Asthma is a long-term pulmonary condition, characterized by inflammation of the lower airways and episodes of airflow obstruction. Asthma severity ranges from intermittent mild symptoms, such as coughs and wheezing, to severe, life-threatening attacks that require immediate hospital treatment. Obstruction of the airway in asthma is generally considered reversible, meaning that the obstruction of the lung can generally be resolved with treatment and in some cases can resolve spontaneously.
Clinical symptoms presented by asthma patients include coughing, shortness of breath, chest tightness and wheezing. The dominant physiological event that leads to these clinical symptoms is airway narrowing. Bronchoconstriction is the contraction of bronchial smooth muscle in response to exposure to a variety of IgE-dependent stimuli such as allergens and non IgE-dependent stimuli including exercise, cold air, and irritants. As the disease becomes more persistent and inflammation more progressive, other factors further limit airflow such us oedema and mucus hypersecretion. These changes are initially reversible but over the years, irreversible structural changes begin to take place. Scarring builds up leading to the thickening of the basement membrane which permanently reduces the airway diameter, this has been associated with a progressive loss of lung function.
Substances that trigger asthma differ from person to person. Extrinsic asthma, is caused by an increase in the production of IgE and airway inflammation in response to allergens whilst Intrinsic asthma is triggered in response to smoke, air pollution, stress, infections and exercise.
Extrinsic:
In some asthma patients, T-helper 2 cells, which are not typically found in normal lungs, are upregulated in the lungs; This increases eosinophil and antibody production therefore promotes inflammation. In other asthma patients low levels of t-helper cells and high levels of neutrophils have been observed. Neutrophils release IL- 8 which plays a key role in asthma, patients with the neutrophilic disease tend to have more severe asthma than patients with the eosinophilic disease.
Intrinsic:
A common intrinsic asthma is Nocturnal asthma. This type of asthma is related to circadian rhythms; at night cortisol and epinephrine levels decrease while histamine levels increase, these changes lead to bronchoconstriction resulting in worsened symptoms.
Asthma can also be exercise induced and may occur as a compensatory mechanism to warm and moisten the airways after exercise in cool and dry conditions.
Moreover, asthma can be drug-induced. Some drugs, for example aspirin can prevent the conversion of prostaglandins. This stimulates the release of leukotriene, a powerful bronchoconstrictor.
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