ACANTHAMOEBA KERATITIS

Acanthamoeba keratitis remains a rare condition and particularly in developing countries. However, the numbers of these infections have been rising significantly in the US and other industrialised nations, and as the populations in underdeveloped areas become more affluent, there is the potential for a geometric increase in cases, said Elmer Tu MD, speaking at Cornea Day during the 26th Asia-Pacific Association of Cataract & Refractive Surgeons meeting in Singapore.

Dr Tu, professor of clinical ophthalmology and director of the cornea service in the Department of Ophthalmology and Visual Science at the University of Illinois Eye and Ear Infirmary, Chicago, US, explained that contact lens wear is the main risk factor for Acanthamoeba keratitis. Therefore, whereas environmental conditions in developing countries are much more conducive to the development of corneal infection overall, the relative rate of Acanthamoeba keratitis has been disproportionately low in these areas compared with developed nations.

“Currently, most cases of Acanthamoeba keratitis in underdeveloped countries have been in patients who are not contact lens wearers. Nevertheless, the level of Acanthamoeba keratitis is still higher than would be expected given the population characteristics,” he said. “Recently, however, we are seeing increases in Acanthamoeba keratitis in Brazil and urban areas of China corresponding with growth in contact lens use. As the number of lens wearers increases in other developing countries in the future, these nations may be facing an enormous health burden from Acanthamoeba keratitis.”

Dr Tu also pointed out that investigations conducted following Acanthamoeba keratitis outbreaks in the US indicate the importance of environmental exposure through contaminated water. Whereas a specific multipurpose contact lens solution was identified as a risk factor, it was used in fewer than half of the cases, and the number of cases of Acanthamoeba keratitis did not return to the baseline level after recall of the lens solution. “Acanthamoeba keratitis is clearly not just a solution problem. Rather, there is an increase in load from the environment that is overwhelming most of the commonly used disinfection systems, which, almost universally, are not very effective against Acanthamoeba cysts,” Dr Tu said. Dr Tu observed that ophthalmologists fully recognise that Acanthamoeba keratitis is a treatment challenge. However, he proposed that part of the problem in managing these cases has been lack of suspicion resulting in delayed diagnosis.

“Thankfully, with increased awareness and diagnostic acumen, outcomes for patients affected by this parasitic infection are improving.” Issues to keep in mind are that in contrast to cases of contact lens-related bacterial keratitis that are more likely in patients wearing soft versus hard contact lenses, the incidence of Acanthamoeba keratitis is similar regardless of lens material type. Clinicians should also be aware that orthokeratology lenses are associated with a very high rate of Acanthamoeba keratitis, and they should recognise there is an appreciable risk of bilateral infection.

“In every large series, about seven per cent to 11 per cent of patients who present with Acanthamoeba keratitis in one eye either have infection in the fellow eye at the time of presentation or develop it within the next few months. The take-home message is to be careful not to concentrate solely on the symptomatic eye,” noted Dr Tu. Discussing some recent research findings regarding treatment issues, Dr Tu noted that in trying to identify factors associated with a poor outcome, univariate analysis showed an interesting association with treatment with an antibiotic not containing benzalkonium chloride (BAK) prior to diagnosis of Acanthamoeba keratitis. Corresponding with that information, results of a recent in vitro study undertaken by Dr Tu and colleagues showed that concentrations of BAK equal to or below those found in commercially available ophthalmic anti-infectives have significant anti-Acanthamoebal activity.

“The activity of BAK against some strains of the parasite even matched that of hydrogen peroxide. I would impart to clinicians that BAK is an effective amoebacidal agent that may have an effect on the patient’s presentation and diagnosis as well as possibly having a therapeutic effect, although the latter requires further study,” he said. Standard medical treatment for Acanthamoeba keratitis that includes propamidine plus a biguanide can be very effective. In addition, systemic voriconazole appears useful in the management of patients with deep corneal infection. Recently published data also indicate that corticosteroid use is largely unnecessary in managing Acanthamoeba keratitis. “However, in patients with severe inflammation, it may be possible to salvage the eye only by adding a corticosteroid or other immunosuppressant,” Dr Tu said. He noted that while there has been excitement about cornea collagen crosslinking (CXL) for treatment of infectious keratitis, this procedure has no in vitro effect on the viability of Acanthamoeba cysts. “CXL is probably not curative for Acanthamoeba keratitis and should not be used as primary therapy. However, it may be a good adjunctive therapy in the future because of its benefit for stabilising the corneal collagen,” he said.