Acanthamoeba keratitis (AK) is a rare condition (prevalence of 1 to 9/100,000 people) caused by a genus of amoeba that is almost ubiquitously found in the environment (Król-Turminska and Olender, 2017; Casero et al, 2017). It is an amphizoic organism that has two distinct life phases; a trophozoite stage is active and reproduces, and a cyst stage is dormant and resistant to stress/disinfection. While AK is rare, 83% to 93% of cases are associated with noncompliant contact lens (CL) use (Casero et al, 2017; Daas et al, 2017).
The main risk factor associated with AK is CL- or non-CL-induced corneal trauma followed by engaging in a behavior that exposes the individual to Acanthamoeba (Król-Turminska and Olender, 2017; Johnston et al, 2009). Specifically, CL noncompliance (topping off solutions, omitting the disinfection step) and water exposure (rinsing lenses/case with tap water, swimming with CLs) are the most commonly cited behaviors (Casero et al, 2017; Brown et al, 2017). We still see many patients who fail to understand the importance of avoiding tap water. And, some GP CL care systems still recommend tap water as part of their cleaning regimens (Zimmerman et al, 2017).
Signs and Symptoms
Early AK usually presents as a unilateral red, watery eye that may have stromal infiltrates, blurry vision, photophobia, and severe pain that is disproportionate to the signs (Casero et al, 2017; Brown et al, 2017). Early AK is also often misdiagnosed as being viral (47.6%); this is problematic because delayed treatment is much more likely to result in permanent vision loss (Daas et al, 2017; Brown et al, 2017).
Late AK frequently has a pathognomonic ring-shaped corneal infiltrate; the eye may even perforate from extreme corneal degradation (Król-Turminska and Olender, 2017). Confocal microscopy is highly specific and sensitive to diagnosing AK; culturing and histological analysis are also commonly used for diagnosis (Daas et al, 2017).
AK pathogenesis begins with the amoeba binding to the corneal epithelium in conjunction with an epithelial break, followed by Acanthamoeba-induced corneal epithelium desquamation with subsequent Bowman’s membrane penetration (Lakhundi et al, 2017). Acanthamoeba then release proteases, which degrade and weaken the corneal stroma (Lakhundi et al, 2017). The amoeba tend to concentrate around corneal nerves, damaging them; this may account for the extreme pain associated with AK (Lakhundi et al, 2017). AK is usually treated with a combination of polyhexamethylene biguanide (kills trophozoites) and chlorhexidine (kills cysts), though results are highly variable, and treatment frequently fails (Hadaś et al, 2017). Recent data suggest that tea tree oil may be a potential new treatment for AK because it is able to penetrate deep into the cornea while having low toxicity and acts against both trophozoites and cysts (Hadaś et al, 2017).
Preventing AK begins with effectively caring for CLs. But, selecting an effective care system is a challenge because the U.S. Food and Drug Administration (1997) is silent with regard to providing Acanthamoeba disinfection standards. Nevertheless, it is not uncommon for care systems to produce a full log reduction in Acanthamoeba cysts and trophozoites (fungal standards) (Kal et al, 2016). The literature suggests that hydrogen peroxide systems are more effective than multipurpose systems are, though discrepancies do exist (Johnston et al, 2009; Kal et al, 2016). Overall, our best method of prevention is to continue to educate our patients about the benefits and dangers of wearing CLs. CLS
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