Clinical Overview of Acanthamoeba Keratitis

Clinical features

Acanthamoeba keratitis occurs when the Acanthamoeba ameba infects the cornea. It likely invades the eye through a physical opening, such as a minor abrasion, in the corneal epithelium, the outermost layer of the cornea. Most people who develop Acanthamoeba keratitis wear contact lenses.

Acanthamoeba keratitis does not produce systemic illness.

Increased risk

People with a greater risk of Acanthamoeba keratitis are those who:

• Improperly store, handle, or disinfect their contact lenses, including using tap water or homemade solutions
• Swim, shower, or use hot tubs while wearing contact lenses
• Come into contact with contaminated water
• Have minor damage to their corneas
• Have experienced previous trauma to their corneas

Unlike non-keratitis Acanthamoeba infections, corneal disease is not associated with immunosuppression.

Symptoms

Symptoms of Acanthamoeba keratitis can include:

• Eye pain
• Eye redness
• Sensation of a foreign body in the eye
• Photophobia (sensitivity to light)
• Decreased visual acuity
• Excessive tearing

Acanthamoeba keratitis is usually unilateral, but it can occur in one or both eyes. A classic feature of the infection is pain that is disproportionate to clinical findings. However, some patients with Acanthamoeba keratitis do not have pain.

Acanthamoeba keratitis can have clinical manifestations similar to other viral, fungal, or bacterial corneal infections. This may cause people with Acanthamoeba keratitis to be misdiagnosed or treated with improper antimicrobial or corticosteroid therapy. These therapies may initially alleviate symptoms but further obscure the clinical picture and diagnosis.

Because the timing of exposure to Acanthamoeba is difficult to assess, and the time required to establish infection is highly dependent on the size of the inoculum, the incubation period for Acanthamoeba keratitis is difficult to determine. It is thought to range from several days to several weeks.

Testing and diagnosis

The first step in diagnosing Acanthamoeba keratitis is to have a high degree of suspicion since the disease can cause permanent vision loss. This is especially important when treating a contact lens wearer with a recent diagnosis of another form of keratitis, such as herpes simplex virus keratitis, who is not responding to therapy.

An early diagnosis offers the best chance of a cure. A diagnosis of Acanthamoeba keratitis is usually based on symptoms, detection of the ameba from a scraping of the eye, or by seeing the ameba by a process called confocal microscopy.

A diagnosis of Acanthamoeba keratitis can be made through isolation of organisms from corneal culture, detection of trophozoites and cysts on histopathology, or detection of Acanthamoeba with a polymerase chain reaction (PCR) test. Confocal microscopy may also assist with diagnosis.

CDC offers diagnostic assistance and testing for Acanthamoeba to physicians and scientists through DPDx and the Free-living and Intestinal Amebas (FLIA) laboratory.

Treatment

Early diagnosis is essential for effective treatment of Acanthamoeba keratitis. The infection can be difficult to treat due to the resilient nature of the cyst form.

Treatment regimens often include a topical cationic antiseptic agent such as polyhexamethylene biguanide (0.02%) or chlorhexidine (0.02%) with a diamidine such as propamidine (0.1%) or hexamidine (0.1%). The duration of therapy may last 6-12 months or longer.

Pain control can be helped by topical cyclopegic solutions and oral nonsteroidal medications. The use of corticosteroids to control inflammation is not often recommended since some studies have found an association between corticosteroid use and poor clinical outcomes.