The Brief Case: A “Fresh” Pair of Contact Lenses

DISCUSSION

Acanthamoeba is a genus of free-living amebae that is ubiquitous in nature and can be found in soil, dust, air, sewage, seawater, tap water, and fresh water sources (1, 3). Acanthamoeba can survive under a wide range of conditions with varying osmolarity, temperature, and pH, including levels of chlorination found in municipal water supplies (3).

Acanthamoeba have both cyst and trophozoite stages. Trophozoites are the active, feeding form of the organism, while cysts are the dormant, environmentally resistant form (3). Trophozoites are pleomorphic and have characteristic fine acanthopodia, which allow for movement, adhesion to surfaces, and attachment to prey, such as bacteria or algae. Under adverse conditions, the trophozoite differentiates into a uninucleate cyst, which is resistant to desiccation, starvation, and other environmental changes (3).

There are more than 24 species of Acanthamoeba. Species that can cause human disease include A. castellanii, A. culbertsoni, A. hatchetti, A. healyi, and A. astronyxis. In an immunocompromised host, Acanthamoeba species cause granulomatous amebic encephalitis as well as infections of the skin, nasopharynx, lung, and kidney. In an immunocompetent host, Acanthamoeba can cause keratitis (3). Other microbial causes of keratitis are listed in Table 1.

Acanthamoeba keratitis, also known as amebic keratitis (AK), is a severe ocular infection in which the leading risk factor is contact lens use (4). The risk is higher with soft contact lens use, as cysts can attach more easily to this type of material. Among the soft contact lenses, Acanthamoeba adheres best to silicone hydrogel and nonionic lenses. Rigid gas-permeable lenses have a lower incidence, as Acanthamoeba can be more easily removed from this hard surface (5).

Poor hygiene, infrequent replacement of storage cases, and overnight contact lens wear further increase the risk (4). Another risk factor is corneal trauma with exposure to soil or water. Acanthamoeba gain access to the lens case by tap water or air, adhere well to hydrophilic plastic lenses, and grow rapidly on lenses if they are worn for prolonged periods and not cleaned properly (1). Acanthamoeba can reach densities as high as 10⁵/ml of contact lens solution (1). Acanthamoeba species produce virulence factors, such as adhesins, neuraminidases, proteases, and superoxide dismutases, that can increase pathogenicity (1).

Common symptoms of AK are pain, photophobia, and increased lacrimation, often out of proportion to exam findings. Early in the infection, a diffuse keratopathy can be found, as the organism remains superficial and can cause epitheliitis or radial keratoneuritis (1). AK is also typically characterized by a ring-like or annular corneal infiltrate (6). As Acanthamoeba trophozoites penetrate deeper into the corneal stroma, however, they can cause stromal keratitis (1).

AK can be diagnosed clinically by confocal microscopy, which demonstrates hyper-reflective, spherical, double-walled cysts (Fig. 1) (1). However, confocal microscopy is often available only in specialized ophthalmology clinics; therefore, definitive diagnosis is established with a corneal scrape specimen for microscopy, culture, and/or nucleic acid amplification testing (NAAT) (1). Swabs are suboptimal for diagnosis using microscopy and culture and should be avoided. To prepare a proper sample for analysis, material obtained via corneal scraping should be placed in 200 μl of sterile saline to prevent desiccation. Contact lens solution can be submitted in the lens case directly to the microbiology and cytology laboratories for processing.

Cysts and trophozoites are most commonly visualized on stained preparations using light microscopy. Various stains, including Giemsa, hematoxylin and eosin (H&E), Papanicolaou, Gomori methenamine silver (GMS), periodic acid-Schiff (PAS), and Masson's trichrome stains, will highlight the organisms. Cysts can also easily be visualized using lactophenol-cotton blue, acridine orange, and calcofluor white preparations (1). Antibody-based techniques, such as indirect immunofluorescence and immunoperoxidase staining, can also be used for identification. Trophozoites measure 8 to 40 μm at their greatest dimension and have bubbly or finely granular cytoplasm and a relatively small single nucleus with a large central karyosome (Fig. 2A and B). Cysts measure 10 to 25 μm and have a characteristic double wall, an inner spherical, hexagonal, polygonal, or star-shaped wall (endocyst), and an outer wrinkled fibrous wall (ectocyst) (7).

The plate culture technique is considered the gold standard for diagnosis (1). Corneal scrapings can be applied onto a nonnutrient or nutrient-poor agar covered with nonmucoid bacteria, such as Escherichia coli. Plates are sealed and incubated at 30°C. Over the next 5 to 7 days, a laboratory technician then uses inverted phase-contrast microscopy to screen the plates daily for signs of Acanthamoeba (1). Cysts begin to form after 2 to 3 days of culture, and therefore, both cysts and trophozoites are generally seen. More recently, various NAATs have been described for detection and identification of Acanthamoeba species in clinical specimens (2, 6).

The preferred therapy for AK is a topical biguanide, such as polyhexamethylene biguanide hydrochloride (PHMB) or chlorhexidine gluconate (CLX). Diamidine derivatives, such as hexamidine isethionate, are also effective treatments but should not be used as monotherapy due to potential resistance (6). Topical instillation should be performed hourly for at least the first 72 h but can be extended to a few weeks. This initial intensive phase of treatment can be followed by four-times-daily dosing of chlorhexidine. Duration typically lasts several months. As these topical agents may cause chemical toxicity in the cornea, long-term follow-up is recommended (6).

Unfortunately, topical biguanides may not be able to penetrate the deep corneal stroma, where the Acanthamoeba organisms can invade. The acid-insoluble proteins and alkali-insoluble polysaccharides of the cystic wall may also render Acanthamoeba resistant to topical treatments (6). If the patient does not respond to therapy, a combination of a biguanide and a diamidine can be used. Topical or oral voriconazole can be administered as second-line therapy as well (6).

If medical therapies fail, therapeutic penetrating keratoplasty may be used if infection spreads to the corneal stroma. This procedure involves excision of localized infection and implantation of donor tissue into an undamaged, immunocompetent bed of tissue. Newer techniques include bipedicle conjunctival flap and cryopreserved amniotic membrane graft, which may improve the quality of the ocular surface. However, therapeutic penetrating keratoplasty remains the optimal treatment option, especially with large corneal perforations (1).

In conclusion, AK should be considered in the differential diagnosis whenever any patient with contact lenses or corneal trauma presents with pain, photophobia, or lacrimation (1). If a diagnosis of AK can be established early with plate culture or NAAT, then treatment with a topical biguanide can be more effective while the infection is early and superficial.