Fungal keratitis is an infection of the cornea (the clear dome covering the colored part of the eye) that is caused by a fungus. Some fungi that have been known to commonly cause fungal keratitis include:
- Fusarium species
- Aspergillus species
- Candida species
Fungal keratitis tends to occur after corneal injury involving plant material or in an agricultural setting, in eyes with chronic ocular surface disease, and increasingly in contact lens wearers. It is usually an indolent process, with the cornea characteristically having multiple stromal abscesses and relatively little epithelial loss. Intraocular infection is common.
Besides fungal keratitis, other types of keratitis include:
- amoebic keratitis, which is an infection with amoeba. It is often caused by Acanthamoeba and usually affects contact lens wearers.
- bacterial keratitis, which is an infection with bacteria
- herpes keratitis, which is an infection with herpes simplex or herpes zoster viruses
- photokeratitis, due to intense ultraviolet (UV) radiation exposure. Examples include snow blindness or welder’s arc eye.
What puts people at risk for fungal keratitis?
The most common way that someone gets fungal keratitis is after experiencing trauma to the eye especially trauma caused by a stick, thorn, or plant.
Risks for developing fungal keratitis include:
- Recent eye trauma, particularly involving plants (for example, thorns or sticks)
- Underlying eye disease
- Weakened immune system
- Contact lens use
A high degree of suspicion from the physician accounts for early diagnosis and treatment, which are paramount for a successful resolution of the fungal keratitis. Corneal ulcers unresponsive to broad-spectrum antibiotics, the presence of satellite lesions, and scanty secretions in a large ulcer are some signs that should raise flags to the attending professional about the possibility of a mycotic agent.
After establishing the patient’s general condition, the examiner should look for evidence of ocular surface disease and determine the amount and type of secretions and lid swelling. The upper eyelid should be everted to exclude a retained foreign body. The examiner should measure the size and depth of the lesion as well as the presence of satellite lesions. The intraocular pressure should also be ascertained. Anterior chamber reaction and evidence of hypopyon should also be recorded. Vitreous reaction if present may suggest intraocular spread of the disease.
Under the slit lamp, early on the lesion might look like an unhealed corneal abrasion with scanty infiltrates and no secretions. With time however, the ulcer develops thicker infiltrates and fuzzy margins. The presence of satellite lesions strongly suggests a fungal infection. Redness and periocular edema are also common. This combined with a history of trauma, especially with vegetable matter, ocular surface disease or chronic use of topical steroids should alert the practitioner to the possibility of a mycotic etiology.
Corneal scrapings are taken from deep within the lesion with a surgical blade or sterile spatula. To perform a corneal biopsy, a dermatological 2mm punch can be used.
For a definitive diagnosis, scrapings taken from deep within the lesion should be made and inoculated in Sabouraud agar. The shortcoming is that it can take up to 3 weeks to grow and identify the organism. For a faster result, smears with special stains such as Gomori, PAS, acridine orange, calcofluor white or KOH should be performed. The drawback is that not all laboratories have these stains available, so, again we might need to rely on the patient’s evolution and the physician’s clinical acumen. If all labs and cultures are negative, corneal biopsy should be considered to obtain a specimen.
Fungal infections can mimic any microbial keratitis secondary to other causes: Bacteria, which is the most common cause of corneal infections; Acanthamoeba, related to swimming with contact lenses and or the use of tap water in their cleaning; or herpes simplex or herpes zoster for which recurrences are frequent. Other conditions such as a retained foreign body, sterile infiltrates, marginal ulcers due to Staphylococcal hypersensitivity or chronic epithelial defect should also be ruled out. Again, a high index of suspicion is important in the diagnosis of fungal keratitis.
Corneal scrapings should be cultured on media suitable for fungi whenever the history or corneal appearance is suggestive of fungal disease. Diagnosis is often delayed and treatment is difficult. Natamycin 5%, amphotericin 0.1–0.5%, and voriconazole 0.2–1% are the most commonly used topical agents. Systemic azoles are probably not helpful unless there is scleritis or intraocular infection. Corneal grafting is often required.
Wearing safety glasses while gardening will diminish the risk of ocular trauma. Also, general hygiene, proper contact lens care, and avoidance of nonessential steroid use should diminish the probability of mycotic infection.