Sinusitis refers to inflammation of the sinus cavities, which are hollow chambers in the bones of the skull. Each sinus is lined by a membrane that produces mucus. There are four pairs of sinuses:Frontal sinusesMaxillary sinusesEthmoid sinusesSphenoid sinuses (not shown in illustration)If the opening from a sinus cavity becomes plugged, the flow of mucus is blocked and pressure builds up, causing pain and inflammation.
Review Date: 1/25/2013
Reviewed By: Harvey Simon, MD, Editor-in-Chief, Associate Professor of Medicine, Harvard Medical School; Physician, Massachusetts General Hospital.