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The paranasal sinuses are air-filled cavities inside the bones of the skull. They are located on either side of the nose, behind and between the eyes, and in the forehead; there is also one further back in the head. They probably function to reduce skull weight and to enhance vocal resonance. They also have a protective function, as they absorb severe impact from the front and prevent it being transmitted to the brain.
They are lined with mucus-secreting cells, and they warm, moisten and filter air breathed in. Air reaches the sinuses through small openings in the bones (ostia) that connect to the nasal passageways. The mucus-producing cells have small hair like fibres (cilia) that beat back and forth to help mucus move towards the ostia and out of the sinuses.
If these openings become blocked, air can't properly pass into the sinuses and mucus can't drain out. Mucus builds up in the sinus, causing pressure or pain. Also, the mucus is an excellent culture medium for bacteria, and infection can result.
Reviewed by Dr Harris Steinman MBChB. (UCT), D.CH (SA), FAAAAI, D.Av.Med.(SAMS), Private specialist at FACTS (Food & Allergy Consulting & Testing Services), February 2015.
(Previously updated by Dr H Steinman, June 2007)
