Chronic rhinosinusitis (CRS) is sinusitis, or literally an inflammation of the sinuses, that lasts for longer than 10 to 12 weeks. Symptoms of CRS include:

• difficulty breathing caused by nasal airway obstruction
• nasal congestion (stuffiness)
• nasal discharge (runny nose)
• head and face pain
• decreased sense of smell

CRS should not be confused with acute rhinosinusitis, or acute sinusitis, which is caused by a bacterial or viral infection. Symptoms of acute sinusitis occur over a shorter time period (usually less than four weeks).

CRS is a very common disease that affects 37 million adults and 7 million children in the United States. Although anyone can have CRS, people who have other respiratory conditions such as asthma or allergies are more likely to have CRS than people who do not. If you have any of these other conditions or think you might have CRS, it is important that you speak to your physician who can provide information about CRS and discuss treatment options if a diagnosis is made.

Until recently, the cause of CRS was not fully understood. In 1999, researchers at the Mayo Clinic in Rochester, Minnesota discovered that fungus plays a major role in CRS. Fungus, like pollen and dust mites, is present in the air and is normally found in the nasal passages of healthy people and those with CRS. However in patients with CRS, it is the body's unusual immune response to the fungus that causes symptoms.

In patients with CRS, cells called lymphocytes identify the fungus as foreign and produce chemical-like substances that start an immune response. The substances that are produced, known as cytokines, serve as signals to other cells to start a defense against the fungus. The cytokines involved in this process are called IL-13 and IL-5. These substances bring in and activate other cells called eosinophils. Eosinophils are also part of the body's natural defense system. When eosinophils locate the fungus in the mucus they break apart (i.e., degranulate) and release a toxic substance that kills the fungus. However, this same substance also injures the nasal tissue. It is this damage to the nasal tissue that causes the symptoms of CRS.

Over time this damage can result in nasal polyps and other changes in the structure of the nasal passages. In addition, the damaged tissue can also be vulnerable to infection by the bacteria that cause acute sinusitus.