An 11 year old girl from Mississippi with no significant past medical history presents to her dermatologist with a lesion of her left eye. The dermatologist notes a polypoid mass measuring 0.6 cm in diameter at the left medial canthus. A biopsy of the lesion is received for histologic evaluation with a clinical impression of pyogenic granuloma. A representative image is shown to the right (figure 1).
What is your diagnosis?
Answer: C. Rhinosporidiosis
Rhinosporidiosis is a rare chronic granulomatous disease that most commonly affects the nasal and nasopharyngeal mucosae, but which may also involve the conjunctiva, skin, oropharnyx, trachea, and urethra, occasionally presenting as disseminated disease (1-3). The causative agent is generally thought to be Rhinosporidium seeberi, once classified as a fungus but currently categorized as a protist of class Mesomycetozoea, comprised of mostly uncultivable aquatic microbes at the animal-fungal boundary (4). It has also been suggested that the causative organism is a cyanobacterium (5).
The disease is encountered in the Americas, Europe, Africa, and Asia, and has its highest incidence in Sri Lanka and southern India, usually affecting but not restricted to adult men (6). The mode of transmission is unknown, but water and soil are believed to be the reservoir for infection, and the incidence of disease is higher among sandworkers, paddy cultivators and people bathing in stagnant waters (7). Of note, the patient in this case reported recently swimming in a local pond.
Presenting symptoms depend on the location of infection, and include nasal obstruction, epistaxis and viscid nasal discharge when the nasal passages are involved. Clinical examination reveals a friable vascular polyp studded with tiny white dots, giving a characteristic “strawberry” appearance (8). Histologic evaluation demonstrates a variably hyperplastic or attenuated epithelium with an underlying dense lymphoplasmacytic infiltrate and scattered intra- and subepithelial cystic structures called sporangia. The sporangia may measure 350 microns in diameter or more, with a thick wall that may measure up to 5 microns, and containing multiple small (2-10 microns) round endospores (figure 2). The sporangia and endospores both stain with H&E, GMS, mucicarmine, and PAS (7, 9).
In addition to the lymphoplasmacytic infiltrate, neutrophils, histiocytes, and occasional giant cells may be seen, as well as subepithelial hypervascularity and foci of necrosis (10) (figure 3). The histologic differential diagnosis includes Coccidioides immitis, which is smaller and negative for mucicarmine, and myospherulosis, a foreign body response to petroleum-based material that does not stain with PAS or GMS. Protothecosis is caused by opportunistic infection by the green algae Prototheca wickerhamii, which is histologically dissimilar from rhinosporidiosis. The mainstay of treatment is surgical removal of lesions, occasionally with dapsone as adjunct therapy, andrecurrenceisaknowncomplication(11,12).
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