What is asthma?
Asthma is a chronic disorder characterized by attacks of reversible narrowing of the airways that limits the flow of air. The main problem in asthma lies in the bronchi, the two tubes that branch from the trachea (windpipe) to carry air to each of the lungs, and the smaller bronchioles which constrict in response to certain irritants. This is the reason that asthma is known as bronchial asthma. The airways are hyper-responsive due to an immune sensitivity and the reaction to allergens are a combination of swelling, excess mucus and contraction of the muscles of the airways.
Asthma is the most common chronic condition seen in children and is more frequently seen in males. It accounts for some 250,000 deaths worldwide but with proper management and immediate treatment in acute exacerbations, asthma is unlikely to be fatal. Asthma has been on the rise globally highlighting the environmental contributor to this airway disease. More than half the cases of childhood asthma resolve significantly or even completely by late adolescence or early childhood.
Pathophysiology of Asthma
How does asthma occur?
Asthma arises as a result of hypersensitivity of the airways. The most common type of asthma, known as atopic asthma, is due to an allergic response. When exposed to an allergen like plant pollen or non-allergenic irritants like smoke, the hyper-responsive airway reacts by :
- swelling of the bronchial walls
- excess mucus production by the bronchial lining
- contraction of the smooth muscle in the wall of the bronchi
These reactions causes the bronchial lumen to narrow and restricts the flow of air through the bronchi.
What is the process behind asthma?
The process behind atopic asthma is well understood. In the allergic constitution, the body produces large amounts of IgE antibodies in response to certain allergens. This immune mechanism is intended to be one of the protective measures but in allergic people, it is an abnormal reaction to an otherwise harmless substance.
These antibodies attach to mast cells, a type of immune cell, in the tissue spaces of the bronchi and bronchioles. When the trigger allergens are inhaled and make contact with the attached antibodies, the mast cells then release a series of immune mediators (chemicals) like histamine and bradykinin. However, this should not downplay the role of other cells like the eosinophils, macrophages and T-lymphocytes as well various cytokines.
Types of Asthma
There are several different types of asthma based on the cause and triggers. This includes :
- Atopic asthma. The most common type of asthma due to immune-mediated hypersensitivity as discussed above. Typically starts in childhood and triggered by allergens such as pollen, dust, animal hair and certain foods. There is usually a strong family history of asthma.
- Non-atopic asthma. Related to viral respiratory tract infections and air pollution which causes hyperirritability of the bronchial tree. Not commonly associated with a family history.
- Drug-induced asthma. Caused by the use of certain drugs, particularly aspirin (aspirin-sensitive asthma) and other NSAIDs. Sometimes arises in the backdrop of another type of asthma which may have gone by unnoticed.
- Occupational asthma. May be immune or non-immune mediated and triggered by exposure to gases, fumes, organic and non-organic dusts and various other chemicals which act as irritants. May also be related to an exacerbation of other types of asthma, like atopic asthma.
- Exercise-induced asthma. This is seen as a variant of certain types of asthma where the condition is exacerbated by asthma. Although seen with atopic asthma or atopy not involving the lungs (allergic rhinitis or allergic dermatitis), it may also be seen in healthy patients with no history of lung disease.
Asthmatic bronchitis is a coexistence of the the two obstructive pulmonary diseases asthma and bronchitis.
Causes of Asthma
The causes of asthma are discussed above but some of the trigger factors include :
- Environmental allergens like pollen, dust, animal hair, certain foods
- Viral respiratory tract infections
- Air pollution
- Airborne irritants like smoke, insecticide sprays, paint fumes
- Certain drugs, especially aspirin
- Occupational exposure to gases, fumes, organic and non-organic dusts and other chemicals
- Emotional stress
- Cold or dry air
Gastroesophageal reflux disease (GERD) is a prominent component of asthma that has been receiving more attention in recent years. Aspiration of the gastric contents may irritate the airways and contribute to an ongoing cycle of hypersensitivity. About 3 out of 4 patients with asthma have GERD and asthmatic adults without a history of allergies, occupational triggers or ongoing exposure to irritants like cigarette smoke should be investigated for GERD.