Acanthamoeba Keratitis

Article Author:
Shaan Somani
Article Author (Archived):
Yasmyne Ronquillo
Article Editor:
Majid Moshirfar
Updated:
10/17/2019 7:19:48 PM
PubMed Link:
Acanthamoeba Keratitis

Introduction

Acanthamoeba is a genus of protozoans that are ubiquitously present in various habitats, including water, air, soil, and dust.[1] First diagnosed in 1974, Acanthamoeba keratitis (AK) is a potentially sight-threatening ocular infection that often carries a poor prognosis due to significant delays in diagnosis.[2] Causes of AK appear to be multifactorial, but most cases have been linked to contact lens wear and their cleaning solutions.[3]

Etiology

The genus Acanthamoeba is within phylum Amoebozoa, subphylum Lobosa and order Centramoebidae.[4] Acanthamoeba species have been shown to thrive in soil and aquatic environments, including ponds, swimming pools, hot tubs, and contact lens solutions. Acanthamoeba species get classified via the 18s rDNA sequence analysis into T1-T12 genotypes.[5] AK most often occurs secondary to species in the T4 genotype.[6] The most common species that cause AK are Acanthamoeba castellani and Acanthamoeba polyphaga.[7]

Acanthamoeba species exist in two forms, an active trophozoite or a dormant cyst. The trophozoite form feeds on bacteria, algae, and yeasts. Trophozoites are also capable of slow locomotion and asexual reproduction.[8][9] The cystic form exhibits minimal metabolic activity and is capable of surviving adverse environmental conditions such as severe changes in temperature or pH, high doses of UV-light, food deprivation, or desiccation.[10]

Epidemiology

The incidence of acanthamoeba keratitis has been rising over the past several decades.[11] Although classically considered a rare cause of keratitis, some studies have found that Acanthamoeba causes approximately 5% of cases of contact-lens-associated keratitis.[12] AK is most common in contact lens wearers, with reported rates in the range of 1 to 33 cases per million of contact lens wearers per year.[1] The wide variation in incidence is thought to be due to regional differences in types of contact lens, contamination of domestic and swimming pool water, and the use of diagnostic tests for AK.[13] Data from the Centers for Disease Control and Prevention indicate that topping off and storing contact lenses in water increases the risk of AK by 4.46 and 4.38 times, respectively.[14] Though poor lens hygiene often gets implicated in the development of acanthamoeba keratitis, the infection may still occur in individuals who properly clean their contact lenses as most available multipurpose solutions are ineffective against Acanthamoeba.[15][16] One study found that only contact solutions containing hydrogen peroxide were effective against Acanthamoeba.[17] Furthermore, AK has been reported in non-contact lens wearers, especially individuals with regular ocular exposure to dust, soil, or contaminated water.[18]

Acanthamoeba keratitis can happen at any age but mainly affects younger and middle-aged individuals.[19] Individuals with compromised immune systems also have an increased risk.[20] There is no known sex predilection.

Pathophysiology

The initial steps of ocular infection involve adhesion of the pathogen to the corneal surface via mannose-binding protein and laminin-binding protein.[21][22] This process, in turn, leads to phagocytosis and release of enzymes and toxins such as neuraminidase, superoxide dismutase, protease, ecto-ATPase, phospholipases, glycosidases, and acanthaporin.[23] Corneal epithelial destruction and apoptosis allow for the invasion of the stroma.[24][25] Once stromal degradation has occurred, the pathogen may penetrate the cornea. Acanthamoeba proteases may induce damage to the corneal nerves, leading to significant pain and the characteristic finding on the slit lamp examination of radial keratoneuritis.[5] Notably, intraocular infection with Acanthamoeba is rare due to the intense response of polymorphonuclear leukocytes in the anterior chamber.[26] Other factors that indirectly contribute to the pathogenicity of Acanthamoeba are its ubiquity, absence of distinct morphology, ability to tolerate a wide range of temperatures and pHs, and ability to reversibly differentiate into a cyst or trophozoite.[23]

Contact lens wear causes microtrauma to the corneal epithelium and upregulation of glycoproteins. Soft contact lenses have a more adherent surface than hard lenses, thereby providing the Acanthamoeba trophozoite greater access to the cornea.[27][28]

History and Physical

A detailed history is a requirement for all individuals with a presentation suspicious for ocular infection. Particular attention should apply to inquire about contact lens wear, recent corneal injury, and exposure to dust, soil, or contaminated water. Acanthamoeba keratitis usually presents unilaterally but may rarely occur in both eyes.[27] The characteristic finding of AK, even in the early stage, is pain out of proportion to clinical findings; there is a thought that this is due to the action of trophozoite-derived proteases.[5] Patients often also frequently report decreased vision, eye redness, foreign body sensation, photophobia, tearing, and discharge. Symptoms may wax and wane between mild and severe.[28] Approximately 75 to 90% of patients with early acanthamoeba keratitis are initially misdiagnosed[8]; therefore, a diagnosis of acanthamoeba keratitis should merit special consideration in patients with several weeks of symptoms that have not improved despite compliance with a daily regimen of topical antibiotics or antivirals. The clinician should consider bacterial superinfection if symptoms worsen despite the initiation of appropriate treatment.

A comprehensive ocular examination is also necessary for patients with suspected ocular infection. Early findings on slit-lamp examination include an epitheliopathy with punctuate keratopathy, epithelial or subepithelial infiltrates, pseudodendrites, and perineural infiltrates.[1] Perineural infiltrates are highly suggestive for AK and have been reported in up to 63% of cases at six weeks, though notably may regress with disease progression.[29] Characteristic findings of late-stage AK on slit-lamp examination include “ring-like” stromal infiltrate and radial keratoneuritis, though satellite lesions, ulceration, abscess formation, anterior uveitis with hypopyon, and epithelial defects are also frequently seen.[28] Of note, the characteristic ring infiltrate is only present in approximately 50% of patients with advanced disease.[23] Signs of very advanced acanthamoeba keratitis include stromal thinning and corneal perforation.[28] As many of these exam findings are non-specific, the clinician must have a high index of suspicion for AK in cases in which the history and other features are highly suggestive.

Evaluation

The plate culture technique is traditionally regarded as the gold standard for the detection of Acanthamoeba, though new developments support an increasing role of polymerase chain reaction (PCR), in vivo confocal microscopy (IVCM), and staining. Corneal scraping or biopsy is required to obtain a sample for culture. As Acanthamoeba trophozoites feed on bacteria, cultures will develop on 1.5% non-nutrient agar plates covered by E. Coli.[30] The rate of positive culture result for Acanthamoeba in the setting of acanthamoeba keratitis is generally low, reported anywhere from 40 to 70%.[31] Samples must be observed daily for up to one week before declaring a negative result.[23] Immunohistological staining with monoclonal antibodies has demonstrated utility, though this technique is both time-consuming and requires the expertise of a skilled microbiologist for interpretation.[32]

PCR has demonstrated significant improvement in the detection of Acanthamoeba over the past decade. The 18s rRNA region is most commonly employed to detect Acanthamoeba in clinical samples. Newer primer sets have demonstrated sensitivity and specificity of 100% and 96%, respectively.[32] Moreover, PCR has the advantages of being widely available, fast, and less labor-intensive. Thus, there is some reason to believe that PCR may become the new gold standard for the diagnosis of acanthamoeba keratitis in the near future.[32]

Some authors recommend that clinicians retrieve patients’ most recent pair of contacts and lens case for culture and/or PCR.[1] However, nearly all contact lens cases, even those of healthy contact lens wearers, produce a positive result for Acanthamoeba on PCR. The absence of Acanthamoeba on a culture of a contact lens case is highly suggestive of a diagnosis other than AK.[23]

IVCM is also an extremely useful tool that allows for the non-invasive examination of individuals cells of the cornea in real-time. IVCM will only reliably detect cysts, which will appear as well-defined, round, double-walled, hyper-reflective bodies. The pooled sensitivity and specificity of IVCM in one study were found to be 85.3% and 100%, respectively. IVCM may also be used to monitor disease progression and response to treatment.[31] However, IVCM is expensive and often not readily available.[33]

Cytology smears is a viable method to detect Acanthamoeba cysts on corneal scrapings or biopsied tissue. Cytology smears have the advantage of being fast, easily performed, and readily available in most facilities. Cytology smears additionally do not require live organisms as with culture or intact DNA as required for PCR.[33] Lactophenol-cotton blue, Giemsa, calcofluor white, and acridine orange stains are among the more widely used options as they produce rapid and accurate results, though the latter two stains require a fluorescent microscope and may lead to false-positives due to staining of cellular debris.[23][34] The silver stain is necessary if the clinician wishes to view the specific cyst morphology. There is some evidence to suggest that staining with H&E is more sensitive and specific than other stains, especially calcofluor white and Giemsa.[34][35] There has been at least one study described in which H&E staining allowed for the diagnosis of AK in the setting of a negative culture.[36]

Many newer diagnostic procedures have employed in diagnosing AK. Loop-mediated isothermal amplification (LAMP) has been demonstrated to have a high sensitivity and specificity comparable to PCR for the diagnosis of AK. In contrast to PCR, the target sequence during LAMP is amplified at a constant temperature of 60 to 65 degrees Celsius, thereby eliminating the need for an expensive thermal cycler. Given its simplicity and low cost, LAMP could serve as an alternative to PCR for future diagnosis of AK.[30] Some novel, non-invasive imaging techniques that researchers have examined include Heidelberg retina tomography II (HRT II) and nuclear magnetic resonance (NMR) spectroscopy, though further studies are needed to demonstrate their sensitivity and specificity.[30]

Treatment / Management

Acanthamoeba trophozoites are frequently sensitive to a variety of medications, including antibiotics, antiseptics, antifungals, and antiprotozoals. However, Acanthamoeba is resistant to most of the listed treatments in the cystic form, thereby allowing for prolonged infection.[1] Diamidines and biguanides are two classes of antiamebics that are often the first-line therapy for acanthamoeba keratitis due to their proven cysticidal effects. When used individually or in combination, the efficacy of these topical antiamebics ranges from 35% to 86%.[29] Both medical and surgical options may be considerations in the setting of treatment-resistant AK.

Diamidines exert their therapeutic effect by altering the structure and permeability of the cell membrane, causing denaturation of cytoplasmic contents. Propamidine-isethionate, hexamidine-diisethionate, and dibromopropamidine are diamidines used for the treatment of acanthamoeba keratitis at 0.1% concentration.[1] Diamidines are generally well-tolerated, but prolonged therapy at the therapeutic level may lead to ocular surface toxicity.[37]

Biguanides similarly cause alterations in cytoplasmic membranes, leading to loss of cellular components and inhibition of enzymes necessary for cell respiration. Polyhexamethylene biguanide (PHMB) and chlorhexidine are the two biguanides most widely used for the treatment of AK. PHMB is traditionally started at a concentration of 0.02% but may increase to 0.06% for AK that is unresponsive or severe at the initial presentation. Likewise, chlorhexidine is usually initiated at a concentration of 0.02% but can be increased to 0.2%, if indicated.[1]

Most modern treatment regimens call for initiation of combination therapy of a biguanide and a diamidine, though there is some newer research to suggest that PHMB 0.08% monotherapy is as effective as combination PHMB 0.02% and propamidine 0.1%.[38] Early intensive treatment is thought to be more effective as cysts have not yet had the opportunity to mature fully.[29] Medications are given on an hourly basis both day and night for the first 48 hours, then reduced to only hourly during the daytime for a period of days to weeks.[1] As the infection subsides, the frequency may be reduced to four times daily, a regimen maintained for six months to a year.[8][39] The goal of this prolonged therapy is to ensure that complete eradication of Acanthamoeba cysts. Premature discontinuation of treatment could allow for any remaining dormant cysts to differentiate into trophozoites. Clinicians should tailor treatment regimens to the individual case to minimize risks of epithelial toxicity.[1]

Neomycin was once used as a first-line treatment of acanthamoeba keratitis due to its anti-trophozoite activity but does not appear to achieve cysticidal levels in vitro.[29] Nonetheless, neomycin prevents bacterial superinfection and reduces bacterial food for trophozoites; for this reason, some physicians elect to include topical neomycin 1% five times daily as part of an initial “triple therapy” as it may demonstrate an additive effect when used in combination with biguanides and diamidines.[8][40]

Approximately 39% of patients with acanthamoeba keratitis fail initial therapy. Individuals with more clinically severe disease or a history of corticosteroid use before diagnosis are more likely to fail initial treatment.[11] Several other medications exist, which may serve as adjuvant treatments with biguanides and diamidines. Miltefosine is a newer antiamebic that recently obtained FDA approval for the treatment of AK. Early case reports support the use of miltefosine as an adjuvant treatment for treatment-resistant AK[41][42]; however, the literature surrounding its use is still growing. Further studies are warranted to clarify its role and establish the optimal timing, dosage, and route of administration.

The therapeutic value of antifungals appears to be limited. Among all antifungals, only newer generation azoles (voriconazole and posaconazole) appear to achieve cysticidal levels in vitro[43]; in vivo, treatment response may not be sustainable.[44] Natamycin demonstrated superior cysticidal effects to PHMB in vitro, but data on its use in animal models is not yet available.[45]

Epithelial debridement may allow for improved penetration of topical medications.[1] If acanthamoeba keratitis remains unresponsive to topical conservative treatment, various surgical options remain available, including penetrating keratoplasty (PK), corneal cryoplasty, amniotic membrane transplantation, and riboflavin-UVA crosslinking.[46][47][48] Notably, PK used to be the first-line therapy for AK before biguanides and diamidines. Now, PK is generally only for patients with significant cataract, fulminant corneal abscess, corneal perforation, or therapy-resistant ulceration with peripheral neovascularization.[29] Topical treatment, as noted above, should continue for up to one year after PK.[8]

At the time of writing, several promising novel therapeutic approaches remain on the horizon. A case series of four patients treated with photorefractive surgery resulted in excellent visual outcomes and no disease recurrence.[49] Other case reports have indicated a potential role for collagen cross-linking.[50] Recent tissue studies have demonstrated a cysticidal effect of three antidiabetic agents (Glimepiride, Vildagliptin, and Repaglinide) against Acanthamoeba castellanii, especially when conjugated to silver nanoparticles.[51] The combination of titanium dioxide and UV-A also demonstrated a synergistic cysticidal effect against Acanthamoeba species when compounded with chlorhexidine in vitro.[52] Clinical trials on the use of these newer therapies in animal models are warranted to evaluate efficacy and safety. Potential areas for future research include the targeting of genetic markers and stem cell research, though this would require a better understanding of the disease at a molecular level.

The development of extra corneal manifestations such as scleritis or limbitis indicates a worse outcome and warrants treatment with anti-inflammatory medications. Most cases of extracorneal inflammation are manageable with oral flurbiprofen 50 to 100 mg, two to three times daily.[29] If scleritis or limbitis remain unresponsive to NSAIDs, high-dose systemic steroids (prednisolone 1 mg/kg/day) and other systemic immunosuppressive agents such as cyclosporine (3.0 to 7.5 mg/kg/day) may be necessary.[29] These medications may warrant continuation for several months to control inflammation and eradicate the pathogen.

Topical corticosteroid use in acanthamoeba keratitis is controversial and discouraged by many physicians. Though topical steroids are usually not required in early disease, their use may be necessary when there is significant anterior segment inflammation.[1] Steroids should be prescribed judiciously due to the potential risk of promoting encystment and increasing the number of trophozoites.[8] Clinicians should never prescribe steroids without concurrent administration of antiamebics, and antiamebic therapy should continue for several weeks after the discontinuation of steroids.[1]

Differential Diagnosis

One study found that clinicians initially misdiagnosed nearly half of all acanthamoeba keratitis cases as herpes simplex keratitis (HSK).[53] The pseudodendrites of early AK may resemble the epithelial dendrites seen in HSK; however, AK pseudodendrites will lack the knot-like widenings at the terminal ends of the erosion as seen in herpetic dendrites.[8] AK may be further distinguished from HSK by the presence of perineural infiltrates and ring infiltrates. Conversely, HSK is more likely to involve the endothelium.[23] Of note, AK is polymicrobial or coinfected with herpes simplex virus in 10 to 23% of cases.[1]

Acanthamoeba keratitis may occasionally be confused with bacterial or mycotic keratitis. Although classically associated with acanthamoeba keratitis, ring infiltrates may also present in both bacterial and mycotic keratitis.[8] In the absence of a superinfection, AK is more likely to present with multifocal, transparent stromal infiltrate rather than monofocal, thicker infiltrates seen in bacterial and mycotic infections.[8] Bacterial keratitis is more likely to extend beyond the cornea and involve the anterior chamber.[23]

Lastly, the multifocal stromal infiltrates of acanthamoeba keratitis may resemble the satellite infiltrates of fungal keratitis. However, AK may be distinguished from fungal keratitis on slit-lamp examination by translucent epithelial defects, perineural stromal infiltrates, and ring infiltrates. Fungal keratitis is more likely to extend beyond the cornea.[23] Moreover, the culture of fungal keratitis would be expected to produce a positive result on Sabouraud agar.

In contrast to patients infected with uncomplicated bacterial or fungal keratitis, patients with acanthamoeba keratitis are more likely to be younger and present with a longer duration of symptoms at the initial presentation.[54] However, acanthamoeba keratitis has been shown to coexist frequently with bacterial and fungal keratitis[55]; therefore, the degree to which patients’ demographic factors should merit consideration in developing a differential diagnosis remains unclear.

Prognosis

The most important prognostic factors for acanthamoeba keratitis are the severity of disease at presentation and the time required to initiate effective therapy.[56] Most patients have a poor prognosis due to significant delays in diagnosis, leading to corneal scarring; however, if treatment starts within three weeks of symptom onset, patients generally have good visual outcomes.[29] The presence of cataracts or involvement outside the cornea indicates a worse prognosis.[1] PK may be performed electively to improve vision, though some authors suggest this should only be performed three months after the infection has resolved and treatment discontinued.[1] There are no reports in the literature to indicate any differences in prognosis of AK secondary to each of the species within the Acanthamoeba genus.

Complications

Common complications of acanthamoeba keratitis include glaucoma, iris atrophy, broad-based anterior synechiae, cataract, and persistent endothelial defect. Rarer complications include scleritis, sterile anterior uveitis, chorioretinitis, and retinal vasculitis.[8] Scleritis occurs in approximately 10% of acanthamoeba keratitis cases and is believed to be secondary to an inflammatory response of unknown etiology rather than a direct invasion of Acanthamoeba.[29] Extracorneal inflammation should be managed with anti-inflammatory treatments, as listed above.

Deterrence and Patient Education

As most cases of Acanthamoeba keratitis occur in contact lens wearers, patients should be educated to avoid wearing contact lenses overnight, using homemade saline solutions, and swimming or showering while wearing contact lenses. The use of daily disposable instead of reusable contact lenses would be expected to reduce the incidence of AK, though this is still unproven.[57] Individuals with regular ocular exposure to dirt, soil, and contaminated water should wear protective eyewear as appropriate. An improved understanding of the pathogenesis of Acanthamoeba would likely be of great value to clinicians.[29]

Pearls and Other Issues

  • Acanthamoeba is a rare cause of keratitis that is both difficult to diagnose and treat. The clinician should have a high suspicion for AK in the setting of keratitis that is not responsive to standard first-line treatments.
  • Most cases of AK develop in contact lens wearers. Though poor lens hygiene is implicated, most multipurpose solutions are ineffective against Acanthamoeba.
  • The classic findings of AK on slit-lamp examination are a ring-shaped infiltrate and radial keratoneuritis. Notably, ring-shaped infiltrates are only present in the advanced stage, and even then, only in 50% of patients.
  • Biguanides and diamidines are the first-line treatment for AK. The use of topical corticosteroids is controversial and discouraged by many physicians.

Enhancing Healthcare Team Outcomes

Most patients with symptoms suggestive of ocular infection will present to their primary care physician or emergency department. Thus, these clinicians must know about acanthamoeba keratitis and its presentation. Direct referral to a cornea specialist rather than other ophthalmologists is associated with 3.2 times lower odds of treatment failure.[11] Communication between the pathologist, the cornea specialist, and the microbiologist is especially vital to employ isolation techniques used to identify Acanthamoeba. The primary care clinicians, including the nurse and pharmacist, should educate the patient on contact lens hygiene maintenance. The patients should remove contact lenses during water-related activities, and the storing solution should be fresh. Also, handwashing before touching the contacts cannot be over-emphasized. At any time, a patient with contact lenses develops eye redness, pain or photophobia, urgent consultation with the eye specialist is a strong recommendation. Pharmacists should also coordinate with the treating clinician regarding medical management, including appropriate agent selection, dosing, and checking for drug interactions. Nursing ophthalmology specialty trained nurses can also verify compliance, monitor therapeutic progress, and be vigilant for adverse events. Pharmacists and nurses need an open communication channel to the prescriber so they can bring any issues to the fore immediately. Only with this type of interprofessional collaboration can acanthamoeba keratitis have optimal results. [Level V]

Close follow-up is required to monitor clinical improvement following initiation of treatment. [Level 3]



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      Image courtesy S Bhimji MD

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