PATHOGENESIS
Microsporidia are intracellular protozoa known to cause ocular infection. Initially recognized as an opportunistic pathogen in individuals with AIDS and those with other forms of immunosuppression, this organism is increasingly reported as the cause of infection in immunocompetent persons in Southeast Asia.
CLINICAL PRESENTATION AND EVALUATION
There are 2 distinct clinical presentations of microsporidial infections, depending on the immune status of the patient. In immunocompetent individuals, a corneal stromal keratitis may develop, and in patients with AIDS, conjunctivitis and an epithelial keratopathy may occur (Fig 10-21). The latter group may also have disseminated microsporidiosis involving the sinuses, respiratory tract, or gastrointestinal tract.
Patients present with symptoms that include ocular irritation, photophobia, decreased vision, and bilateral conjunctival injection with little or no associated inflammation. Stromal keratitis is caused by agents of the genus Nosema, whereas the genera Encephalitozoon and Septata have been associated with keratoconjunctivitis. In the keratoconjunctivitis variant, corneal findings include superficial nonstaining opacities described as “mucoid” in appearance, along with dense areas of fine punctate fluorescein staining. The corneal stroma remains clear, with minimal or no iritis.
With light microscopy using the Brown and Hopps stain, small gram-positive microsporidial spores may be identified in conjunctival epithelial cells (Fig 10-22). Transmission electron microscopy (Fig 10-23), immunofluorescence antibody techniques, or elaborate tissue culture techniques may also be used.
MANAGEMENT
Restoration of immune function can lead to resolution of microsporidial keratitis. Although there is no definitive treatment, topical fumagillin has been used to successfully treat microsporidial keratoconjunctivitis, with little toxic effect. In severe cases of Vittaforma corneae (formerly Nosema corneum) infection, granulomatous inflammation may lead to necrotic thinning and perforation. PK may then become the only available treatment for severe stromal thinning. In general, medical regimens require long-term use, and recurrence is common after treatment discontinuation. More recent cases have been reported to be self-limited or responsive to a wide array of commercially available topical ophthalmic antibiotics.
Joseph J, Sridhar MS, Murthy S, Sharma S. Clinical and microbiological profile of microsporidial keratoconjunctivitis in southern India. Ophthalmology. 2006;113(4):531–537.
Loh RS, Chan CM, Ti SE, Lim L, Chan KS, Tan DT. Emerging prevalence of microsporidial keratitis in Singapore: epidemiology, clinical features, and management. Ophthalmology. 2009;116(12):2348–2353.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.