Sinusitis refers to inflammation of the sinus cavities, which are moist, hollow spaces in the bones of the skull. There are four pairs of sinuses:
Sphenoid 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.
Seth Schwartz, MD, MPH, Otolaryngologist, Virginia Mason Medical Center, Seattle, Washington. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M. Health Solutions, Ebix, Inc.