Histopathological Diagnosis of Fungal Sinusitis and Variety of Its Etiologic Fungus

Fungal sinusitis is divided into two categories depending on mucosal invasion by fungus, i.e., invasive and noninvasive. Invasive fungal sinusitis is further divided into acute and chronic disease based on time course. Noninvasive fungal sinusitis includes chronic noninvasive sinusitis (fungal ball type) and allergic fungal sinusitis. Chronic noninvasive sinusitis is the most predominant fungal sinusitis in Japan, followed by allergic fungal sinusitis. Invasive fungal sinusitis is rare. Hyphal tissue invasion is seen in invasive fungal sinusitis. Acute invasive fungal sinusitis demonstrates hyphal vascular invasion while chronic invasive fungal sinusitis usually does not. Fungal tissue invasion is never found in noninvasive sinusitis. A fungal ball may exist adjacent to sinus mucosa, but its hyphae never invade the mucosa. Fungal balls sometimes contain conidial heads and calcium oxalate, which aid in identifying the fungus in the tissue. Allergic fungal sinusitis is characterized by allergic mucin that is admixed with numerous eosinophils and sparsely scattered fungal elements. Histopathology is important in classifying fungal sinusitis, especially in confirming tissue invasion by the fungus.