Loay Nawaz Rahman

Imperial College Healthcare NHS Foundation Trust, London, United Kingdom.

Introduction 

Acanthamoeba keratitis involves inflammation of the cornea, the thin protective transparent layer at the front of the eye, caused by the protozoa Acanthamoeba. It is rare but can be sight-threatening with an annual incidence of 1.4 per million per annum, and typically affects contact lens wearers (1).

Aetiology

Acanthamoebae are free-living protozoan that are frequently found in soil and water such as in swimming pools, tap water, shower water and contact lens solutions. They can cause infections in the eye, skin and central nervous system. The most common species of Acanthamoebae that cause keratitis are A. castellani and A. polyphaga (2).

Risk factors

More than 90% of Acanthamoeba keratitis cases are associated with contact lens use, particularly soft lenses (2). The contact lens may be exposed to contaminated lens solution, tap water or through use while bathing and swimming. The Acanthamoeba can also enter the eye from soil, vegetation and ocular trauma.

Pathophysiology

The Acanthamoeba survives between the contact lens and the surface of the eye. As soft contact lenses are more adherent to the surface of the cornea than hard lenses, the Acanthamoeba can survive and bind to the glycoproteins on the corneal surface. It can then invade the eye leading to inflammation of the cornea (keratitis) that may progress to endophthalmitis. Contact lens use increases the expression of glycoproteins on the corneal surface, resulting in it being more prone to invasion (2).

Clinical Presentation

As Acanthamoeba keratitis is rare, it is often misdiagnosed in the early stages as bacterial, viral or fungal keratitis. It should always be considered in patients with contact lens use or recent corneal trauma. Typical symptoms include severe pain (although can be painless in the early stages), reduced visual acuity, redness of the eye, photophobia and epiphora.

Slit lamp examination may reveal epithelial or subepithelial infiltrates, pseudodendrites (‘dendritic’ lesions not caused by herpes simplex virus) and punctate keratopathy (fine punctate corneal epithelial damage). These are also present in herpes simplex keratitis and can therefore lead to a misdiagnosis.

As the disease progresses and the corneal inflammation becomes deeper, a central or paracentral white ‘ring infiltrate’ can form on the cornea. Overtime, the opacification can develop throughout the cornea, as shown in figure 1. It can lead to corneal ulceration and endophthalmitis with formation of a hypopyon. If left untreated, it can lead to permanent visual impairment or blindness (3).

Investigations

Diagnosis can be made by taking corneal scrapings that are sent for microscopy, culture and sensitivity (MC&S) to identify any Acanthamoeba growth. The contact lenses are also sent for MC&S.

Management

Once it is suspected or diagnosed, it must be treated urgently as it can lead to permanent vision problems. Management of Acanthamoeba Keratitis revolves around the use of biguanides, such as chlorhexidine or polyhexamethylene biguanide (PHMB), and diamidines such as Brolene (propamidine isetionate). These drops can be combined with systemic analgesics for pain management, topical steroids to reduce inflammation, and topical antibiotics to prevent secondary bacterial infection (4).

Guidelines

Management guidelines for Acanthamoeba keratitis differ between hospitals. The following is based on guidelines from Imperial College Healthcare NHS Foundation Trust (5).

First line management

First line management is dual therapy. This involves a combination of either Polyhexamethylene Biguanide (PHMB) (0.02% drops) or Chlorhexadine (0.02% drops) with Brolene (0.1% drops). The eye drops need to be applied every hour, day and night, for the first 48 hours. Following this, they are applied hourly in the day time for 72 hours, then every 2 hours in the day time for 3-4 weeks. Continuous treatment may be necessary for weeks or months.

Second line management

If there is no improvement then triple therapy (PMHB (0.02% drops) + Chlorhexadine (0.02% drops) + Brolene (0.1% drops)) should be considered as well as a discussion with infectious diseases.

Conclusion

Acanthamoeba keratitis is a rare condition that commonly affects contact lens users. It is a sight threatening condition that can often be misdiagnosed and therefore early assessment, diagnosis and treatment are paramount.

References

1. Microbial keratitis (Acanthamoeba sp.) [Internet]. College-optometrists.org. 2021 [cited 7 November 2021]. Available from: https://www.college-optometrists.org/clinical-guidance/clinical-management-guidelines/microbialkeratitis_acanthamoebasp 

2. Auran, James D. M.D.; Starr, Michael B. M.D.; Jakobiec, Frederick A. M.D. Acanthamoeba Keratitis, Cornea: Volume 6 – Issue 1 – p 2-26 

3. Lorenzo-Morales J, Khan N, Walochnik J. An update on Acanthamoeba keratitis: diagnosis, pathogenesis and treatment. Parasite. 2015;22:10. 

4. Acanthamoeba Keratitis Treatment [Internet]. American Academy of Ophthalmology. 2021 [cited 7 November 2021]. Available from: https://www.aao.org/current-insight/acanthamoeba-keratitis-treatment 

5. Acanthamoeba Keratitis. Adult and Children Treatment of Eye Infections and Ophthalmology Handbook. Imperial College Healthcare NHS Foundation Trust. 2021.