American Academy of Ophthalmology
In an attempt to find a cause for the rise in Acanthamoeba keratitis (AK), Thebpatiphat et al.1 conducted a retrospective consecutive case series of 20 eyes with AK between 1995 and 2005. They found that multiple-use contact lens wear continues to be an important risk factor, and that multipurpose solutions do not have adequate cysticidal effect, even when used properly. Other risk factors were exposure to well or contaminated water, swimming while wearing contact lenses and overnight wear.
In this series, herpes simplex virus (HSV) keratitis was the leading misdiagnosis, and the worst outcomes were seen in cases of ring ulcers and deep stromal involvement. Thebpatiphat et al conclude that because of the increasing use of contact lenses (especially extended wear) and the concomitant rise in AK, early diagnosis and effective early treatment are paramount.
A study by Lee et al.2 evaluated the efficacy and toxicity of current treatments for AK, including polyhexamethylene biguanide (PHMB) and chlorhexidine. When the minimal cysticidal concentration and the keratocyte toxicity of these therapies were compared, chlorhexidine was found to be less cytotoxic than PHMB, although it maintained effective cysticidal properties.
Although this study was performed in vivo, it aids in our clinical understanding and approach to this difficult-to-treat infectious agent. Perhaps the first-line treatment after prevention should be the less toxic biguanide, chlorhexidine with or without other classes of anti-amoebic medications such as propamidine and neomycin.
Whether steroids should be added to the treatment regimen for AK remains controversial. An in vitro assay conducted by McClellan et al.3 in 2001 found that the addition of dexamethasone leads to conversion to trophozoite and increased infectious load. However, the researchers did not examine the effect of steroid treatment with concomitant treatment with amoebicidal agents.
In an earlier retrospective review, Park et al.4 showed that topical steroid use extends AK treatment time of but does not have a statistically significant effect on the clinical outcome. However, they noted that topical steroids may help treat the inflammatory response associated with AK, and they can also be used in eyes requiring penetrating keratoplasty (while concomitantly treating with anti-amoebic medications such as biguanides, propamidine or neomycin). Maxitrol (neomycin sulfate, polymyxin B sulfate and dexamethasone) can be useful in this application. Again, chlorhexidine would be a less toxic and effective adjunctive amoebicidal drug of choice.