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 sinuses
- Maxillary sinuses
- Ethmoid 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.