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Causes and Risk Factors

In contrast to the nasal passages that are heavily colonized with bacteria, the paranasal sinuses are generally free from harmful bacteria or other pathogens (DeMuri 2009). However, the drainage openings (ostia) that allow the sinuses to empty into the nasal cavity are relatively small, and are thus vulnerable to becoming blocked easily (Merck 2006; Osguthorpe 2001). When this drainage system is blocked, the stagnant mucus begins to accumulate, allowing bacteria and other pathogens to colonize in the sinus cavity, resulting in inflammation and infection (ie, sinusitis) (NIH 2012A; Merck 2006).

Blockage of the ostia can occur as a result of direct mechanical obstruction, or injury that causes swelling in the nose (NIAID 2012; DeMuri 2009). The following table represents potential causes of ostia blockage (Leung 2008; NIH 2012; DeMuri 2009):

Swelling Factors

  • Viral upper respiratory tract infection (ie, common cold)
  • Allergies (eg, hay fever)
  • Cystic fibrosis
  • Chemical inhalation (eg, tobacco smoke)
  • Immune disorders
  • Facial injury
  • Changes in atmospheric pressure (eg, flying, scuba diving)
  • Overusing nasal decongestant sprays

 Mechanical & Anatomical Obstructions

  • Deviated septum
  • Nasal polyps
  • Foreign body
  • Congenital deformity
  • Tumor
  • Nasal bone spur

Although there are multiple risk factors that can contribute to ostia obstruction, allergic inflammation and viral upper respiratory infections (URIs) are the most significant (DeMuri 2009). Infection with a common cold virus is the most frequent cause of viral sinusitis (Mayo Clinic 2012b; Balkissoon 2010). Bacterial sinusitis is much less common, arising as a complication of viral sinusitis in about 0.5-2% of cases (Piccirillo 2004; Leung 2008).  

Other conditions that reduce the clearance of mucus from the sinuses can also contribute to sinusitis (DeMuri 2009). For instance, the common cold virus appears to impair mucus clearance from the sinuses by disrupting the structure and function of the cilia (AAFP 2008; DeMuri 2009). This increases the chances of developing sinusitis, particularly in the maxillary sinuses where the direction of drainage is against gravity (Leung 2008; AAFP 2008).

Since the function of cilia is largely dependent on the quality and quantity of the surrounding mucosal fluid, diseases that dry out the mucosal layer or affect its viscosity (eg, cystic fibrosis) may also contribute to sinusitis (DeMuri 2009; NIH 2012). Ostia blockage is also associated with an increase in mucosal viscosity because the trapped mucus begins to lose its water content. Likewise, sinus inflammation independently thickens sinus secretions through the release of inflammatory debris (NIAID 2012; DeMuri 2009).

In rare cases, fungi can cause sinusitis (NIAID 2012). People with abnormal sinus structures or those with weakened immune systems are more vulnerable to fungal sinusitis (NIAID 2012; Mayo Clinic 2012b; Riechelmann 2011). Between 6 and 9% of all resistant rhinosinusitis cases that require surgery are attributable to fungal infection (Schubert 2009). Unfortunately, surgical treatment is usually needed, since evidence suggests that antifungal treatment is of little to no benefit in the management of chronic rhinosinuisitis due to fungal infection (Sacks 2011, 2012; Isaacs 2011).