Disorders that cause swelling of the membranes of the nose, such as allergic rhinitis or viral respiratory infections, are the most common cause because the swelling prevents fluid from draining out of the sinus normally. A deviated nasal septum or other obstruction of the nose may also trap fluid in a sinus. Occasionally, swimming or immersion of the head in water may allow water and bacteria to enter the sinus, causing irritation and infection. The fluid trapped in the sinus may then become infected with bacteria, viruses, or fungi. Dental infections such as tooth abscess may spread into the sinus and infect it directly.
Acute sinusitis is most commonly the result of a bacterial infection.
Chronic sinusitis is much less common than acute sinusitis. When sinusitis recurs frequently, or lasts for a prolonged time, it is classified as chronic . The pain of sinusitis results from the inflammation itself or from pressure within the sinus from accumulation of undrained fluid. A CBC shows elevated white blood cell count . A sinus or head X-rays , cranial CT scan , or cranial MRI shows sinusitis or thickening of the membranes of the sinus.