Acanthamoeba Keratitis
Organism: Acanthamoeba are free-living protozoa that have been isolated from soil, vegetable material, and fresh, marine and chlorinated water. They have also been isolated from humidifiers, dialysis units and heating, venting and air conditioner units (HVAC). The organisms are unicellular and exist in two forms. The trophozoite form is 15 to 45 microns long with a granular cytoplasm containing mitochondria and has a central round nucleus with prominent nucleolus. It is in the trophozoite form that the motile organism is able to feed on bacteria, fungi and other unicellular organisms. In response to a harsh environment the trophozoite form is able to encyst in a wall of cellulose. The cyst form measures 10 to 25 microns and is resistant to extreme environmental conditions including temperate extremes and desiccation, antimicrobial agents and even chlorine.
Epidemiology: Acanthamoeba keratitis is an uncommon but severe infection. Up to 85% of current cases occur in contact lens wearers, many of whom use tap water, well water, homemade saline solutions or saliva to clean or store the lens. It has been associated with all types of contact lens, including soft, rigid gas permeable and disposable types. Minor trauma to the corneal epithelium likely plays an important role in the pathogenesis and allows infection to progress.
Clinical Manifestations: In the early stages of infection patients usually complain of severe pain, irritation, blurred vision or a foreign body sensation in the eye. Often patient will report a history of minor trauma to the affected eye which might explain why most cases are unilateral. Examination reveals conjunctivitis, corneal ulceration, radial keratoneuritis and stromal infiltrates. The presence of ring form stromal infiltrates is highly suggestive of infection with Acanthamoeba.
Diagnosis: Infections with Acanthamoeba are frequently misdiagnosed as bacterial, fungal or herpes simplex keratitis. For superficial infections a diagnosis can be made by direct microscopic examination or culture of the organism. Wet mounts obtained from corneal scrapings may show the trophozoites and cysts when stained with a variety of methods including acridine orange, Giemsa, or lactophenol cotton blue to name a few. While typically not necessary to establish a diagnosis, corneal biopsies may show the organisms when stained with methenamine silver, periodic acid-Schiff (PAS) or even hematoxylin and eosin. Culture of Acanthamoeba is done on a non-nutrient agar plate coated with a sea of E. coli. The diagnosis is established by observing the trails the Acanthamoeba leave in the E.coli as they gobble up the bacteria (tasty!). Morphologically Acanthamoeba species have a spiny pseudopod known as an acanthapodia along with a contractile vacuole. When the trophozoite form has finished up all of the E.coli on the plate it encysts and the double-walled cysts can be identified.
Treatment: Successful treatment requires early diagnosis and aggressive management. Combination therapy is recommended and includes combinations of topical antiseptics and systemic antimicrobials. The patient presented received a combination of topical chlorhexidine, hexamidine and oral itraconazole. Treatment failures may be due to resistance, poor penetration of the drug into the ulcer and induced encystment from subcysticidal drug levels.
