Small, hair-like projections called cilia must beat in unison to propel mucus outward, expelling bacteria and other particles. The mucous membranes themselves must be intact, and the sinus passages must be open to allow drainage and the circulation of air through the nasal passage. If one or more of these processes or factors are amiss, an infection called sinusitis occurs. For example, a number of conditions may impede the flow of air through the nasal passage, directly obstruct the sinuses, or cause mucus to thicken and stagnate. In such cases, drainage of secretions and the flow of air are blocked.
Secretions build up, encouraging the growth of certain bacteria.
The resulting infection, swelling, and inflammation create further blockage, which may cause the sinuses to close up completely. Sinusitis is usually defined as acute, recurrent acute, or chronic. Acute sinusitis lasts no longer than eight weeks or occurs less than four times a year with each attack lasting no longer than ten days. Acute sinusitis can be successfully treated with medications, leaving no residual damage to the mucous linings. Recurrent acute sinusitis involves more frequent episodes but leaves no significant damage.