Acanthamoeba keratitis is a severe eye infection affecting the cornea, the clear outer layer of the eye. It is caused by Acanthamoeba, a microscopic, single-celled organism found in water and soil. This infection can lead to significant vision loss or blindness if not diagnosed and treated promptly, especially for contact lens wearers.
Understanding Acanthamoeba Keratitis
Acanthamoeba is a free-living amoeba that exists in two forms: an active trophozoite and a dormant, resistant cyst. These organisms are widespread, found in water sources like tap water, swimming pools, hot tubs, and lakes, as well as in soil and dust. The infection, keratitis, occurs when these amoebae invade the cornea, often through a minor abrasion or compromise in its outermost layer, the epithelium.
Individuals primarily contract this infection through exposure to contaminated water or improper contact lens hygiene. Contact lens wearers are at the highest risk, accounting for over 80% of cases in the United States. Risk factors include:
Showering, swimming, or using hot tubs while wearing lenses.
Using tap water or homemade solutions to clean lenses or cases.
Not replacing lens cases regularly.
The amoeba can adhere to contact lens surfaces and transfer to the cornea, especially if the corneal surface has microtrauma from lens wear.
Recognizing the Symptoms
Symptoms of Acanthamoeba keratitis can vary and often mimic other common eye infections, making early diagnosis challenging. A characteristic sign is severe eye pain, which can be disproportionate to visible signs of infection. This intense pain is sometimes due to the amoeba grouping around corneal nerves.
Other common symptoms include eye redness, blurred vision, and sensitivity to light (photophobia). Patients may also experience a persistent sensation of something in their eye, along with excessive tearing. In later stages, a visible ring-shaped infiltrate may appear on the cornea, though this is only present in about 50% of patients. These symptoms can last for weeks or months and may fluctuate in severity.
Diagnosis and Treatment Approaches
Diagnosing Acanthamoeba keratitis is challenging due to its overlapping symptoms with other eye conditions. A comprehensive eye examination is performed, followed by specific laboratory tests. Corneal scraping, where a small sample of corneal tissue is collected, is a common diagnostic procedure. This sample is then analyzed under a microscope or cultured on a special medium with E. coli to encourage amoeba growth.
Confocal microscopy, a non-invasive imaging technique, is also used for diagnosis, allowing real-time examination of the cornea to detect the amoeba. Polymerase chain reaction (PCR) tests, which detect the amoeba’s DNA, are also available and can be a useful adjunct. A combination of these methods provides the most accurate and rapid identification.
Treatment for Acanthamoeba keratitis involves a prolonged regimen of topical antiseptic drops, for several months to a year. Common medications include polyhexamethylene biguanide (PHMB) and chlorhexidine, biguanides that disrupt the amoeba’s cell wall. These medications are applied hourly or every few hours, then gradually tapered as the infection improves. Early and consistent treatment is important for better outcomes, as untreated infections can lead to permanent vision loss or the need for a corneal transplant.
Preventing Infection
Preventing Acanthamoeba keratitis focuses on meticulous contact lens hygiene. Always wash your hands thoroughly with soap and water and dry them completely before handling contact lenses. This step helps prevent the transfer of microorganisms from your hands to your lenses.
To prevent infection:
Use only fresh, sterile contact lens solution for cleaning, rinsing, and storing your lenses. Never use tap water, distilled water, or homemade saline solutions, as these can contain Acanthamoeba.
Clean your lens case with fresh solution, not water, and allow it to air dry upside down with the caps off.
Replace your contact lens case regularly, every one to three months, to prevent bacterial and amoebic buildup.
Avoid wearing contact lenses while swimming, showering, or using a hot tub.
Never sleep in your lenses unless specifically approved by your eye care professional.