Sinuses are air filled hollow cavities in the skull. These are present in four pairs – two behind the forehead, two on either side of the bridge of the nose, two behind the eyes and the two largest ones behind the cheek bones.
The sinuses are lined by mucous membranes that produce mucus to lubricate the insides. The sinuses help in warming and moistening the inhaled air. The air from the nose passes through small channels into the sinuses and circulates back into the nasal cavity from which they reach the lungs.
Normally the mucus is drained into the nasal cavity and the sinuses are free of mucus or any form of microorganisms like viruses and bacteria.
The responsibility of clearing the sinuses of these microbes and inhaled pollutants lies with the mucus and with tiny hairs called cilia over the mucous membranes. These cilia are in constant motion and they move to remove the foreign microbes. 1-7
Sinusitis is caused when there is inflammation of the mucous membranes of the sinuses. Causes include viral infection, bacterial infection and so forth.
Viral infection as a cause of sinusitis
The common cause of such inflammation is infection or more commonly upper respiratory tract infection.
The most common cause of sinusitis is the cold virus or influenza (flu). The virus affects the nasal and the sinus mucous membranes and leads to increased mucus formation and inflammation that blocks the nose and the sinuses.
In addition, there may be a secondary bacterial infection of these sinuses that may lead to formation of pus and worsen the situation.
Bacterial and other infections as a cause of sinusitis
Infection may also be bacterial or may be fungal. An infected tooth may also cause the sinuses to become infected although this is rare.
Among bacteria the common causes are Streptococcus pneumonia (in 20 to 43% cases), Haemophilus influenza (in 22-35% cases).
Others include Streptococcus species (3-9%), Moraxella catarrhalis (2-10%), Staphylococcus aureus (4%), anaerobes (5%), and Haemophilus species (8%).
Allergic reactions that lead to sinusitis
Allergic reactions lead to stimulation of excess mucus production that blocks the channels draining the mucus into the nose and may lead to inflammation of the nose and the sinuses.
Allergic rhinitis, seasonal allergic rhinitis, hay fever and asthma are common conditions that may lead to sinusitis.
Pollutants that lead to sinusitis
Pollutants in air, smoke, chemicals, household detergents, disinfectants may all be inhaled and these may lead to irritation of the sinuses and may cause inflammation and sinusitis. Smokers are at a higher risk of sinusitis.
Anatomical defects and sinusitis
Anatomical defects that lead to narrowing of the nose passages like polyps (bulbous benign tumours or growths of the mucous membranes of the nose), deviated or distorted nasal septum (leading to narrowing of one nasal cavity and widening of the other) and a bony spur or growth of the nasal bone.
Children with large adenoids are also at risk of recurrent sinusitis. Mucus can build up behind the narrowed areas. This stagnation and lack of mucus movement leads to a predisposition to sinusitis.
Conditions that may make you more prone to sinusitis
Some conditions do not allow normal movement of the cillary cells or the tiny hairs that remove microbes from the sinuses. This includes Cystic fibrosis – a genetic condition that causes thick, sticky mucus to build up within the body and prevents cillary clearance of the microbes.
Other conditions include ciliary immobility like Cilliary immotile syndrome or Kartagener’s syndrome.
Other risk factors for sinusitis
The following factors predispose to sinusitis in many individuals:
those with nasal dryness
after insertion of nasogastric tubes
mechanical ventilation for assistance in breathing
injuries to the nose and cheeks
Those who have low immune functions for example with HIV AIDS are at higher risk of getting sinusitis. Children in day care are also at a higher risk.
Reviewed by April Cashin-Garbutt, BA Hons (Cantab)