In a series of patients with laboratory-proven acanthamoeba infection and a subsequent corneal swabbing performed, the median microbial clearance time was 6–7 weeks in our unadjusted model, with an IQR of approximately 4–12 weeks. Better initial visual acuity was significantly associated with shorter microbial clearance times. The use of corticosteroids before presentation and stromal involvement appeared to lengthen the duration of infection, though these associations did not reach statistical significance.
The long duration of corneal infection in this study is consistent with other reports. A previous study noted viable acanthamoeba organisms as long as 23 months after initial presentation in a severe case, non-responsive to anti-acanthamoebal therapy.18
Treatment durations have been reported to last approximately 13 weeks; however, to our knowledge, median duration of active infection has never been characterized.19
In our patient population, the IQR of clearance times was 22–82 days (approximately 4 to 12 weeks), indicating that by 13 weeks of treatment, at least 75% of our patients would have cleared their infection.
We recognize that the median microbial clearance time from the unadjusted survival analysis is likely biased, as patients with worse disease would be more likely to have been recultured. Indeed, the patients recultured in our series had worse 6-month visual acuity than those patients not recultured and were more likely to require a penetrating keratoplasty. Therefore, we created a model to predict clearance time, given significant predictive factors as determined by univariate analyses, and used this model for all cases, whether they had been cultured more than once or not. This resulted in a decreased clearance time estimate of approximately 5–6 weeks for the single and repeat culture groups combined. However, these results should be interpreted with caution, as they were derived from a specific model of clearance time. An unbiased estimate of clearance time would require that all patients with acanthamoeba keratitis be cultured at regular intervals, regardless of disease severity. In the absence of these data, we have attempted instead to account for the inherent biases in the data using statistical methods.
The sensitivity of acanthamoeba culture and smear are unknown, but even the combination of the two tests is almost certainly imperfect.17
In addition, initiation of anti-acanthamoebal therapy could reduce the sensitivity of culture and smear, as topical therapy might clear superficial infection but not deeper infection. In other words, some repeat cultures were likely false negatives, leading to an underestimate of the median clearance time. We estimated the clearance time, assuming that a portion of our repeat cultures had been misclassified as negative. This model found a slight increase in clearance time. The true-negative predictive value and sensitivity of culture and smear in our facility is unknown, hence, if the actual sensitivity of the test were lower than our assumed value of 83%,17
the expected clearance time would be longer. In a sensitivity analysis to address this possibility, we found that if the sensitivity of smear and culture was reduced to 60%, the clearance time increased by approximately 10 days. If the actual sensitivity of smear and culture were <60%, the clearance time estimates would increase more; in fact, if the sensitivity of culture were in reality as low as 10%, as has been claimed in reports using confocal as a gold standard,1
then our method would be unable to estimate a clearance time. However, it should be noted that culture-negative cases diagnosed with confocal clear, on average, quicker than culture-positive cases.1
Future studies could examine the expected clearing time of cysts on confocal.
A weakness of culture and smear as diagnostic techniques for acanthamoeba is that they may not identify stromal cysts too deep to be collected by swabbing. Confocal microscopy might be able to identify many more cases of acanthamoeba keratitis than are identified by culture and smear.1
The analysis did not include diagnoses made with confocal microscopy, a technique, which in theory allows the identification of deeper stromal cysts, and could increase the overall sensitivity to detect acanthamoebal infection, and potentially extend the estimated clearance time.7, 17, 20, 21
This study should be repeated with confocal microscopy to determine if the clearance time estimates change with different diagnostic techniques. Identification of acanthamoeba infection might also depend on the quantity of organisms, which we did not take into account in our statistical analyses.
Worse initial BCVA was significantly associated with longer clearance times in a univariate model. In this model, each Snellen line (0.1 logMAR) corresponds to approximately a 10% increase in clearance time. Patients who present with worse BCVA likely have been infected for longer and have more severe disease than those who have better initial BCVA, and may therefore require more time to clear the infection. In a prediction model, patients with excellent vision at baseline (20/20) had shorter clearance times than patients with worse vision at baseline.
The prior use of topical corticosteroids appeared to lengthen the time until acanthamoebal clearance, though this association was not statistically significant. Our observation is nevertheless consistent with other reports, which have shown that use of topical corticosteroids before acanthamoeba diagnosis and treatment may lengthen the duration of infection.7, 22, 23
We also found that eyes with stromal involvement at presentation seemed to have longer microbial clearance times compared with those without stromal involvement, though this too lacked statistical significance. This finding is consistent with previous studies that have shown an association between deep stromal keratitis and worse disease outcomes.3, 7
In this report, we estimate that the median clearance time of acanthamoeba from corneas treated with standard anti-acanthamoebal medications is 42.5 days (IQR 22–82 days). This indicates that 50% of our cases cleared between 4 and 12 weeks, though clearly the duration of treatment should be tailored for each individual case. Decreased visual acuity at presentation was significantly associated with a prolonged duration until microbial clearance. The duration of acanthamoebal infection was longer in those treated with topical corticosteroids compared with those who were not, though this association was not significant. This study provides evidence supporting the use of extended treatment protocols, especially for those presenting with decreased vision. The median clearance time found in this study is shorter than treatment durations typically reported in the literature.19
Note that this estimate is of the median, and that 50% of cases would be longer than 42 days. Because of this, it would be prudent to give antimicrobial therapy for longer than the median clearance time. As clearance is apparently correlated with severity, treatment duration may be tailored according to the response of each individual patient. Limitations of this study include a small sample size, and that there was likely a bias in our estimates. However, we attempted to correct for bias due to case selection or misclassification error. A larger, prospective study could be carried out to confirm the findings of this study, and might include confocal microscopy to allow identification of stromal infection with a sterile surface. In addition to clinical healing time and visual acuity, time to microbiological cure could be considered as an outcome for future clinical trials of antiparasitic agents.