Causes

Causes

Bacteria are the most common direct cause of acute sinusitis. (Other organisms might be the infecting agents in less common cases.) The ability of bacteria or other organisms to infect the sinuses, however, must first be set up by conditions that create a favorable environment in the sinus cavities. Sinusitis is most often an acute condition, which is self-limiting and treatable. In some cases, however, the inflammation in the sinuses persists or is chronic do begin with. The causes for such chronic sinusitis cases are sometimes unclear.

Conditions That Cause Acute Sinusitis

The typical process leading to acute sinusitis starts with a flu or cold virus. Viruses themselves do not usually cause sinusitis directly and are implicated in only about 10% of sinusitis cases. Instead, they set the stage by causing inflammation and congestion in the nasal passages (called rhinitis) that leads to obstruction in the sinuses. This creates a hospitable environment for bacterial growth, which is the direct cause of sinus infection. In fact, rhinitis is the precursor to sinusitis in so many cases that expert groups now refer to most cases of sinusitis as rhinosinusitis.

Rhinosinusitis tends to involve the following sinuses:

• The maxillary sinuses (behind the cheekbones) are the most common sites.
• The ethmoid sinuses (between the eyes) are the second most common sites affected by colds.
• The frontal (behind the forehead) and sphenoid (behind the eyes) sinuses are involved in about a third of cold-related cases.

Nearly everyone with colds has inflamed sinuses. These inflammations are typically brief and mild, however, and most people with colds do not develop true sinusitis.

Conditions That Cause Chronic or Recurrent Sinusitis

Chronic or recurrent acute sinusitis typically results from one of the following conditions:

• Untreated acute sinusitis that results in damage to the mucous membranes
• Chronic medical disorders that cause inflammation in the airways or persistent thickened stagnant mucus (such as diabetes, AIDS or other disorders of the immune system, hypothyroidism, cystic fibrosis, Kartagener's syndrome, and Wegener's granulomatosis)
• Structural abnormalities