The more common infection caused exclusively by Acanthamoeba is amebic keratitis. This is a nonfatal but painful and vision-threatening infection of the cornea of an otherwise healthy person.^17–21 The earliest reports of Acanthamoeba keratitis were associated with corneal trauma and subsequent exposure to contaminated water. However, a clear association of this disease with contact lens usage has subsequently unfolded.^22–28 Transmission is presumed to occur through contamination of contact lens cases and contact lenses from environmental sources. Hundreds of cases of this disease have been reported worldwide, with estimates of 80% to 90% of cases occurring in contact lens users.^6,8,19,29 The actual number of cases of Acanthamoeba keratitis occurring worldwide is undoubtedly much higher because of unpublished data, nonstandardized reporting systems, and misdiagnoses.

MORPHOLOGY AND LIFE CYCLE

The life cycle of Acanthamoeba consists of growth and replication by simple binary fission with two alternating morphologic forms: (1) trophozoite and (2) cyst. In the trophozoite stage, amebae are actively feeding and phagocytosing a variety of other microorganisms, especially bacteria and yeast. Mitotic division occurs at distinct intervals, depending on nutrient availability and conducive environmental conditions. Depending on the species, trophozoites range in length from 10 to 50 μm and usually display a broad, clear hyaline zone at the anterior end. Locomotion is slow, usually directional, and is caused by the flow of cytosolic contents from the endoplasm into the ectoplasmic region. During locomotion, the anterior hyaline region produces a large number of slender, needlelike pseudopodial projections. These projections are known as acanthopodia, and are a defining characteristic of the family. Cytoplasmic vacuoles are prominent and mostly concentrated in the posterior half of the ameba. A single, large contractile vacuole, important for osmoregulation, is usually seen. Amebae are usually uninucleate, but multinucleate forms are not rare. The nucleus contains a large, dense, and centrally located nucleolus surrounded by a clear halo. During mitosis, the nuclear membrane and nucleolus disappear, and a typical spindle is formed.³⁰

A variety of adverse environmental or nutritional changes may trigger the transformation of the trophozoite into the double-walled cyst characteristic of the genus.³¹ Acanthamoeba cysts readily survive cold or dry environmental conditions, as well as exposure to many antimicrobial agents and disinfectants. They are variable in shape and size (range, 15 to 30 μm), depending on the species. The inner wall (endocyst) varies in shape from circular to polygonal or stellate, whereas the outer wall (ectocyst) is usually wrinkled. The ectocyst usually follows the contour of the endocyst, with intermittent junctions (ostioles) appearing between the two, giving rise to the polygonal to stellate appearance of the cyst. These ostioles remain plugged by opercula until excystation is initiated. During excystation the ameba detaches itself from the inner cyst wall and dissolves the operculum, escaping through the ostiole.

TAXONOMY

Considerable confusion exists regarding the taxonomy of those free-living amebae that have been implicated as opportunistic pathogens. N. fowleri is unique among this group in that it is an ameboflagellate; it can be readily differentiated from the others by its ability to produce a motile, flagellated stage, a feature typical of the order Schizopyrenida (Table 1).³² Pathogenic amebae that do not produce a flagellate stage have been grouped within the order Amoebida, and within either the Hartmannella or Acanthamoeba genus, depending on characteristics such as nuclear division and cyst morphology.^30,33–36 More recently, Page^37,38 separated these two genera on the basis of locomotive form, behavior, and cyst characteristics. The two have also been shown to be distinct antigenically.³⁹ Many species now classified as Acanthamoeba were known as Hartmannella in older publications. The taxonomy of Balamuthia and other amebae in the order Leptomixida is uncertain, awaiting further studies for clarification (see Table 1).

TABLE 1. Taxonomic Classification of Some Free-Living Amebae

To date, at least 20 species of Acanthamoeba have been described, based primarily on subtle differences in size and cyst morphology. As a practical exercise, however, it is difficult to differentiate among these species because of the subjective nature and unreliability of the described diagnostic characteristics. Methodologic differences alone are substantial, but even the examination of clonal populations reveals considerable variability in cyst sizes and morphology. Pussard and Pons⁴⁰ categorized all Acanthamoeba species into three major groups on the basis of cyst morphology and size (Table 2):

TABLE 2. Categorization of Described Species of Acanthamoeba Based on Cyst Morphology^9,40

  Group I consists of four species with large trophozoites and cysts. The cysts usually have smooth ectocysts and are greater than 18 μm in diameter. The endocysts are stellate and the opercula are usually located at the level of the ectocysts.
  Group II comprises 10 species whose cysts have wrinkled, rippled, and irregular ectocysts measuring less than 18 μm in diameter. The endocyst may be stellate, polygonal, triangular, or sometimes rounded without prominent rays (arms). The opercula are present within the depressions of the ectocysts. Most of the species implicated in the etiology of Acanthamoeba infection belong to Group II.
  Group III consists of five species having thin ectocysts, either gently rippled or smooth, measuring less than 18 μm. The endocysts are usually rounded or may have three to five rays.

Despite attempts to simplify the identification of acanthamoebae, taxonomic status of known species remains questionable. Recent studies employing molecular-based techniques (e.g., isoenzyme profiles, mitochondrial DNA and 18S rDNA restriction fragment length polymorphisms, ribosomal gene sequence analysis) may offer approaches to identification that represent an improvement over traditional morphologic studies.^41–45

ECOLOGY

Acanthamoebae are among the most common protozoa in soil and water ecosystems worldwide. They are specialized predators of bacteria, fungi, and other protozoa and play an important role in nutrient cycling in those habitats in which their food sources thrive. Encystation allows the amebae to survive untoward environmental changes, including temperature extremes, desiccation, lack of nutrients, increase in salinity, and disinfection. Acanthamoebae have been isolated from a variety of environmental habitats, although their preferred habitat appears to be soil, which provides a source of oxygen, water, optimum survival temperature, and a continuous bacterial food supply.^46,47 Soils high in organic matter content, and subsequently in numbers of bacteria, will contain the greatest numbers of amebae.^48,49 Vertical distribution studies⁵⁰ have shown that they are present in the littoral and profundal sediments of pond and lake waters, respectively. Acanthamoebae are also known to be present in tap water, faucet aerators and shower heads, and well water, among other sources that may be epidemiologically linked to human infections.^42,43,45

ACANTHAMOEBA KERATITIS

Among the 20 or more named species of Acanthamoeba that have been distinguished by cyst morphology, immunofluorescent antibody testing, or isoenzyme profiles,⁵¹ 5 have been repeatedly identified by culture or immunofluorescent antibody testing as being responsible for keratitis: Acanthamoeba castellanii, Acanthamoeba culbertsoni, Acanthamoeba hatchetti, Acanthamoeba polyphaga, and Acanthamoeba rhysodes (see Table 2).

The development of Acanthamoeba keratitis is associated with mild antecedent trauma in the majority of cases.^29,52 This can be in the form of vegetable matter, insects, paint thinner, wind-blown contaminants, sawdust, or contact lens wear in which microabrasions of the cornea may develop.^53,54 Exposure of the damaged corneal epithelium to amebic trophozoites or cysts may result in their attachment and subsequent invasion.

The earliest manifestation of Acanthamoeba keratitis is frequently the development of a dendritiform epithelial pattern (Fig. 1).^55–57 This dendritiform keratitis may represent epithelial infection before the development of any stromal involvement. This contention is supported by the rapid resolution of symptoms after wide epithelial débridement.^56,58 A dendritiform epithelial pattern presenting early in the course of Acanthamoeba keratitis may explain why the great majority of patients have undergone treatment for herpes simplex keratitis before the correct diagnosis was established.^26,56 Dendritiform lesions of Acanthamoeba keratitis do not truly mimic herpes simplex dendrites, which form frank ulcerations of the cornea; rather, amebic keratitis causes an edematous and necrotic appearance of the involved epithelium. Mottled epithelial staining or frank epithelial defects may be seen; however, an intact epithelium is frequently noted. The dendritiform pattern of Acanthamoeba keratitis does share some similarity to that of herpes zoster infection, which is composed of swollen cells, often lined with mucus, that give it a linear, gray, plaquelike character. On initial presentation, the epithelium may be intact despite a mottled or dendritiform appearance, but the epithelium frequently breaks down repeatedly.^56,57,59 Variable, persistent, or recurrent epithelial erosions characterize this infectious process.

Moore and associates⁵⁷ described radial keratoneuritis as a presenting sign in Acanthamoeba keratitis. Radial keratoneuritis is characterized by linear, radial, branching infiltrates found in the midstroma, which appear to be located along corneal nerves. These infiltrates begin paracentrally and extend to the limbus in a radial pattern.

Conjunctival injection and chemosis are present as a rule. Adenopathy may or may not be present. Acanthamoeba keratitis is characterized by a fluctuating, nongranulomatous inflammatory reaction in the anterior chamber. McClellan and Coster⁶⁰ cultured Acanthamoeba organisms from an anterior chamber paracentesis in a patient with severe keratitis. Elevated intraocular pressure and cataract, presumably secondary to the anterior segment inflammation, may be seen. Mannis and colleagues⁵⁹ reported that 39% of cases demonstrated sufficiently pronounced anterior segment inflammation to produce hypopyon. Severe pain, often out of proportion to the degree of anterior segment inflammation, characterizes this disorder, particularly as the keratitis becomes chronic.^24,57,59,61 Patients who present with epithelial involvement alone may not complain bitterly of pain.⁵⁶ A distinguishing feature of Acanthamoeba keratitis is the prominent scleral inflammatory component. Although anterior scleritis may be part of the clinical presentation, the severe ocular pain characteristic of Acanthamoeba keratitis suggests that scleritis may occur commonly in chronic cases of Acanthamoeba keratitis. In some reported cases, the ocular pain became so severe that retrobulbar alcohol injections had to be given.⁶²

A ring-shaped stromal infiltrate is characteristic of advanced infection and is nearly pathognomonic of Acanthamoeba keratitis (Fig. 2).⁶¹ The infiltrate is often most dense in its peripheral aspects, forming a characteristic ring. This annular infiltrate may be segmental or circumferential, may be progressive, often involves stromal thinning or furrowing, and may be associated with a variable overlying epithelial defect. The arcuate or ringlike infiltrate is due to polymorphonuclear leukocyte infiltrates generated by chemotaxis after antigen-antibody precipitation,⁶³ analogous to the infiltrate described by Wessely⁶⁴ after injecting horse serum into the cornea.⁶⁵

NONKERATITIC OCULAR DISEASE

Nodular scleritis has been reported in 14% of cases of Acanthamoeba keratitis.⁵⁹ Significant scleral ectasia was reported in one patient, who was not treated because of a delayed diagnosis.⁶⁶ Posterior scleritis and optic neuritis have been noted in association with Acanthamoeba keratitis.^59,67,68 Acanthamoeba was seen histopathologically in the ciliary body of one patient, in whom fatal meningoencephalitis developed.²³ Amebic organisms have also been implicated as a cause of exudative chorioretinopathy.⁶⁹

The potential for spread of Acanthamoeba keratitis to the posterior segment appears to be low. There have been many reported cases of corneal perforation or keratoplasty performed in the face of active infection, including penetrating keratoplasty performed in conjunction with cataract extraction, in which posterior segment involvement has not become evident.

Stehr-Green and associates¹⁹ found a nearly equal distribution of Acanthamoeba keratitis cases among males and females. Males, however, account for a minority of soft contact lens wearers in the United States, suggesting that males may adhere less stringently to recommended disinfection procedures or may increase their risk of corneal trauma or exposure to polluted water in some manner.

The presence of bacterial or fungal contamination within the contact lens care system may be an important element for the survival and growth of Acanthamoeba organisms. Donzis and co-workers⁸⁶ analyzed bacterial and fungal contamination of the contact lens care systems of 10 patients who also had Acanthamoeba species detected within their care systems. Gram-negative bacteria were found in all 10 care systems, and Pseudomonas species were found in 6. Fungi were also isolated in 6 of the 10 care systems. Of interest was the finding that Acanthamoeba organisms were found only in contact lens cases or solutions that also had concomitant bacterial or fungal contamination. Larkin and colleagues⁸⁷ found that 7% of asymptomatic contact lens wearers had storage cases that were contaminated with free-living amebae; six of seven patients with Acanthamoeba-contaminated storage cases had significant numbers of contaminating bacteria. The most recent assessment of contact lens storage contamination showed that 81% of storage cases were contaminated: 77% grew bacteria, 24% fungi, and 8% Acanthamoeba species.⁸⁸ Bacterial cocontamination of contact lens systems harboring Acanthamoeba has been well documented by several additional studies.^89,90 Acanthamoeba organisms display selective bacterial feeding, particularly nonfermentative gram-negative organisms and coliforms.⁹¹ Although Pseudomonas aeruginosa has been found to co-contaminate contact lens cases, this organism produces a toxin that is highly lethal to A. castellanii and A. polyphaga; therefore, Acanthamoeba species and P. aeruginosa may be mutually exclusive eye pathogens.⁹² The presence of bacteria and possibly fungi within the contact lens care system may be an important food source for acanthamoebae, resulting in their enhanced growth. Clearly, the attack rate of Acanthamoeba keratitis in no way approximates the 7% to 8% incidence of contact lens case co-contamination with Acanthamoeba and bacterial species.

By use of the scanning electron microscope, Acanthamoeba organisms have been shown to adhere to the surface of contaminated hydrogel contact lenses.⁵³ Several subsequent studies^93–95 have shown that both Acanthamoeba cysts and trophozoites can firmly adhere to unworn soft contact lenses. The adherence of these organisms to contact lenses may play a significant role in the pathogenesis of Acanthamoeba keratitis.

Likewise, spontaneous Acanthamoeba keratitis is rarely seen in persons who do not wear contact lenses or who have not experienced antecedent corneal trauma. Prior macroscopic or microscopic corneal trauma appears to be necessary for attachment or invasion of trophozoites introduced secondary to the use of contact lenses or ocular solutions, or to the eye's exposure to water or airborne particulates (fomites) that contain amebic cysts. Once introduced to the eye, trophozoites attach to the injured epithelium, producing cytolysis.¹⁰⁰ Multiplication of the organisms and extension into the corneal stroma produces edema, cellular disruption, and necrosis. Tissue damage is exacerbated by the inward migration of inflammatory cells and concomitant release of proteases, cationic proteins, and cytokines.¹⁰¹

Attachment of trophozoites to cellular substrates is an important first step in the development of infection. The presence of intact cytoskeletal element function is essential for pseudopod formation and attachment, and inhibition of this function correlates with a significant decrease in the in vitro cytopathic effect.¹⁰⁰ After cellular contact between an ameba and a host cell, adhesionlike factors have been demonstrated to exist that can be inhibited by mannose or other carbohydrates.¹⁰² Ultrastructural studies demonstrate that cell attachment is mediated through the formation of incomplete desmosome junctions with the appearance of plaquelike maculae.¹⁰³

The role of phagocytosis or trogocytosis in disease progression is unclear, although Acanthamoeba will consume intact, living, cultured human corneal epithelial cells and stromal keratocytes as a food source in vitro.^104,105

Cytolysis appears to play a more prominent role in pathogenesis than does phagocytosis.^100,106,107 The production of membrane-associated or membrane-secreted hydrolytic enzymes (e.g., phospholipase A, neuraminidase, plasminogen activator, collagenase) has been associated with invasion of Acanthamoeba and may play an important role in the disruption of intercellular matrix and host cell membranes.^{39,107a–109} Calcium channels also appear to be important for the cytolytic capabilities of Acanthamoeba, as they are with Entamoeba histolytica. Pretreatment of Acanthamoeba trophozoites with calcium channel blockers resulted in a significant decrease in their in vitro cytopathic effect.¹⁰⁰

Bacteria may need to be present in order for some strains of Acanthamoeba to establish infection or exhibit other virulence characteristics. Recent studies have revealed that a variety of bacteria and fungi are inevitably present in contact lens cases containing free-living amebae, and are important for enhancing growth of the amebae.^86–91,110 In a rat model of amebic keratitis, corneal invasion was shown to be dependent on the co-inoculation of Corynebacterium xerosis.^{101,106,111,112} A variety of other bacteria, including nonculturable endosymbionts, are commonly found in association with Acanthamoeba originating from ocular and nasal specimens, although no epidemiologic link to the development of keratitis or any other disease has been detected.^113–117

Whatever methods are used for the detection of Acanthamoeba, the observer must be familiar with the morphologic characteristics of trophozoites and cysts, and must have culture or other reference material available for comparative study. Similarity of amebae to inflammatory and epithelial cells in wet mount and stained preparations may make differentiation especially difficult. Corneal scrapings, biopsy imprint smears, and fluid from contact lens cases can be directly examined using a variety of stains and optical techniques. Direct wet-mount examination of corneal scrapings or other fluid specimens for rapid identification of trophozoites or cysts is a common laboratory procedure. Such specimens may be examined by brightfield microscopy, although the use of phase contrast or differential-interference contrast microscopy is preferred. Examination is made for cells suggestive of trophozoites on the basis of size, nuclear characteristics, and presence of vacuoles and acanthopodia (Fig. 3, left). Cysts are more readily recognized by their typical double-walled structure: the ectocyst wall often appearing wrinkled and the endocyst polygonal or stellate (see Fig. 3, right).

A variety of stains may be used on dried and fixed specimens, including Giemsa, Wright's, Wheatley's trichrome, and hematoxylin and eosin. Gram's stain is not an optimal one because the organisms stain faintly and resemble other cellular elements. The fluorochrome calcofluor white is especially useful in identifying amebic cysts because of the presence of complex polysaccharides in the cyst wall, and it offers enhanced sensitivity for detection in methanol-fixed specimens (Fig. 4).¹¹⁸ Also, this stain is readily available in most diagnostic laboratories, where it is commonly used for fungal examinations. Direct fluorescent antibody staining is both sensitive and specific for Acanthamoeba, but is not currently available from commercial sources.¹¹⁹

Biopsy tissues may be submitted for routine histology. Both trophozoites and cysts are readily recognized on histologic sections stained with hematoxylin and eosin or periodic acid-Schiff, among other stains (Fig. 5).

Cultivation of acanthamoebae from clinical specimens remains the most sensitive and specific diagnostic procedure available. Although acanthamoebae will grow on a variety of enriched bacteriologic culture media, contaminating bacteria can mask the growth of acanthamoebae on these media, making their detection difficult. Use of a 1.5% non-nutrient agar plate overlaid with a lawn of live Escherichia coli or other coliform bacilli is a simple and highly effective medium for the recovery of all Acanthamoeba species.^10,120 Corneal scrapings, triturated tissues, or fluid specimens are applied directly to the center of the plates, which, in turn, are placed in a sealed bag to maintain humidity and incubated at ambient temperature. Plates are examined microscopically daily for up to 10 days using the low-power (× 10) objective with the condenser lowered or, ideally, with an inverted microscope for the presence of trophozoites and their tell-tale trails left in the bacterial lawn (Fig. 6). Detection of growth is subsequently confirmed by examining the organisms in wet-mount preparations.¹⁰

Recently, tandem scanning confocal microscopy for in vivo identification of Acanthamoeba within the corneal epithelium and anterior stroma has been used successfully as an alternate rapid diagnostic method.^121–123 Investigators employing this diagnostic technique have been able to visualize the highly reflective Acanthamoeba organisms in the corneal tissues of patients as well as animal models. The technique has also proved useful for following the course of the infection during and after therapy.

Wide epithelial débridement may be curative in early cases of Acanthamoeba keratitis involving the epithelium alone.^56,58,128 Wide débridement not only provides adequate epithelium for culture and staining, but also debulks the load of infectious organisms and enhances delivery of topical medications.^56,124,126

Acanthamoeba are difficult to treat because of their ability to encyst under adverse conditions; the cysts are considerably more resistant to treatment than are trophozoites. Furthermore, if Acanthamoeba keratitis proceeds without early treatment, the organisms invade progressively deeper into the corneal stroma, where they presumably feed on keratocytes.¹²⁵ This may further limit the success of topical agents because drug concentrations may be reduced in the deeper target tissues.

Although a wide variety of drugs have been used clinically in the treatment of Acanthamoeba keratitis, novel approaches to chemotherapy of Acanthamoeba keratitis continue to evolve.

APPROACHES TO MEDICAL MANAGEMENT

Cationic Antiseptic Agents

In vitro studies have shown that the cationic antiseptic agents chlorhexidine and polyhexamethyl biguanide (PHMB) have the best amebicidal and cysticidal activity of the drug classes studied to date.^129,130 The cationic antiseptics exert their effect by disrupting membrane function.^129,131,132 Chlorhexidine 0.02% and PHMB 0.02% have been effective clinically as primary therapy^{126,130,133–137} as well as in cases where medical failures have occurred with other agents.¹²⁹ PHMB 0.02% (200 μg/mL) and chlorhexidine 0.02% (200 μg/mL) each have minimal cysticidal concentrations approximating 2 μg/mL.^129,130 The topical solutions used have concentrations that are 100 fold greater than the minimal cysticidal concentrations for the Acanthamoeba species.

Clinically, corneal epithelial toxicity has been minimal for both chlorhexidine 0.02% and PHMB 0.02%. In vitro bathing of epithelial and endothelial surfaces of rabbit cornea with 0.05% chlorhexidine digluconate in Ringer's saline caused swelling of the corneal stroma.¹³⁸ At concentrations equal to or greater than 0.2%, chlorhexidine has toxicity to skin, conjunctival and corneal epithelial cells, and fibroblasts, but at concentrations of 0.02% it appears safe for mucosal cells.¹³⁹ Mammalian cells are unaffected at a chlorhexidine concentration of 0.02%, and there is no apparent ocular toxicity.^132,140

PHMB has been shown to be nontoxic to mammalian epithelia at concentrations of up to 20%,¹³⁸ and has lacked corneal toxicity clinically.^{126,135,136,141} Chlorhexidine may have less nonspecific binding to corneal tissue than PHMB, allowing for greater drug availability.¹³²

When studied in vitro, an additive effect was seen between chlorhexidine and propamidine, and between PHMB and neomycin, meaning that the result of the two compounds used together was equivalent to their sum when used separately.¹³⁰ Chlorhexidine and PHMB are slightly synergistic when used individually in combination with pentamidine, meaning that the result with the two compounds was greater than the additive response.¹³⁰ Alexidine is another bis-biguanide, which has excellent amebicidal activity in in vitro studies, and clinical efficacy remains to be studied.¹⁴²

Aromatic Diamidines

The earliest successful treatment of Acanthamoeba keratitis included the diamidine propamidine isethionate (Brolene 0.1%).¹⁴³ Most successful treatment regimens for Acanthamoeba keratitis have included propamidine in conjunction with other agents.^{24,56,58,62,63,118,126,130,133–137,141,143–146}

Other aromatic diamidines that have proved effective against both forms of Acanthamoeba include pentamidine isethionate, dibromopropamidine, diminazene aceturate, hydroxystilbamidine isethionate, and hexamidine diisethionate.^{130,142,147–150} The diamidines act as inhibitors of S-adenosylmethionine decarboxylase in Acanthamoeba¹⁵¹ or interact directly with the ameba's nucleic acids.^152,153 In human neutrophilic granulocytes, pentamidine exerts its effect by inhibition of co-factors¹⁵⁴ or cytoplasmic enzymes,¹⁵⁵ and the diamidines may exert similar effects on Acanthamoeba as well.

In general, the diamidines are well tolerated by ocular tissues when applied topically; however, prolonged treatment with Propamidine has led to toxic keratopathy, which cleared gradually after discontinuation of the drug.¹⁵⁶

Aminoglycosides

The aminoglycosides paromomycin and neomycin have been used effectively in conjunction with other topical drugs.^25,56,62,143 The aminoglycosides have shown variability among strains^149,157 and are largely ineffective against cysts.^130,135,148

Neomycin and the diamidines propamidine, pentamidine, and diminazene aceturate have an additive effect when used in combination.¹³⁰ Much like the cationic antiseptics chlorhexidine and PHMB, neomycin disrupts the organism's plasmalemma, and thereby may facilitate entry of an effective drug, such as an aromatic diamidine. The topical use of neomycin is often limited in duration because of the frequent development of a toxic or hypersensitivity reaction.¹⁵⁸

Imidazoles

The imidazoles miconazole^{24,56,58,82,84,124,144,149,159} and clotrimazole^{143,145,149,160–162} have been used clinically with some success. However, the effect is likely to be amebastatic rather than amebacidal.^130,163

Ketoconazole appears to be the most effective synthetic imidazole, but it must be given systemically.^{61,75,84,143,161} It has significantly greater side effects than the triazole antifungal drug itraconazole, which is given systemically but requires only once-daily dosing.¹²⁴

Whereas topical miconazole invariably leads to toxic epitheliopathy, topical clotrimazole is generally well tolerated. In a rabbit model, no epithelial toxicity was seen, although a mild conjunctival reaction was noted.¹⁶⁴

Dimethylsulfoxide

When used as a treatment regimen against Acanthamoeba cysts only, propamidine isethionate 0.1%, miconazole 1%, and neomycin 1% were found not to be cysticidal. When combined with dimethylsulfoxide, however, propamidine isethionate was clearly cysticidal even in low concentrations, suggesting that dimethylsulfoxide may act as a carrier for propamidine, thereby increasing its penetration into the normally drug-resistant cyst form of Acanthamoeba.¹⁶⁵

Corticosteroids

In vitro studies of Acanthamoeba using broth suspensions demonstrated that dexamethasone concentrations of 15 μg/mL and greater inhibited morphogenesis—namely, the conversion of cysts to trophozoites and vice versa.¹⁶⁶ This finding had significant therapeutic implications; however, a rabbit model of Acanthamoeba keratitis failed to confirm the in vitro studies. In fact, the number of corneal infiltrates and the degree of collagen necrosis of the stroma were enhanced with corticosteroid use.¹⁶⁷

Recent successful therapeutic regimens for the treatment of Acanthamoeba keratitis minimize the need for concomitant treatment with topical corticosteroids. Pain can be controlled by topical nonsteroidal agents.¹²⁶ Topical corticosteroids should be limited to specific indications, including limbitis, scleritis, and uveitis.

Conclusions Regarding Medical Management

In vitro and clinical studies have established the efficacy of the biguanide cationic antiseptics chlorhexidine and PHMB. Chlorhexidine digluconate 0.02% and PHMB 0.02% clearly have the lowest minimum trophozoite amebacidal concentrations and the lowest minimal cysticidal concentrations of any pharmaceutical class studied to date. Slight synergy has been found between the cationic antiseptics and pentamidine. Additive effects were observed between the cationic antiseptics plus propamidine or neomycin. The cationic antiseptics and, to a lesser extent, neomycin induce membrane disruption, which may facilitate the entry of effective drugs, such as an aromatic diamidine.^130,132

Recommended therapy would include chlorhexidine digluconate 0.02%, or PHMB 0.02% in combination with propamidine isethionate 0.1%, given hourly day and night for the first 3 days after wide epithelial débridement. Neomycin solution may also be used as part of triple therapy. This loading dose would be followed by an intensive treatment phase in which topical medications are each given every 2 hours while awake and every 4 hours at night for 4 to 7 days. A maintenance phase would follow, in which drops are applied every 4 hours for 7 to 21 days. Finally, a tapering phase would begin, in which a cationic antiseptic may be given alone or in conjunction with propamidine 3 to 4 times daily for up to 4 months, depending on the clinical course. It is during the maintenance phase that any drug causing toxicity may be discontinued, as long as chlorhexidine or PHMB therapy is maintained.^126,132

APPROACHES TO SURGICAL MANAGEMENT

Cryotherapy

In vitro studies of Acanthamoeba keratitis showed that cryotherapy killed trophozoites, but not cysts.¹⁶⁸ Early isolated clinical reports had mixed results when cryotherapy was used as treatment.^84,168 Lindquist and associates⁵⁶ reported one case, and Binder¹⁶⁹ reported five additional cases in which cryotherapy to the infected host cornea, coupled with medical and surgical treatment, successfully eliminated viable organisms in all cases.

It is well documented that Acanthamoeba grow on living or dead bacteria.³¹ Although bacteria or fungi may provide a food source for Acanthamoeba harbored in contact lens cases, Acanthamoeba have been shown to consume intact, living, cultured human corneal epithelial cells and stromal keratocytes as a food source in vitro.^105,170 In early infection with Acanthamoeba, wide epithelial débridement may be effective because it reduces the food supply in addition to debulking the number of organisms. Cryotherapy may be further effective by killing the stromal keratocytes, thereby eliminating the major food supply for Acanthamoeba organisms deep within the corneal stroma. Furthermore, Acanthamoeba cysts that are relatively tolerant to freezing may, in fact, be made more susceptible to medical treatment after freezing.¹⁷¹

Penetrating Keratoplasty

The role and timing of penetrating keratoplasty in the treatment of Acanthamoeba keratitis has been repeatedly debated. Several reports advocated early penetrating keratoplasty because of the difficulties associated with medical therapy.^{71,72,80–82,84,172} However, recurrence of Acanthamoeba infection in the graft and complications of grafting have dampened the enthusiasm for early penetrating keratoplasty.^24,82,84,173

The success of currently available medical treatment suggests that in the absence of a fulminant corneal abscess, perforation, or heavy infection of a recent graft, surgery in the presence of active Acanthamoeba infection is contraindicated until a medical cure has been achieved.^125,173

Results of penetrating keratoplasty for optical reasons in quiescent, medically cured Acanthamoeba keratitis eyes is excellent.^56,173

CONTACT LENS CARE SYSTEMS

Thermal disinfection has been shown to be the most reliable system for eradicating acanthamoebae.^53,54 This involves heating the contact lenses to 80°C for a minimum of 10 minutes according to US Food and Drug Administration guidelines.^54,174 Thermal disinfection does reduce the lifespan of soft contact lenses, and therefore other modalities have been proposed.

Hydrogen peroxide has gained widespread usage as a disinfecting agent. It is toxic to human tissue and therefore needs to be neutralized before the contact lens can be reapplied to the eye. This can be accomplished, however, by enzymatic means or with a metal catalyst that neutralizes hydrogen peroxide to water. Contact lens care systems using 3% hydrogen peroxide and a metal platinum catalyst are ineffective in eradicating Acanthamoeba organisms. The metal catalyst, which is present from the very onset of the disinfection process, neutralizes the hydrogen peroxide long before satisfactory disinfection can occur.^53,175 Contact lenses must be immersed in 3% hydrogen peroxide for a minimum of 2 hours before neutralization to eradicate Acanthamoeba cysts effectively.^175,176

Hydrogen peroxide disinfection systems must involve a two-step system in which a minimum of a 2-hour exposure to 3% hydrogen peroxide is achieved before a chemical neutralizing agent is applied.^88,175 Only then can effective amebacidal, bactericidal, and fungicidal exposure be achieved.

PRESERVATIVES IN CONTACT LENS CARE SYSTEMS

Various preservatives used in contact lens solutions have been shown to be effective amebacidal and bactericidal agents. Chlorhexidine (0.001% and 0.005%) disinfection regimens are very effective against Acanthamoeba organisms, with an effective exposure time as short as 30 minutes. Solutions preserved with 0.004% benzalkonium chloride were effective with exposures of 1 hour,^175,176 whereas the 0.001% concentration required an 8-hour exposure.

Polyaminopropylbiguanide (0.0015%) was amebacidal at 30 minutes; at a concentration of 0.00005%, which is found in a commercial contact lens disinfection solution, it was completely ineffective against Acanthamoeba.¹⁷⁶

Thimerosal (0.004%) in combination with 0.1% edetic acid (EDTA) was effective at 4 hours. However, 0.004% thimerosal alone or 0.001% thimerosal with or without EDTA was ineffective; 0.001% polyquaternium, 0.1% sorbic acid, and 0.1% EDTA were also completely ineffective.

Disinfection regimens requiring exposure times of greater than 1 hour increase the possibility of patient noncompliance. Because noncompliance with manufacturers' recommendations has been found to be a significant risk factor in the etiology of Acanthamoeba keratitis, the simplest regimens have distinct advantages.^19,85 Although the use of disposable soft contact lenses may reduce the need for careful disinfection of contact lens cases, Acanthamoeba keratitis continues to be detected among disposable contact lens wearers. Bacon and co-workers¹²⁶ found that 28 of 64 patients (44%) who acquired Acanthamoeba keratitis while using contact lenses were wearing disposable contact lenses.

Chemical contact lens disinfection systems using 0.005% chlorhexidine, 0.0015% polyaminopropylbiguanide, or 0.004% benzalkonium chloride with 1-hour exposures or 3% hydrogen peroxide with exposures of at least 2 hours before neutralization are effective amebicidal and bactericidal agents. Patients need to be educated that contact lens cases must be regularly disinfected as well. This can be accomplished by a 1-minute exposure to greater than 70°C moist heat, which will kill all Acanthamoeba trophozoites and cysts.¹⁷⁷

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