PROTOZOAL KERATITIS

ACANTHAMOEBA KERATITIS
Acanthamoeba keratitis has recently gained importance because of its increasing incidence, difficulty in diagnosis and unsatisfactory treatment.
Etiology
Acanthamoeba is a free lying amoeba found in soil, fresh water, well water, sea water, sewage and air. It exists in trophozoite and encysted forms.

Mode of infection. Corneal infection with acanthamoeba results from direct corneal contact with any material or water contaminated with the organism.
Following situations of

contamination have been described:
1. Contact lens wearers using home-made saline (from contaminated tap water and saline tablets)is the commonest situation recognised for acanthamoeba infection in western countries.
2. Other situations include mild trauma associated with contaminated vegetable matter, salt water diving, wind blown contaminant and hot tub use.
Trauma with organic matter and exposure to muddy water are the major predisposing factors in developing countries.
3. Opportunistic infection. Acanthamoeba keratitis can also occur as opportunistic infection in patients with herpetic keratitis, bacterial keratitis,
bullous keratopathy and neuroparalytic keratitis.

Clinical features
Symptoms. These include very severe pain (out of proportion to the degree of inflammation), watering, photophobia, blepharospasm and blurred vision.

Signs. Acanthamoeba keratitis evolves over several months as a gradual worsening keratitis with periods of temporary remission. Presentation is markedly variable, making diagnosis difficult. Characterstic features are described below :
1. Initial lesions of acanthamoeba keratitis are in the form of limbitis, coarse, opaque streaks, fine epithelial and subepithelial opacities, and radial kerato-neuritis, in the form of infiltrates along corneal nerves.
2. Advanced cases show a central or paracentral ring-shaped lesion with stromal infiltrates and an overlying epithelial defect, ultimately presenting
as ring abscess (Fig. 1). Hypopyon may also be present.

Fig. 1. Ring infiltrate (A) and ring abscess (B) in a patient
with advanced acanthamoeba keratitis.

Diagnosis
1. Clinical diagnosis. It is difficult and usually made by exclusion with strong clinical suspicion out of the non-responsive patients being treated for herpetic,
bacterial or fungal keratitis.
2. Laboratory diagnosis. Corneal scrapings may be helpful in some cases as under:
i. Potassium hydroxide(KOH) mount is reliable in experienced hands for recognition of acanthamoeba cysts.
ii. Calcofluor white stain is a fluorescent brightener which stains the cysts of
acanthamoeba bright apple green under fluorescence microscope.
iii. Lactophenol cotton blue stained film is also useful for demonstration of acanthamoeba cysts in the corneal scrapings.
iv. Culture on non-nutrient agar (E. coli enriched) may show trophozoites within 48 hours, which gradually turn into cysts.

Treatment
It is usually unsatisfactory.
1. Non-specific treatment is on the general lines for corneal ulcer .

2. Specific medical treatment includes:
(a) 0.1 percent propamidine isethionate (Brolene) drops;
(b) Neomycin drops; 
(c) Polyhexamethylene biguanide (0.01%–0.02% solution);
(d)chlorhexidine; (e) other drugs that may be useful are paromomycin and various topical and oral imidazoles such as fluconazole, itraconazole and miconazole. Duration of medical treatment is very large (6 months to 1 year).

3. Penetrating keratoplasty is frequently required in non-responsive cases.

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