Description of the condition
Acanthamoeba, a genus containing at least 24 species of free-living amoebic protozoa (single-celled organisms), is ubiquitous in nature, existing both in soil and in nearly all water sources and supplies. Worldwide, Acanthamoeba has been found in seawater, lakes, rivers, and streams and is commonly found in water supplies, such as tap and bottled water, drinking fountains, eye wash stations, dental units, and dialysis machines. Acanthamoeba can be isolated from the upper respiratory tracts of humans, 50% to 100% of whom have antibodies to it (Alizadeh 2001). The life-cycle of Acanthamoeba is characterized by two stages: 1) a motile, feeding, and replicating trophozoite form, which is the most common form found in water, and 2) a double-walled dormant cyst (Park 2002). Trophozoites feed on a variety of organisms, including cyanobacteria, bacteria, fungi, and other protozoa. Trophozoite encystment allows the organism to survive adverse conditions such as nutrient deficient environments, extreme temperatures and pH, desiccation, and chemical exposure. In the resilient double-walled cyst form, Acanthamoeba can survive for years under such adverse conditions (Biddick 1984; Brandt 1989; Neff 1969). Trophozoites and cysts vary in size from 25 to 50 µm. Acanthamoeba reproduce by asexual binary fission.
Despite its near universal presence, Acanthamoeba infection in humans is relatively uncommon. The combination of corneal epithelium barrier disruption, whether from trauma or from contact lens wear, and exposure to a sufficient inoculum of Acanthamoeba substantially increases the risk of keratitis (inflammation of the cornea) (Garate 2006; Ibrahim 2007). There are a number of steps that take place after exposure, before keratitis results. First, a pathogenic trophozoite attaches to the outer corneal epithelial cells via binding proteins. The attached trophozoite then secretes an enzyme that desquamatizes (peels back) the corneal epithelial cells allowing the trophozoite to invade the middle stromal layers of the cornea. Once inside the stroma, the trophozoite secretes collagenolytic factors that dissolve the stromal matrix, mediating an inflammatory response. Corneal cell death and keratitis result from the inflammatory response (Alizadeh 2005). Acanthamoeba infection can result in the loss of the eye if unresolved. Acanthamoeba castellanii (T4 genotype) is the most common reported species to cause Acanthamoeba infection, accounting for 94.3% of keratitis cases and 79.3% of those with skin, lung, and/or brain infections (Awwad 2007; Walochnik 2000).
Acanthamoeba keratitis (AK) was first described in the early 1970s with a dramatic increase in observed cases occurring in the early to mid-1980s (Naginton 1974; Stehr-Green 1989). This rise was associated with the increasing use of contact lens wear. In 1985, the US Centers for Disease Control issued a report to alert ophthalmologists about the association between contact lens wear and this difficult infection (US DHHS 1986). The development of AK in contact lens users is much more strongly related to poor lens hygiene and contaminated water than is bacterial keratitis, for which the overnight wear of lenses remains the dominant risk factor (Dart 2008). Swimming in contact lenses, irregular or inadequate disinfection of contact lenses, cleaning the lens case with tap water, minor corneal trauma, and exposure to contaminated water are all additional risk factors for developing AK (Radford 2002; Stehr-Green 1987).
Contact lens wear remains the most common risk factor for development of AK. In recent case series, contact lens wear was reported in 80% to 86% of cases (Butler 2005; Claerhout 2004; Radford 2002). Generally, AK is a rare infection and comprises less than 5% of contact lens-related microbial keratitis (Butler 2005; Claerhout 2004). The incidence of AK is estimated at 0.33 to 1.0 per 10,000 hydrogel contact lens wearers per year (Seal 2003). Extended wear soft contact lenses (SCLs), which are worn overnight, and daily wear SCLs, which require daily cleaning and overnight storage, carry higher risks of infection than daily disposable lenses, which are not worn or stored overnight (Butler 2005). The incidence of AK in rigid contact lens wearers is 9.5 times lower than for SCL wearers (Seal 2003). Recently, however, patients using rigid contact lenses overnight for orthokeratology, a clinical technique to reshape the cornea to correct refractive errors, have displayed high rates of AK (Watt 2005).
Contact lens storage cases may be colonized with Acanthamoeba, regardless of whether the user develops infection. For example, studies examining contact lens cases from individuals in the UK asymptomatic for any ocular infection have shown that between 4% and 7% of lens storage cases were colonized with Acanthamoeba (Seal 1999). The unfortunate fact is that most commercially available contact lens disinfection solutions are ineffective against Acanthamoeba (Hiti 2005; Johnston 2009; Radford 2002). Further, contaminated contact lens disinfection solutions have been directly linked to outbreaks of Acanthamoeba infection among contact lens wearers (Acharya 2007; Stehr-Green 1987; Yoder 2012).
There is a 10 to 15 times higher rate of AK reported in the UK than that noted in the US: as few as 0.15 per million in the US (Stehr-Green 1989) to as high as 1.4 per million in the UK (Radford 2002; Seal 2003). Rates of AK in other countries fall somewhere between the US and UK rates, with diverse rates among geographic locations (Watson 2012). Additionally, certain regions of the US have higher rates of AK compared with other regions. Differences in national and regional rates have been attributed to water supply factors, including the colonization rate of pathogens in the domestic water supply, the higher prevalence of bacteria at the ends of the water distribution system, and the subsequent use of these waters to store SCLs and to prepare the home made lens care saline solutions (Gray 1995; Kilvington 2004; Lindsay 2007).
Presentation and diagnosis
The most common symptom of AK is severe pain, which is out of proportion to the clinical signs and associated with photophobia, blurry vision, and tearing. AK usually presents in one eye, although disease may be bilateral in contact lens wearers (Kim 2010). Absence of pain does not exclude diagnosis. Later, patients may develop ring infiltrates, hypopyon, and/or diffuse inflammation.
Diagnosis and treatment of Acanthamoeba are difficult. The most important step in diagnosing AK is to suspect it. Early diagnosis and prompt delivery of appropriate medical therapy is essential to secure a good prognosis. If effective therapy is delayed for three weeks or more the prognosis deteriorates (Claerhout 2004; Tu 2008). AK should be considered in any case of corneal trauma complicated by exposure to soil or contaminated water and in all contact lens wearers, especially in those with significant pain or poor response to first-line therapy for bacterial or herpes simplex virus (HSV) keratitis. Even when there has been a positive culture for another organism, Acanthamoeba may still be present since 10% to 23% of cases of AK may be polymicrobial or co-infected with HSV (Mathers 1997; Sun 2006).
Confirmation of infection, even when the diagnosis is suspected, is also challenging. Diagnosis is made much more easily when the disease is in its early stages and superficial in the cornea. Confocal microscopy is a noninvasive tool that allows visual confirmation of cysts in the eye and may aid in the diagnosis of Acanthamoeba. The cystic form of Acanthamoeba appears as a double-walled, hexagonal, hyperreflective structure that is 10 to 25 μm in diameter (Rezaei Kanavi 2012; Winchester 1995). The trophozoite form and surrounding swollen nerves also can be seen by confocal microscopy, but it may be difficult to differentiate the trophozoites from normal corneal keratocyte nuclei. The trophozoite form may appear as a pear-shaped structure 15 to 45 μm in diameter with a surrounding bright halo (Rezaei Kanavi 2012). Corneal scrapings also may demonstrate the cyst and trophozoite forms. Culturing the scrapings on non-nutrient agar overlaid with Escherichia coli may show characteristic trails (markings) as trophozoites move across the plate to feed on the bacteria (Borin 2013; Dart 2009; Winchester 1995). Corneal scrapings stained with Giemsa, periodic acid–Schiff (PAS), hematoxylin and eosin (H&E), calcofluor white, or acridine orange stains may be used to identify the cyst and trophozoite forms. Once deeper involvement of the stroma occurs, a corneal biopsy may be necessary.
The sensitivity and specificity of diagnosis using confocal microscopy compared with the conventional method of corneal scrapings has been reported to be as high as 88% and 91%, respectively (Vaddavalli 2011). Identification of Acanthamoeba by polymerase chain reaction (PCR) showed a sensitivity of 84% and a specificity of 100%; it was positive in 16 of 19 epithelial samples (84%) compared to 10 of 19 (53%) culture-positive (Lehmann 1998). Neither confocal microscopy nor PCR can be expected to distinguish between viable and nonviable Acanthamoeba cysts; this leaves culture of corneal scrapes or biopsies as the only certain way of identifying persistent culture-positive disease. Unfortunately, a negative culture result does not confirm that all the viable organisms have been eradicated unless a substantial corneal excision biopsy has been taken.
In non-contact lens wearers the diagnosis of AK is more difficult and often takes longer than in contact lens wearers as the suspicion for this infection is usually lower (Sharma 2000; Speer 2003), despite comprising 3% to 15% of AK cases in the UK and US (Stehr-Green 1989). Due to delays in diagnosis, non-contact lens wearers tend to have worse visual outcomes than those with contact lens-associated AK. AK in non-contact lens wearers is usually associated with trauma, exposure to contaminated water or soil, and agricultural work (Stehr-Green 1989; Sun 2006).
Description of the intervention
The goals of medical therapy in AK include the eradication of viable cysts and trophozoites along with rapid resolution of the associated inflammatory response. The first medical cure was noted in the mid-1980s through the use of topical propamidine and neomycin (Wright 1985). Propamidine, a diamidine anti-amoebic, is commonly administered as eyedrops (0.1%), but may also be applied as an ointment (0.15%). Hexamidine, usually administered as 0.1% eyedrops, is another type of diamidine anti-amoebic used for treating AK. Biguanides are another class of anti-amoebics that may be used. Two biguanides that are in use are polyhexamethylene biguanide (PHMB) 0.02% to 0.06% and chlorhexidine 0.02% to 0.2%. Other topical drugs that have been used for the treatment of AK include antibiotics (e.g., aminoglycoside, neomycin) and antifungals (e.g., azole, itraconazole, metronidazole, voriconazole). Oral itraconazole has been used in severe cases to prevent the potential spread of trophozoites into adjacent tissues (Sun 2006; Thebpatiphat 2007). There are currently no drugs licensed for the treatment of AK in the US. Brolene (propamidine isethionate 0.1%; Sanofi-Aventis, Australia) is licensed in Australia.
Although the current anti-amoebics show low or very low minimal cysticidal concentrations, the diamidines and biguanides are currently the most effective cysticidal anti-amoebics in vitro and their use is supported by multiple case series (Dart 2009). However, the clinicopathologic correlation between the in vitro sensitivities and the clinical outcome has been less than satisfactory, with treatment failing despite the use of topical preparations at 10 to 60 times the minimal cysticidal concentration for biguanides in vitro. Although the reasons for this difference are not well understood, it is possible that therapeutic intrastromal concentrations are not being achieved, that drugs may bind to tissue components or that they may be inactivated in vivo, or that organisms in vivo intrinsically are more resistant than in vitro. Repeated epithelium debridement is used in some centers to improve drug penetration (Pérez-Santonja 2003; Sun 2006).
The role of steroids in treating AK is controversial and unnecessary in most cases diagnosed early, as these cases usually respond rapidly to anti-amoebic drugs (Dart 2009). Further, steroids should not be started until most organisms have been killed by using anti-amoebic drugs for a few weeks. Steroids may be helpful in ameliorating persistent inflammation of the cornea, anterior chamber, or sclera. In cases where the inflammation has spread to the limbus (limbitis) or sclera (scleritis), oral nonsteroidal anti-inflammatory treatment (NSAIDs) or even oral steroid might be indicated (Dart 2009). Scleritis can be severe and result in scleral necrosis and uncontrollable pain in which enucleation is indicated.
In cases of corneal perforation or corneal abscess not responding to medical treatment, therapeutic keratoplasty (corneal transplantation) is indicated. Optical penetrating keratoplasty for the purposes of improving vision should be delayed until the infection has been treated and there is no evidence of recurrence for a few months after stopping the medication (Awwad 2007).
Cryotherapy can be used for refractory cases, but persistent corneal edema with endothelial failure may occur (Oh 2010). In vitro studies of AK have shown that cryotherapy kills trophozoites but not cysts unless combined with medical treatment (Matoba 1989).
How the intervention might work
When Acanthamoeba face environmental stress the trophozoites transform to thick-walled cysts. Thus, persistent or recurrent infection often is related to the presence of Acanthamoeba cysts (Pérez-Santonja 2003). Because of the transformation to cysts in the human body, effective medical treatment of Acanthamoeba infection demands cysticidal drugs (Hammersmith 2006; Hay 1994).
Cataract, iris atrophy, glaucoma, and peripheral ulcerative keratitis have all been reported as complications of severe and prolonged AK and have been attributed to toxicity from the use of topical biguanides, diamidines, or both (Herz 2008; Murthy 2002). Although a possibility, it is currently uncertain that anterior segment ocular complications are drug-induced. Investigators of several studies have reported using topical biguanides without any apparent ocular complications (Dart 2009). It may be likely that complications result more from immune reactions to the infection than from drug toxicity.
Why it is important to do this review
Our goal is to review and summarize the medical management of AK systematically. Medical management of AK includes a few classes of anti-amoeba agents, none of which are licensed for use in AK in the US. There are no formal guidelines or standard of care in treating this infection and most of the current treatment regimens are based on personal experience.