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Acute sinusitis (rhinosinusitis) is inflammation of the paranasal sinuses, most commonly caused by viral upper respiratory infection. Bacterial superinfection occurs in a minority of cases and is distinguished by specific clinical features that guide antibiotic prescribing decisions.
Mucosal edema from viral infection or allergen exposure obstructs the sinus ostia, impairing mucociliary clearance and creating conditions for bacterial overgrowth. Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis are the most common bacterial pathogens. Chronic sinusitis (>12 weeks) involves distinct inflammatory mechanisms often including biofilm formation and eosinophilic inflammation.
The IDSA clinical decision rule identifies bacterial sinusitis by: (1) persistent symptoms >10 days without improvement, (2) severe symptoms (high fever ≥39°C, purulent nasal discharge, facial pain) for ≥3 consecutive days, or (3) worsening symptoms after initial improvement (double sickening). Imaging is not routinely recommended for uncomplicated cases. Differentials include allergic rhinitis, nasal polyps, and dental abscess. Complications — orbital cellulitis, cavernous sinus thrombosis, meningitis — require urgent imaging and surgical evaluation.