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

Chronic or recurrent acute sinusitis can be a lifelong condition.

The Role of Bacteria. The role of bacteria or other infectious agents is complicated in chronic sinusitis. They may play a direct, an indirect, or, in some patients, infectious agents may not play any role at all. For example, one study reported the following for patients with chronic sinusitis who had not responded to antibiotics:

  • 30% had no evidence of bacteria in their passageways.
  • 20% had bacteria unrelated to infection.

Inflammatory Response, Allergies, and Asthma. The absence of bacterial organisms as a causal factor in many cases suggests that some instances of chronic sinusitis may be due to a continuing inflammatory condition. Such on-going inflammation may have been triggered immune factors that were produced in response to injuries from acute sinusitis. 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.

Severe asthma (which is often associated with allergies) and chronic sinusitis often overlap, although the relationship is unclear. Between 53 - 75% of children with asthma caused by allergies have sinus abnormalities, and various studies have shown that between 17 - 30% of asthmatic patients develop true sinusitis. In fact, chronic sinusitis may actually be the cause of asthma in some cases.

Abnormalities of the Nasal Passage. Abnormalities in the nasal passage can cause blockage and thereby increase the risk for chronic sinusitis. Some abnormalities include:

  • Polyps (small benign growths) in the nasal passage block mucus drainage and restrict airflow. Polyps themselves may be consequences of previous sinus infections that caused overgrowth of the nasal membrane.
  • Enlarged adenoids can lead to sinusitis.
Adenoids picture
Adenoids are masses of tissue located high on the posterior wall of the pharynx. They are made up of lymphatic tissue, which trap and destroy pathogens in the air that enter the nasopharynx.
  • Cleft palate
  • Tumors
  • Deviated septum (a common structural abnormality in which the septum, the center section of the nose, is shifted to one side, usually the left)
Deviated septum picture

Click the icon to see an image of a deviated septum.

Bacterial Sinusitis

The bacteria most commonly implicated in sinusitis include:

  • Streptococcus pneumoniae (also called pneumococcal pneumonia or pneumococci). This bacterium is found in between 20 - 43% of adults and children with sinusitis.
  • H. influenzae (a common bacterium associated with many upper respiratory infections). This bacterium colonizes nearly half of all children by age 2, and causes about 25% of sinusitis cases in this group. Studies have reported the presence of this bacterium in 22 - 35% of adult sinusitis patients.
  • Moraxella catarrhalis. Over 75% of all children harbor this bacterium, which causes about 25% of sinusitis cases.

Other possible bacterial culprits include:

  • Other streptococcal strains
  • Staphylococcus aureus

Fungal Sinusitis

While fungi are an uncommon cause of sinusitis, the incidence of such infections is increasing. At least 5 - 10% of chronic rhinosinusitis patients actually have a form of allergic fungal sinusitis. Some scientists believe that chronic sinusitis may be an immune disorder, rather than an infectious disease. Breakthrough research released in 2004 supports this theory. Researchers from the U.S. National Institute of Allergy and Infectious Diseases exposed immune cells from patients with chronic sinusitis and healthy volunteers to four common types of fungi: Alternaria, Aspergillus, Penicillium, and Cladosporium. Among the study’s findings:

  • Cells from the sinusitis patients released inflammation-inducing proteins called cytokines including interferon-gamma, interleukin-5 (IL-5), and interleukin-13 (IL-13).
  • Cells from healthy patients released very little interferon-gamma and no IL-5 or IL-13.
  • The Alternaria fungus caused the most extreme immune response.
  • The researchers also compared nasal secretions from the two groups and found no difference in fungi levels.
  • These findings suggest that some people who suffer from chronic sinusitis have an extreme immune and inflammatory response to fungi and may benefit from anti-fungal treatment.

Fungi involved in sinusitis include:

  • Aspergillus is the most common cause of all forms of fungal sinusitis.
  • Other fungi include Curvularia, Bipolaris, Alternaria, Dreschslera, Cryptococcus, Candida, Sporothrix,Exserohilum, and Mucormycosis.
  • There have been a few reports of fungal sinusitis caused by Metarrhizium anisopliae, which is used in biological insect control.

There are four categories of fungal sinusitis:

  • Acute or invasive fungal sinusitis - This infection is most likely to affect people with diabetes and compromised immune systems.
  • Chronic or indolent fungal sinusitis - This form is generally found outside the US, most commonly in the Sudan and northern India.
  • Fungus ball (mycetoma) - This fungal sinusitis is noninvasive and occurs usually in one sinus, most often the maxillary sinus.
  • Allergic fungal sinusitis - This form typically occurs because of an allergy to the fungus Aspergillus (rather than being caused by the fungus itself). In such cases, a peanut butter-like fungal growth occurs in the sinus cavities that may cause nasal passage obstruction and the erosion of the bones.

Fungal infections can be very serious, and both chronic and acute fungal sinusitis require immediate treatment. Fungal ball is not invasive and is nearly always treatable.

Fungal infections should be suspected in people with sinusitis who also have diabetes, leukemia, AIDS, or other conditions that impair the immune system. Fungal infections can also occur in patients with healthy immune systems but they are far less common.