Microsporidial Stromal Keratitis

Microsporida are spore-forming, obligate eukaryotic protozoan parasites that belong to the phylum Microspora. The ophthalmic manifestations of ocular microsporidiosis exhibit characteristic clinical features depending on the genus involved. With the genus Encephalitozoon, infection is limited to the epithelial cells of the cornea and conjunctiva, producing a diffuse punctate epithelial keratoconjunctivitis. With the genera Nosema and Microsporidium, the infection typically involves the corneal stroma, including the keratocytes.¹

To date, only 5 case reports of microsporidial stromal keratitis have been published.^2-6 We report an additional case caused by Vittaforma corneae (formerly known as Nosema corneum), which was confirmed by histological analysis and electron microscopy.

A 65-year-old white man was referred with a 4-month history of progressive redness, pain, and decreased vision in the right eye. The patient denied any history of herpes zoster, vesicular eruptions, trauma, and contact lens wear. Ophthalmic examination revealed a best-corrected visual acuity of 20/40 OD and 20/20 OS. Findings from an examination of the left eye were normal. The right eye showed moderate diffuse conjunctival hyperemia with a central area of stromal infiltration and edema without suppuration (Figure 1). The anterior chamber had 1+ cells and flare with a 3% hypopyon inferiorly. Findings from the remainder of the ocular examination were unremarkable. The patient was treated with 1% prednisolone acetate and 1% trifluridine, both 6 times a day in the right eye for presumed herpes simplex stromal keratitis. Conjunctival and corneal swabs and scrapings collected for bacterial, chlamydial, fungal, and herpesvirus cultures were all negative for microorganisms.

Over the next 2 months, the visual acuity improved to 20/25 OD, with gradual resolution of the stromal edema. Four weeks later, while the patient was taking a tapering regimen of prednisolone acetate and trifluridine, stromal keratitis recurred with a disciform appearance associated with central epithelial defect as well as superficial corneal vascularization superiorly (Figure 2). Visual acuity was 20/200 OD. Repeated cultures revealed only a few colonies of Staphylococcus epidermidis. Despite appropriate antimicrobial treatment, a persistent epithelial defect ensued with progressive stromal thinning centrally. Ten months after onset of symptoms, the right cornea perforated. The patient underwent a penetrating keratoplasty of the right eye (8.25-mm donor size to 7.75-mm recipient size). The keratectomy specimen was sent for histopathological examination.

Microscopically, the corneal epithelium showed intracellular edema with subepithelial bullae (Figure 3).Bowman's layer was mostly absent. The stroma revealed marked areas of scarring and vascularization with a neutrophilic infiltrate, involving all layers of the stroma. Descemet's membrane was wrinkled and folded. The endothelium was attenuated with a retrocorneal fibrovascular membrane, containing few neutrophils. The corneal stroma showed round to oval organisms measuring 2.0 to 4.0 µm. These organisms were mostly located within the cytoplasm of keratocytes and macrophages, as well as extracellularly. Gram stains showed gram-positive spores, some of which exhibited a polar granule. These organisms stained positively with periodic acid–Schiff and Grocott-methenamine silver methods but were acid-fast negative.

Transmission electron microscopy 1-µm sections revealed scattered sporoblasts, displaying a distinct cell wall with dikaryotic arrangement of the nuclei, which is characteristic of Vittaforma corneae⁷ (Figure 4). A diagnosis of microsporidial stromal keratitis caused by Vittaforma corneae was made.

Six months after the penetrating keratoplasty, the visual acuity improved to 20/25 OD. A blood sample tested with an enzyme-linked immunosorbent assay was negative for antibodies to human immunodeficiency virus. The subsequent postoperative course was complicated by severe microbial keratitis secondary to Pseudomonas aeruginosa infection, leading to perforation. A second penetrating keratoplasty was performed in the right eye approximately 1 year after the first grafting. Histopathological examination of the keratectomy specimen showed acute necrotizing and suppurative keratitis with stromal thinning. No evidence of microsporidial organisms was present.

At the last follow-up examination, approximately 15 months after the second penetrating keratoplasty, the graft remained clear, and visual acuity was 20/40 OD.

Well-documented cases of microsporidiosis involving the corneal stroma are rare. Only 2 genera, Nosema and Encephalitozoon, are known to cause ocular infections.⁶ Deep stromal infections of the cornea have been described in association with trauma leading to stromal scarring and vascularization, ² with atraumatic perforated corneal ulcer manifesting with keratouveitis and hyphema, ³ and with foreign body and persistent corneal ulcer.⁴ In addition, spontaneous stromal keratitis mimicking herpes simplex virus infection has also been described.⁵ Lastly, stromal microsporidiosis has also been reported in a patient with persistent stromal keratitis, which was initially managed as a case of herpes simplex stromal keratitis.⁶ Likewise, our patient was first seen with stromal keratitis resembling herpes simplex virus infection, which ultimately progressed to corneal thinning and perforation, leading to a penetrating graft. Thus, microsporidial ocular infections should be considered in the differential diagnosis of culture-negative stromal keratitis refractory to conventional medical treatment.

Full-thickness corneal transplantation appears to be the only procedure that has proven to be successful for the treatment of deep stromal microsporidiosis. In one of the reported cases, ⁶ a deep lamellar keratoplasty was unable to prevent the recurrence of the disease. Furthermore, the use of both topical fumagillin and oral albendazole failed to control the infection, even after lamellar keratoplasty. Of the 4 case reports of corneal microsporidiosis, ^4-6 including our own, no recurrences of microsporidial infection occurred at the last follow-up examination following a full-thickness penetrating corneal graft. Therefore, one should consider performing a full-thickness penetrating graft rather than a lamellar graft to avoid the possibility of recurrences in the graft.

Identification of microsporidial ocular infections require cytologic and histopathological examination of corneal or conjunctival biopsy specimens combined with transmission electron microscopy, which is essential for accurate species characterization. Distinguishing features, including the clinical findings and ultrastructural features of Encephalitozoon hellem and Vittaforma corneae, are depicted in Table 1. In our patient, dikaryotic arrangement of the nuclei was found, which is characteristic of Vittaforma corneae.^6,7

The authors have no relevant financial interest in this article.

This study was supported in part by grants from the Retina Research Foundation, Houston, and Research to Prevent Blindness, Inc, New York, NY. Dr Font is the recipient of the Senior Investigator Award from Research to Prevent Blindness, Inc.

Corresponding author and reprints: Ramon L. Font, MD, Ophthalmic Pathology Laboratory, Cullen Eye Institute, Baylor College of Medicine, Houston, TX 77030 (e-mail: rfont@bcm.tmc.edu).