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Trends point to strategies for management of parasitic keratitis
Although relatively rare, cases of Acanthamoeba keratitis have been on the rise for more than a decade, for reasons not entirely understood.
Take-home:
Although relatively rare, cases of Acanthamoeba keratitis have been on the rise for more than a decade, for reasons not entirely understood.
By Nancy Groves; Reviewed by Jeremy Keenan, MD, MPH
San Francisco-Acanthamoeba keratitis (AK) is one of the most feared infections in ophthalmology; patients are at risk of permanent visual impairment and blindness. Though rare, the number of cases is much higher than little more than a decade ago, and questions persist not only about its etiology but also about the best approaches to prevention and treatment, said Jeremy Keenan, MD, MPH.
“We’re seeing more Acanthamoeba now-in the recent past-than we did before,” said Dr. Keenan, associate professor, Francis I. Proctor Foundation, University of California, San Francisco. “This has been borne out in several different observational epidemiologic studies.
“It’s not really clear why we’re seeing more now, but these studies suggest that Acanthamoeba keratitis is a relatively larger public health problem than it used to be and that we should devote more energy to try to find out why this is the case and how we can prevent the infection from happening,” he added.
Explaining the increase
Data show that the yearly number of cases at 13 sentinel ophthalmology clinics and laboratories in the United States gradually increased from 22 in 1999 to 43 in 2003. Prevalence (both culture-confirmed and non culture-confirmed cases) then more than doubled from 2003 to 2004, when 93 cases were reported, signaling the beginning of a marked increase that peaked at 170 cases in 2007. Despite fluctuations, the prevalence has remained higher than before the pre-outbreak levels. Although aggregated numbers are not available for recent years, the numbers of culture-proven cases identified at the Proctor Foundation Microbiology Laboratory have remained elevated through 2013.
Interpreting these figures to explain the increase in cases has been a challenge, in part due to limited studies on the incidence of AK, Dr. Keenan said.
Testing some theories
The population at highest risk for infection is contact lens wearers, who constitute at least 85% of cases. Increasing numbers of contact lens wearers means that more individuals will develop the infection, even if the proportion of affected users within the entire contact lens wearing population remains stable.
Acanthamoeba is a widespread environmental pathogen commonly found in soil, water, air, and manmade systems, such as heating and air conditioning units, and nearly everyone is exposed to it. Yet, since contact lens wear is the number-one risk factor, it is possible that contact lenses alter ocular surface immunity and increase susceptibility to infection, Dr. Keenan said.
Changes in the materials used to make lenses or in cleaning solutions are also possible factors, as demonstrated by an association between cases of AK and a contact lens care solution (Complete MoisturePlus, Advanced Medical Optics) found in mid-2007. The solution was voluntarily recalled, but the continuing elevated case rate suggests that this contact lens solution was not the only causative factor in the AK epidemic.
Detection bias
Another theory is detection bias. Some experts believe that the rate of AK was always high, but because physicians did not suspect it was the cause of patients’ symptoms, they did not conduct tests that would have diagnosed it. Now, with tools such as confocal microscopy, it is easier to see suspected acanthamoeba cysts in vivo and then conduct tests to identify them, Dr. Keenan said.
“It’s possible that we’re overdiagnosing it now that we have this new tool,” he said.
However, like other theories, the use of confocal microscopy cannot in itself explain the number of cases since this would result in a much higher proportion of non culture-confirmed cases, which currently represent only a small portion of the total.
Prevention and treatment strategies
Better understanding of AK may lead to more effective prevention and treatment approaches. Wearing daily disposable contact lenses is not a foolproof strategy for preventing bacterial infections, although the effect on development of AK specifically is not known, Dr. Keenan said.
However, using disposable lenses does remove one variable from the list of potential risk factors.
Lens wearers who do not use disposable lenses may reduce their risk of AK by choosing a hydrogen peroxide disinfecting solution. Removing lenses before they go to sleep or go swimming is another highly recommended hygiene tip.
Dr. Keenan also has seen cases of AK in children who wear specially designed hard contact lenses at night, as part of an orthokeratology treatment for nearsightedness, and for this reason does not recommend this therapy.
Treatment of AK can be tricky, as there are no FDA-approved medications specifically for this infection, Dr. Keenan said.
Physicians instead rely on antiseptic medications that act against a variety of infections. Biguanide agents, such as chlorhexidine and polyhexamethylene biguanide, are the mainstay of treatment and have been found to be the most effective for killing acanthamoeba cysts in vitro. Diamidine agents, such as propamidine and hexamidine, are second-line agents, with demonstrated but inferior cysticidal activity.
Because AK is a serious infection, physicians tend to use multiple agents simultaneously. However, using several different drops may mean that patients get less of the more effective drug and also may increase the risk of complications, such as cataract and glaucoma.
“It’s not really clear how we should be doing this,” Dr. Keenan said.
Topical corticosteroid therapy must be managed carefully, since steroids have a role in treatment of AK by reducing inflammation but could aggravate the condition if not administered concurrently with anti-amoebic therapy. Most corneal specialists would treat first with 2 to 4 weeks of anti-amoebic therapy and then add a topical corticosteroid to this regimen only if the patient has active inflammatory changes.
Jeremy Keenan, MD, MPH
P: 415/476-6323
Dr. Keenan did not report any financial disclosures.
