Causes

Bacteria are the most common direct cause of acute sinusitis. (Other organisms might be the infecting cause 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 is lasting, or is chronic do begin with. The causes for such chronic sinusitis cases are sometimes unclear.

Upper Respiratory Infections

The typical process leading to acute sinusitis starts with a flu or cold virus. Over 85% of people with colds have inflamed sinuses. These inflammations are typically brief and mild, however, and only between 0.5 - 10% of people with colds develop true sinusitis. Instead, colds and flu set the stage by causing inflammation and congestion in the nasal passages (called rhinitis ), leading 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, other disorders of the immune system, hypothyroidism, cystic fibrosis, Kartagener's syndrome, and Wegener's granulomatosis)
• Structural abnormalities
• Allergic reaction to fungi

Chronic or recurrent acute sinusitis can be a lifelong condition.

Inflammatory Response, Allergies, and Asthma

The absence of bacterial organisms as factor in many cases suggests that some instances of chronic sinusitis may be due to a continuing inflammatory condition. Many of the immune factors observed in people with chronic sinusitis resemble those that appear in allergic rhinitis, suggesting that sinusitis in some individuals is due to an allergic response.

Allergies, asthma, and sinusitis often overlap. Those with allergic rhinitis (so-called hay fever and rose fever) often have symptoms of sinusitis, and true sinusitis can develop as a result of the mucus blockage it causes. A causal association, however, has not been proved, and many experts believe allergies themselves rarely predispose to sinusitis. People with chronic sinusitis may also have an allergic reaction to fungal organisms.