Pathophysiology of Asthma

What is the pathophysiology of asthma?

Asthma is a common health problem defined by severe inflammation of respiratory tubes, the constriction of respiratory smooth muscle and moments of bronchoconstriction.

The World Health Organization emphasises the extent of the illness which affects around 235 million individuals globally. Asthma can be categorized into two prominent areas: non-allergic and allergic. They are triggered in different ways, however both cause bronchoconstriction.

Bronchoconstriction

When an asthma attack takes place, inflamed airways react to certain triggers such as smoke, pollen or dust. The airways produce mucus and begin to reduce in size, leading to coughing and breathing difficulties. The bronchi react to this by contracting into a spasm (an asthma attack). Inflammation takes place thereafter, with more constriction and mucus. Bronchospasm will usually cease after one to two hours, however in around half of individuals there may be a late response with bronchoconstriction and inflammation continuing after three to 12 hours.

The bronchus carries out functioning through the autonomic nervous system, which also initiates reflexively. Nerve endings are within the inner impulses which travel to the vagal center, thereafter reaching the vagal pathway to again reach the bronchial small airways. These nerve endings release Acetylocholine, which is a result of the over production of inositol 1,4,5- trisphosphate in the bronchial smooth muscle cells which leads to bronchoconstriction, the muscles shortening.

Bronchial inflammation

In those with asthma and those without, inhaled allergens that travel to the inner airways are consumed by a cell known as the antigen-presenting cells (APCs). These cells then show the allergens to another immune system cell named TH0. In those without asthma, this cell will ignore the allergen molecules. However, in those who have asthma, the cells change into a different type of cell called TH2. It is yet to be fully explained as to why this takes place.

There is a hypothesis, the 'hygiene' hypothesis, that speculates that if a child is exposed to bacteria in early life, has a limited amount of antibiotics, lives in a larger family and within a country environment, this will stimulate a TH1 response and prevent that child from getting asthma. Asthma is linked with a procoagulant state in the bronchoalveolar space.

Pathogenesis

The main issue in asthma appears to be immunological: young individuals in the early stages of asthma present signs of excessive inflammation within their airways. Epidemiological findings enable hints as to the pathogenesis, the amount of people suffering from asthma is increasing globally, especially in affluent countries.

Sleep apnea

It has been noted that a high number of patients who have both asthma and sleep apnea are likely to improve when sleep apnea has been treated.

Gastro-esophageal reflux disease

Gastro-esophageal reflux disease is often common in difficult to control asthma, however, treating it has not been found to improve asthma. When both as suspected, an Esophageal pH Monitoring protocol is needed in order to confirm the diagnosis and plan a care pathway.

During pregnancy

Around four to eight out of every 100 pregnant women are affected by asthma. During pregnancy, there is an immunological shift because of hormone changes. At times, high estrogen levels can reduce the ability of the natural killer cells (TH1) to produce inflammatory cytokines and the production of anti-inflammatory cytokines. These, as discussed previously, play a vital part in the pathophysiology of asthma.

Studies have further found a connection between the exposure to air pollution in women with asthma and early labor. Findings suggested that women with asthma do have a higher risk of preterm birth. These asthmatic episodes and the risk of preterm labor were found to be associated with exposure to carbon monoxide and nitrogen dioxide pollutants.

Research also looked into when women were most likely to develop asthma. Findings noted that women were at higher risk of developing when exposed to allergens before conception and thereafter, during pregnancy. Living near busy roads have been found to increase levels of asthma.

In another piece of research, data was collected from 6,000 children within a Californian school. They found that a high exposure to prenatal air pollution was connected to an increased rate of asthma during childhood.

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Last Reviewed:
June 14, 2017
Last Updated:
October 10, 2017
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