Eye

• Rare, serious parasitic infection of the cornea occurring mostly in contact lens wearers
• Acanthamoeba are free living, ubiquitous protozoa found in soil and freshwater (tap water, hot tubs, swimming pools)

• ICD-10
  • B60.13 - keratoconjunctivitis due to Acanthamoeba
  • H16.8 - other keratitis

• 85% of cases occur in contact lens wearers (CDC: Sources of Infection and Risk Factors [Accessed 17 November 2023])
• Risk factors include poor contact lens hygiene (e.g., wearing lenses overnight or during swimming / showering) and orthokeratology (specially fitted lenses) (J Clin Med 2021;10:942)

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Contributed by Gabrielle Yeaney, M.D.

Contributed by Gabrielle Yeaney, M.D.

• Herpetic keratitis:
  • Ulceration, may or may not have Cowdry A intranuclear inclusions in corneal epithelium at edge of ulcer (may be IHC HSV1 / HSV2 positive)
  • Lymphocytes, plasma cells, vascularization
  • Multinucleated giant cells around Descemet membrane
• Mycotic (fungal) keratitis:
  • Granulomatous, chronic nongranulomatous or rarely purulent inflammation
  • May have associated hypopyon
  • PAS or GMS positive for yeast or hyphal forms

A 25 year old man with orthokeratology (specially fitted lenses) presents with severe pain in his left eye. The slit lamp examination shows a pattern of radial perineural and ring infiltrate in cornea. Corneal biopsy following negative direct smear examination on corneal scrapes shows focal ulceration of the surface with loss of epithelium; infiltrate of inflammatory cells, mainly polymorphonuclear cells and macrophages, are shown in the image above. What is the most likely diagnosis?

1. Acanthamoeba keratitis
2. Bacterial keratitis
3. Herpetic keratitis
4. Mycotic (fungal) keratitis

A. Acanthamoeba keratitis. H&E sections at high power show several Acanthamoeba infiltrating corneal stroma in varying states of degeneration. Microscopic features on corneal biopsy also include ulceration, necrosis, neutrophilic and macrophage infiltrates; may show nonnecrotizing granulomatous inflammation. Encysted forms may be highlighted with PAS or GMS stains. Answer C is incorrect because a corneal biopsy for herpetic keratitis shows inflammatory infiltrates of mainly lymphocytes, plasma cells with stromal vascularization on H&E sections. Multinucleated giant cells around Descemet membrane are also identified. Answer D is incorrect because a corneal biopsy for mycotic (fungal) keratitis shows granulomatous, chronic nongranulomatous or rarely purulent inflammation on H&E sections; PAS or GMS positive for yeast or hyphal forms. Answer B is incorrect because microscopic findings of infiltration of the stroma by polymorphonuclear leukocytes occur in acute bacterial keratitis. The endothelium is often damaged and may have associated hypopyon and corneal perforation. Gram stain often shows bacteria in the stroma bordering the inflammatory infiltrate.

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Reference: Acanthamoeba keratitis

A 52 year old man who is a soft contact lens wearer presents with left eye pain and vision changes. Ophthalmic exam shows a ring shaped corneal stromal defect. Corneal biopsy reveals necrosis, neutrophilic infiltrates as shown in the image above. Which of the following is the best choice of stain to confirm the diagnosis?

1. AFB Fite
2. AFB Ziehl-Neelsen
3. GMS
4. Gram
5. Mucicarmine

C. GMS. GMS staining will highlight the encysted forms of amoeba in this description of Acanthamoeba keratitis. Answer A is incorrect because AFB Fite is used for Nocardia and would not stain amoebic forms. Answer B is incorrect because AFB Ziehl-Neelsen identifies acid fast organisms such as tuberculosis but would not stain amoebic forms. Answer D is incorrect because Gram stain is typically for bacterial organisms and would not be helpful in amoebic keratitis. Answer E is incorrect because mucicarmine is a stain for mucin and highlights cryptococcal forms, for example but is not helpful in this case of Acanthamoeba keratitis.

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Reference: Acanthamoeba keratitis