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Acanthamoeba Keratitis

What is Acanthamoeba Keratitis?

Acanthamoeba Keratitis (AK) is a rare infection in the cornea of the eye. The cornea is a transparent layer that covers the front of the eye. A condition in which the cornea becomes inflamed is called keratitis. AK is caused by an organism called acanthamoeba, normally a free-living organism and harmless to humans. In rare instances, acanthamoeba can cause an opportunistic infection, meaning that an organism that is usually harmless causes an infection when an opportunity presents itself, such as with a weakened immune system. AK is more common in contact lens users, but has been reported in non-contact lens users as well. Symptoms include light sensitivity (photophobia), pain, and excess tearing. If AK is not treated, it can lead to loss of vision. There are some types of acanthamoeba that are resistant to traditional antibiotic therapy.

 

Acanthamoeba Keratitis (AK) is a rare infection in the cornea of the eye. The cornea is a transparent layer that covers the front of the eye. A condition in which the cornea becomes inflamed is called keratitis. AK is caused by an organism called acanthamoeba, normally a free-living organism and harmless to humans. In rare instances, acanthamoeba can cause an opportunistic infection, meaning that an organism that is usually harmless causes an infection when an opportunity presents itself, such as with a weakened immune system. AK is more common in contact lens users, but has been reported in non-contact lens users as well. Symptoms include light sensitivity (photophobia), pain, and excess tearing. If AK is not treated, it can lead to loss of vision. There are some types of acanthamoeba that are resistant to traditional antibiotic therapy.

Acknowledgement of Acanthamoeba Keratitis has not been added yet.

The worldwide prevalence of AK is between 1 to 30 individuals per one million individuals. Since it was first described in the 1970s, its prevalence has been increasing dramatically because of the increase in the use of contact lenses. In most AK cases, affected individuals report contact lens use.

Name Abbreviation
AK Acanthamoeba Keratitis

Acanthamoeba is a type of organism that has at least 24 different species that can be found in soil and water sources and supplies. Humans are exposed to these organisms on a daily basis, and normal corneal barriers do not allow for an infection to occur. In its active form, acanthamoeba can feed on other organisms such as bacteria and fungi by engulfing and digesting any small particle it encounters. When food sources are low for more than a few days, the organism transforms into an inactive or dormant cyst that can resist extreme conditions. The cyst remains viable for years, and once it is exposed to a food source, it becomes active again.  

When corneal barriers are disrupted as a result of contact lens use, trauma, and poor hygiene, acanthamoeba takes advantage of this opportunity to infect the cornea. It engulfs and digests the cells of the cornea. When the active form of the organism attaches to the outer layer of the cornea, it secretes chemicals that degrade corneal cells and allow the acanthamoeba to invade the middle layers of the cornea. Once inside the cornea, the organism causes the immune system to initiate an immune response known as inflammation.

Inflammation is a critical immune response and can damage the body if it persists. The chronic inflammation caused by acanthamoeba leads to keratitis and corneal cell death. The organism can feed on the cells of the cornea (keratocytes) by engulfing the cells and digesting them. Once the cornea is fully invaded, acanthamoeba cluster around the nerves of the cornea which is believed to contribute to the severe pain experienced by affected individuals.

Contact lens use increases the risk of infection significantly because it causes microscopic injuries to the cornea. Additionally, it leads to the increased presence of molecules on the surface of corneal cells that the organism can adhere to. The organism can inhabit the lens through other sources such as tap water. It multiplies rapidly and comes into contact with the cornea through the insertion of a contact lens. AK can also occur in individuals who do not use contact lenses, such as corneal trauma, exposure to contaminated water, and poor hygiene.

The earliest signs of AK are usually non-specific and involve mild to severe pain, light sensitivity, blurred vision, and excessive tearing. While AK is typically found in one eye, it can affect both eyes in contact lens users.

The presence of white blood cells in the cornea is known as corneal infiltrate and presents as white opacities on the cornea. The corneal infiltrate surrounds the corneal nerves in a radial distribution (radial perineural keratitis) which is common in early-stage AK. As the infection progresses, a white ring can form on the cornea known as a ring infiltrate which is a common characteristic of AK. Inflammation of the white of the eye (scleritis), the border of the cornea, and the white of the eye (limbitis) are common.

The anterior chamber is the space between the cornea and the colored part of the eye (iris) that is filled with an aqueous fluid. In some severe cases of AK, the anterior chamber may become inflamed. As white blood cells accumulate in the anterior chamber, a hypopyon forms. A hypopyon is the layering of white blood cells in the anterior chamber which can be clearly visible to the naked eye. In some cases, cataract(s) may occur. Glaucoma is also frequently reported. Glaucoma is an eye condition where there is an increase in pressure inside the eye. It can damage the optic nerve and lead to vision loss. If glaucoma is untreated, it could lead to the thinning of the cornea or even corneal rupture (corneal perforation).

 

Diagnosis of AK is based on clinical observations and laboratory findings. AK may be suspected in any case of corneal trauma in all contact lens users or in individuals with recent exposure to contaminated soil or water.

Diagnosis is easier in the early stages when the infection is still superficial or on the surface level. Microscopy allows imaging of the microscopic structure of the eye and visualization of the organisms for a noninvasive and tentative diagnosis. However, to confirm the diagnosis with certainty, corneal samples must be obtained and investigated using laboratory techniques. If acanthamoeba is detected in corneal samples, then the diagnosis is confirmed. In some cases, clinical evidence is highly suggestive of AK but the organism is not detected. In such cases, a clinical diagnosis can be made. For contact lens users, it might be helpful to investigate the contact lens or its storage case. Detection of the organism alone on the contact lens does not confirm the diagnosis because acanthamoeba can be present on contact lenses of unaffected individuals. If the organism is not detected on the contact lens, AK is unlikely.

In vivo confocal microscopy (IVCM) can be used for a tentative diagnosis of AK. IVCM is a noninvasive imaging technique that visualizes the individual corneal cells and their inflammatory response. If the organism is present in the cyst form, it presents with a distinct structure in IVCM. The active form is more difficult to differentiate from normal cells of the cornea. The gold standard to confirm the diagnosis of AK is to obtain corneal scrapings and culture the sample.

A culture is a method for multiplying microbial organisms by allowing them to reproduce under controlled laboratory conditions in a specific medium. The corneal sample is cultured in the presence of the bacteria  E. coli. If acanthamoeba is present, they will move across the plate to feed on the bacteria and leave characteristic markings that can be observed by a microscope.

In advanced cases with deeper infections, a corneal biopsy may be necessary. A biopsy is similar to corneal scraping. A piece or sample of the cornea is taken for testing. It involves a deep incision to obtain a sample deep in the cornea.

Another method that is being increasingly used is the detection of acanthamoeba from corneal scraping with a polymerase chain reaction (PCR). For PCR-based diagnosis, the genetic material (DNA) of the cells obtained from corneal scraping is extracted. The PCR is used to create many more copies of DNA target sequences. The amplified genetic sequences are then analyzed to identify the organism to which they belong to. If acanthamoeba is present, its DNA is detected through this method. In contact lens users, their contact lenses and storage cases may also be samples and investigated by PCR or microbial cultures.

 

The treatment goal of AK is to eliminate all forms of organisms and to decrease inflammation. There is currently no single agent that is effective against the active organisms and the cysts. Treatment involves several methods or therapies. Cysts are highly resistant to treatment.

The antimicrobial drug combination therapy includes antibiotics, antiseptics, antifungals, and antiprotozoals. Cysticial drugs can be effective against the highly resistant cysts but can have many risks and side effects.

In some affected individuals, keratectomy may be appropriate. It is the removal of damaged tissues and cells on the cornea surface, followed by the immediate administration of antimicrobial drugs.

For inflammation, anti-inflammatory medications or eyedrops can be used, such as non-steroidal anti-inflammatory drugs (NSAIDS) or steroids. The use of steroids is not preferred by some doctors because it suppresses the immune system from fighting the infection off. Some doctors recommend the use of steroids for inflammation after the organism has been eliminated.

Corneal thinning and perforation have been observed in some severe cases, and corneal transplant may be required. After successful treatment, the immune system can be weakened. Affected individuals must be wary and prevent recurrent infection.

 

Prognosis depends on the stage at which the diagnosis is made. Early diagnosis and treatment increase the likelihood of a good visual outcome. The later treatment is initiated, the poorer the prognosis.

Tips or Suggestions of Acanthamoeba Keratitis has not been added yet.

Lorenzo-Morales J, Khan NA, Walochnik J. An update on Acanthamoeba keratitis: diagnosis, pathogenesis and treatment. Parasite. 2015;22:10. Available from doi: 10.1051/parasite/2015010


Illingworth C, Cook S. Acanthamoeba Keratitis. Survey of Ophthalmology. 1998;42(6):493-508. Available from https://doi.org/10.1016/S0039-6257(98)00004-6


Jiang C, Sun X, Wang Z, Zhang. Acanthamoeba Keratitis: Clinical Characteristics and Management. The Ocular Surface. 2015;13(2);164-168. Available from https://doi.org/10.1016/j.jtos.2015.01.002

Maycock N, Jayaswal R. Update on Acanthamoeba Keratitis: Diagnosis, Treatment, and Outcomes. Cornea. 2016; 35(5):713–720. Available from DOI: 10.1097/ICO.0000000000000804

Page MA, Mathers WD. Acanthamoeba keratitis: a 12-year experience covering a wide spectrum of presentations, diagnoses, and outcomes. J Ophthalmol. 2013;2013:670242.

Alkharashi M, Lindsley K, Law HA, Sikder S. Medical interventions for acanthamoeba keratitis. Cochrane Database Syst Rev. 2015;2(2):CD010792. Published 2015 Feb 24. doi:10.1002/14651858.CD010792.pub2

Panjwani N. Pathogenesis of acanthamoeba keratitis. Ocul Surf. 2010;8(2):70-9. Available from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3072032/

Zhong J, Li X, Deng Y, et al. Associated factors, diagnosis and management of Acanthamoeba keratitis in a referral Center in Southern China. BMC Ophthalmol. 2017;17(1):175. Published 2017 Oct 2. doi:10.1186/s12886-017-0571-7

 

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Last updated 27 Dec 2008, 05:56 AM

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