Radiologic studies are essential in the diagnosis of severe, refractory, nosocomial, or complicated cases of sinusitis but are not routinely necessary. Because of technical difficulties in obtaining valid cultures, the bacteriology of sinusitis has been incompletely defined. Nasal cultures are unreliable because they correlate poorly with actual sinus fluid. Sinus punctures reveal that the most common pathogens in acute sinusitis are Hemophilus influenzae, pneumococci, streptococci, and Moraxella catarrhalis . Staphylococcus aureus has been isolated in nasal cultures from some patients with acute sinusitis, but the organisms are probably contaminants rather than the true causative agents.
Chronic sinusitis is usually a polymicrobial infection.
Anaerobes are also important in chronic sinusitis because they predominate in brain abscesses associated with sinus infections. Viruses seldom cause clinical sinusitis. In immunocompromised patients, sinusitis may also be caused by Candida, Alternaria, Pseudomonas, Nocardia, Le gionella, atypical mycobacteria, and certain parasites. Finally, noninfectious processes must be considered in the differential diagnosis of sinusitis. Allergic or vasomotor rhinitis is by far the most common noninfectious cause of sinus symptoms.