuries.

Cause/Risk Factor	Description	Contribution to Risk	Sources
Contact lens use	Particularly soft lenses, poor hygiene	Most significant risk factor	1 3 4 5 7 8 12
Contaminated water	Tap water, swimming pools, hot tubs	Source of Acanthamoeba	3 5 12 14
Minor corneal trauma	Small injuries to the eye surface	Facilitates entry	1 3 4 9 12
Poor lens care	Homemade saline, improper disinfection	Increases risk	4 18
Non-lens wearers	Ocular trauma, environmental exposures	Less common, but possible	3 9 12

Table 3: Major Causes

Contact Lens Use—The Primary Risk Factor

Over 85% of AK cases are associated with contact lens wear, especially among young adults and women. Soft contact lenses, in particular, are implicated due to their higher permeability and the tendency of users to clean them with tap water or wear them while swimming 4 5 7 8 12.

Improper contact lens hygiene—such as using homemade saline, reusing solutions, or not cleaning lens cases—dramatically increases the risk 4 18. Swimming, showering, or using hot tubs while wearing lenses exposes the eyes to Acanthamoeba, which thrives in water sources 3 5 12.

Non-Contact Lens-Related Causes

Although less common, AK can also occur in people with no history of contact lens use. Minor trauma to the eye (e.g., from dust, plant material, or foreign bodies) can create an entry point for Acanthamoeba, especially in agricultural settings or where contaminated water is present 1 3 9 12.

Environmental Ubiquity

Acanthamoeba is found worldwide in water, soil, dust, and even air. Its cyst form is highly resistant to disinfectants and adverse conditions, making complete eradication nearly impossible 3 11 12. This resilience explains why even meticulous lens hygiene cannot eliminate all risk, though it greatly reduces it.

Other Contributing Factors

• Young age and female gender: Higher prevalence in younger females, possibly due to higher rates of contact lens use 5.
• Intracorneal microbes: Presence of bacterial or fungal endosymbionts within Acanthamoeba may affect clinical presentation and severity 9.

Treatment of Acanthamoeba Keratitis

Treating AK is challenging due to the organism's ability to form resistant cysts and the often-delayed diagnosis. Early and aggressive therapy is vital for a good outcome, and treatment can be lengthy—often lasting months. Both medical and surgical approaches may be required in advanced cases.

Treatment	Description	Typical Use/Notes	Sources
Biguanides	PHMB or chlorhexidine eye drops	First-line, cysticidal	2 3 14 15 16
Diamidines	Propamidine isethionate, hexamidine	Used in combination therapy	2 3 14 15
Antibiotics	Neomycin, sometimes added	Limited efficacy against cysts	2 3 18
Antifungals	Miconazole, others; occasionally used	Not core therapy	18
Corticosteroids	For inflammation/immune response	Use is controversial	15
Surgical therapy	Keratoplasty (corneal transplant), cryotherapy	For advanced, unresponsive cases	2 17 18
Amniotic membrane	Transplant for corneal healing	Adjunct in severe cases	2 14
Cross-linking	Riboflavin-UVA (experimental/adjunct)	Limited evidence	2

Table 4: Treatment Approaches

Medical Therapy—The Cornerstone

Biguanides
Polyhexamethylene biguanide (PHMB) or chlorhexidine eye drops are the backbone of AK therapy. These agents are effective against both the trophozoite and cyst forms of Acanthamoeba, with PHMB often favored for its low toxicity and proven cysticidal activity 2 3 14 15 16. Treatment is intensive, with drops administered hourly at first, then tapered over months.

Diamidines
Propamidine isethionate (Brolene) or hexamidine is often combined with biguanides for synergistic effect 2 3 14 15. Combination therapy is usually required for up to 12 months 2.

Other Medications
Neomycin and antifungals like miconazole may be added in some regimens, though their efficacy is limited against cyst forms 2 3 18.

Corticosteroids
Their use is debated: while they can reduce severe inflammation, steroids may also suppress immune responses needed to clear infection. They are generally reserved for severe inflammatory complications unresponsive to antiamoebic therapy 15.

Surgical Interventions

When medical therapy fails—especially in cases of deep stromal involvement, progressive disease, or corneal perforation—surgery may be necessary:

• Penetrating keratoplasty (corneal transplant): Removes infected tissue and restores vision in advanced cases, though repeat procedures may be needed 2 17 18.
• Amniotic membrane transplantation and corneal cryotherapy: Used as adjuncts to promote healing or control infection 2 14.
• Riboflavin-UVA cross-linking: An emerging approach; evidence is limited but may play a role in refractory cases 2.

Duration and Prognosis

With early diagnosis and aggressive therapy, most patients retain useful vision, but treatment often lasts six months or more 14 15. Delay in therapy, deep stromal involvement, or need for multiple surgeries worsens prognosis 4 17.

Prevention

Given the challenges of treating AK, prevention is critical:

• Adhering to strict contact lens hygiene.
• Avoiding exposure to water (swimming, showering) while wearing lenses.
• Never using tap water or homemade solutions for lens care 3 5 12.

Conclusion

Acanthamoeba keratitis is a rare but potentially blinding infection that demands prompt recognition and persistent therapy. Early diagnosis is the single most important factor for a good outcome, as is meticulous adherence to treatment regimens.

Key Takeaways:

• Symptoms: Severe pain, photophobia, epithelial defects, ring infiltrate, and stromal infiltrates are characteristic; pain often exceeds visible findings.
• Types: Most cases are caused by genotype T4, but other genotypes can also cause disease; presentations range from superficial epithelial to deep stromal and advanced intraocular involvement.
• Causes: Primarily linked to contact lens use and poor hygiene, but can occur after minor trauma or exposure to contaminated water.
• Treatment: Relies on prolonged topical therapy with biguanides and diamidines, with surgery reserved for severe or unresponsive cases.

By focusing on education, early detection, and rigorous prevention strategies, the risk of this vision-threatening infection can be minimized. If you suspect Acanthamoeba keratitis, seek urgent ophthalmic evaluation—early action saves sight.