Acute rhinosinusitis involves inflammation and infection of the paranasal sinus mucosal lining. The sinuses are air-filled cavities (maxillary, ethmoid, frontal, sphenoid) lined with pseudostratified ciliated columnar epithelium that produces mucus continuously cleared by mucociliary transport through the ostia into the nasal cavity. Obstruction of sinus ostia from mucosal edema (viral URI, allergic rhinitis) traps mucus within the sinus cavity, creating a stagnant, anaerobic environment ideal for bacterial growth. Common pathogens include Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis. Complications arise from direct extension of infection through thin bony walls. The ethmoid sinuses are separated from the orbit by the lamina papyracea (paper-thin bone), and infection can spread to cause preseptal cellulitis, orbital cellulitis, subperiosteal abscess, or orbital abscess (Chandler classification). Frontal sinusitis can extend intracranially through the posterior table causing epidural abscess, subdural empyema, or brain abscess. Cavernous sinus thrombosis can result from retrograde thrombophlebitis through valveless facial and ophthalmic veins.