Bronchitis is the result of an infection in the inner walls of the bronchi or airways in the lungs. This infection leads to inflammation, irritation and scarring of the inner mucosal membranes of the airways.
Pathology of bronchitis
There is hypertrophy of the mucus-secreting bronchial glands. This means there is an increase in size and number of these glands. This leads to an increase in the bronchial wall thickness. This thickness is associated with inflammation.
There may be other features like formation of ulcers, infiltration by inflammatory cells like neutrophils, loss of the tiny hair like cilia on the mucosal surfaces, invasion by bacteria etc. There may be areas where the epithelium is changed due to inflammation and scarring.
Causes and risk factors
Some of the causes and risk factors associated with bronchitis include:-
Infection is more often than not caused by either a virus or bacteria, although viral bronchitis is much more common. In most cases acute bronchitis results from the same viruses that cause the common cold or influenza (flu).
The viruses spread by travelling in the tiny droplets of secretions that come out of the nose and mouth when someone coughs or sneezes. They are present in the infected person’s secretion in millions and are inhaled by healthy persons who catch the viral infection.
These droplets typically spread about 1m and can hold suspended in the air for a while. They may be inhaled while they are suspended. They can then land on surfaces where the viruses survive for up to 24 hours. Anyone who touches these surfaces can spread the virus further by touching something else.
Common surfaces that carry common cold and flu viruses include keyboards, phones, ATMs, door handles, children’s toys etc. People usually become infected by picking up the virus on their hands from these contaminated objects and then placing their hands near their mouth or nose.
Age wise there is a difference in possible causative virus infection:-
In patients younger than one year, respiratory syncytial virus, parainfluenza virus, and coronavirus are the most common causes.
In patients one to 10 years of age, parainfluenza virus, enterovirus, respiratory syncytial virus, and rhinovirus predominate.
In patients older than 10 years, influenza virus, respiratory syncytial virus, and adenovirus are most frequent.
Time of the year also determines the commonest type of infection. For example:-
Parainfluenza virus, enterovirus, and rhinovirus infections most commonly occur in the fall.
Influenza virus, respiratory syncytial virus, and coronavirus infections are most frequent in the winter and spring
Air pollutants or irritants
Bronchitis can be exacerbated and triggered by breathing in irritant substances that includes air pollutants, smoke, smog, chemicals, grain dust, fabric fibres, ammonia, strong acids or chlorine and dust etc.
Smoking as well as passive exposure to cigarette smoke is one of the most important risk factor associated with bronchitis. Smoking is the main cause of chronic (long-term) bronchitis.
Reviewed by April Cashin-Garbutt, BA Hons (Cantab)