Cytologic and differential diagnosis of rhinosporidiosis.

Rhinosporidiosis is a mycotic infection caused by Rhinosporidium seeberi. The fungus occurs in tissues as spherules measuring 0.25-3 mm. The spherules contain endospores. Diagnosis is usually made histologically on biopsy specimens from polypoid lesions on the mucous membranes of the nasopharynx, larynx, trachea, bronchus and conjunctiva. In our experience two cases of rhinosporidiosis were diagnosed by cytology. The cytologic features are typical. On direct examination the spherules are well-circumscribed, globular structures with several endospores within. The spherules show great variability in size, up to 10-fold. The diameter ranges from 30 to 300 microns. Permanent stains for detecting R seeberi are Giemsa, Gridley and toluidine blue. Numerous mycotic infections (Coccidioides immitis, Histoplasma capsulatum, Mucor, Aspergillus, Blastoschizomya capitatus, Paracoccidioides brasilienses, Cryptococcus neoformans) can be definitively diagnosed or strongly suspected on cytology. In immunocompromised patients it is important to commence the diagnostic study on unstained material. By direct examination R seeberi organisms are identified readily by their brown color. Much more information can be gained from material stained with special stains, especially periodic acid-Schiff, in differentiating R seeberi from Coccidioides immitis.