Keratitis, Acanthamoeba

Keratitis refers to the inflammation of the cornea, which is the clear, dome-shaped surface that covers the front of the eye. The cornea is crucial for vision as it focuses light entering the eye, and any damage to it can result in serious visual impairment. Keratitis can be caused by various factors, including infection, injury, or underlying medical conditions, and one of the most concerning causes of keratitis is a microbial infection, particularly Acanthamoeba keratitis (AK).

Acanthamoeba is a type of free-living amoeba commonly found in soil, water, and even in air. While Acanthamoeba infections are rare, they are serious and can lead to severe pain, vision loss, and in some cases, the need for a corneal transplant. Understanding the nature of keratitis, particularly Acanthamoeba keratitis, its causes, symptoms, diagnosis, treatment, and prevention strategies is essential for both medical professionals and the general public.

What is Keratitis?

Keratitis is an inflammation of the cornea that can result from a variety of factors. The inflammation can lead to redness, pain, sensitivity to light, blurred vision, and even corneal scarring if not treated properly. Keratitis is broadly classified into two types: infectious and non-infectious. Non-infectious keratitis might result from trauma, dry eyes, or autoimmune diseases like rheumatoid arthritis, while infectious keratitis is caused by viruses, bacteria, fungi, or parasites.

Among the various infectious agents, Acanthamoeba, a protozoan, is a notable cause of keratitis. Acanthamoeba keratitis is often more challenging to diagnose and treat compared to other forms of infectious keratitis due to the organism’s unusual nature and its resistance to conventional antibiotics.

What is Acanthamoeba Keratitis?

Acanthamoeba keratitis (AK) is a rare, but serious, infection of the cornea caused by the Acanthamoeba species. These amoebae are free-living organisms commonly found in the environment, especially in water sources such as lakes, rivers, and swimming pools, as well as in soil, dust, and air. Despite being ubiquitous, Acanthamoeba can cause keratitis primarily in people who wear contact lenses, especially if the lenses are not cleaned or handled properly.

Acanthamoeba keratitis typically manifests in individuals with weakened immune systems, contact lens users, or those exposed to contaminated water. The infection is notorious for its prolonged course and its ability to cause significant corneal damage, leading to severe vision impairment or even blindness if not treated promptly.

Epidemiology of Acanthamoeba Keratitis

Acanthamoeba keratitis is a rare condition, but its incidence has been increasing over the years, particularly in contact lens wearers. Studies suggest that AK accounts for approximately 1% of all microbial keratitis cases in contact lens users. While the infection can affect anyone, certain risk factors make individuals more susceptible:

1. Contact Lens Use: People who wear contact lenses, especially soft lenses, are at an increased risk. Improper lens hygiene, prolonged wear, exposure to water while wearing lenses (such as swimming), and inadequate cleaning of lenses all contribute to the risk of infection.
2. Immune Suppression: Individuals with compromised immune systems, such as those with HIV/AIDS or those undergoing chemotherapy, are more vulnerable to Acanthamoeba infections. The body’s ability to fight off the infection is reduced, making it easier for the amoeba to invade the cornea.
3. Exposure to Contaminated Water: Swimming or using hot tubs while wearing contact lenses is a significant risk factor. The amoeba can thrive in water sources such as lakes, pools, and even tap water, which can lead to infection if it comes into contact with the eye.
4. Trauma or Injury to the Cornea: Any injury to the cornea, such as from an accidental scratch or foreign object, can make the eye more susceptible to infection, including Acanthamoeba.

Causes and Pathophysiology of Acanthamoeba Keratitis

Acanthamoeba keratitis is caused when the Acanthamoeba organisms come into contact with the corneal epithelium, usually through contaminated contact lenses or water. The pathogenesis of AK involves the amoeba adhering to the corneal surface, where it can invade the tissue and cause damage.

Acanthamoeba exists in two forms: the trophozoite (active form) and the cyst (inactive form). The trophozoite form is the one that causes infection. It can enter the corneal epithelial cells, leading to tissue destruction, inflammation, and the formation of corneal ulcers. The cyst form is more resistant to environmental stressors, such as heat and chlorine, and can survive in contaminated water sources for extended periods.

Symptoms of Acanthamoeba Keratitis

The symptoms of Acanthamoeba keratitis can be quite severe and often resemble those of other types of keratitis, making early diagnosis challenging. Common symptoms include:

• Severe eye pain: This is often disproportionate to the apparent degree of infection.
• Redness of the eye: Affected individuals may experience significant conjunctival redness.
• Blurry vision: Vision may be blurry or impaired due to corneal swelling or ulcers.
• Sensitivity to light (photophobia): The cornea becomes sensitive to light, causing discomfort.
• Tearing or discharge: Excessive tearing or a yellowish discharge may occur.
• Swelling of the eyelids: In some cases, the eyelids may swell as a response to the infection.
• Foreign body sensation: A feeling that something is stuck in the eye.

The symptoms tend to worsen over time if the infection is not addressed promptly, and severe cases can result in permanent scarring, vision loss, or the need for corneal transplantation.

Diagnosis of Acanthamoeba Keratitis

Diagnosing Acanthamoeba keratitis can be quite difficult because the symptoms can mimic other forms of keratitis. A thorough clinical examination, along with laboratory tests, is required to confirm the presence of the infection.

1. Corneal Scraping and Microscopic Examination: A sample of the corneal tissue is taken for examination under a microscope. The presence of Acanthamoeba trophozoites or cysts can be confirmed using specific staining techniques such as Giemsa or calcofluor white.
2. Polymerase Chain Reaction (PCR): PCR is a highly sensitive method used to detect Acanthamoeba DNA in corneal samples. It helps to confirm the diagnosis, particularly in cases where microscopy may not reveal clear results.
3. Culture: Acanthamoeba can be cultured from corneal scrapings. This method allows for the isolation and identification of the organism, although it can be time-consuming and may not always yield results.
4. Confocal Microscopy: This advanced imaging technique allows for in vivo examination of the corneal tissue, providing a clearer picture of the infection and its progression.
5. Corneal Biopsy: In rare cases, a biopsy of the cornea may be required to confirm the diagnosis when other tests are inconclusive.

Treatment of Acanthamoeba Keratitis

Treatment of Acanthamoeba keratitis can be challenging due to the organism’s resistance to many conventional antibiotics. However, with early detection and appropriate treatment, the prognosis can improve significantly.

1. Antimicrobial Therapy: The mainstay of treatment involves the use of antimicrobial agents that target the Acanthamoeba organism. These include:
   • Biguanides (e.g., chlorhexidine, polyhexamethylene biguanide): These are effective against the trophozoite stage of Acanthamoeba.
   • Aminoglycosides (e.g., neomycin, gentamicin): These can be used in combination with biguanides for synergistic effects.
   • Azoles (e.g., itraconazole, ketoconazole): These are antifungal agents that can also exhibit some efficacy against Acanthamoeba.
   • Propamidine isethionate: This drug has been shown to be effective against Acanthamoeba cysts and is often used as part of combination therapy.
2. Topical Medications: Topical treatment is the main approach, with frequent application of the prescribed antimicrobial agents throughout the day. Treatment typically lasts for several weeks or even months.
3. Corneal Transplantation: In severe cases where corneal scarring occurs or vision loss is imminent, a corneal transplant may be required. This procedure involves replacing the damaged cornea with a healthy donor cornea.
4. Steroid Treatment: Steroids may be used cautiously to reduce inflammation, but their use is controversial, as they may suppress the immune system and make the infection worse.
5. Adjunctive Therapies: In some cases, the use of hyperbaric oxygen therapy has been explored as an adjunct to antimicrobial therapy, particularly for severe or resistant cases.

Prevention of Acanthamoeba Keratitis

Preventing Acanthamoeba keratitis primarily involves proper hygiene and care when handling contact lenses. Key prevention strategies include:

1. Proper Contact Lens Hygiene: Regular cleaning of lenses with an appropriate disinfecting solution is essential. Avoid using tap water or saliva to clean lenses.
2. Avoiding Water Exposure: It is crucial not to wear contact lenses while swimming, showering, or using hot tubs, as this increases the risk of contamination.
3. Disposing of Contact Lenses as Recommended: Follow the manufacturer’s instructions for how long lenses should be worn before replacing them.
4. Regular Eye Exams: Routine eye check-ups are essential to detect any potential issues with contact lens use or eye health.
5. Avoiding Contact Lens Wear During Sleep: Contact lenses should be removed before sleep, unless specifically designed for overnight wear.

Acanthamoeba keratitis is a rare but severe condition that primarily affects contact lens users. Early diagnosis and treatment are crucial in preventing permanent vision damage or loss. With proper hygiene, caution around water exposure, and timely intervention, the risk of Acanthamoeba infection can be significantly reduced. Nonetheless, ongoing research into more effective treatments and preventative measures remains important to combat the challenges posed by this infectious disease.

By promoting awareness and understanding of Acanthamoeba keratitis, individuals can make informed decisions about their eye care, minimizing the risk of this debilitating condition and ensuring better overall eye health.