Causes of asthma Part 2-By Dr.Karim
Causes Of Asthma
The main component of bronchial asthma is Air flow obstruction
This air flow obstruction is due to 3 factors:
A-Contraction of the airway smooth muscle(Bronchoconstriction)
Smooth muscle tissue is a type of muscle tissue found in blood vessels, digestive tract, some sphincters and bronchioles. This type of muscle tissue is innervated by the autonomic nervous system thus it is involuntary acting.
In a normal individual, bronchial smooth muscle tissues relax causing dilatation of the airways when more oxygen supply is needed as in fight or flight, they contract causing bronchoconstriction when there is no demand for more oxygen as during rest and sleep.
In an asthmatic, there is an exaggerated bronchoconstriction and airway narrowing on exposure to allergens. This is due to hyperesponsiveness of their airways ( This was discussed in the previous article).
B-Increased secretion of the epithelial glands
Airways are lined by epithelial cells and epithelial glands that secrete mucus which moistens and protects the airways and serves as a barrier that prevents infection and tissue injury.
In a normal individual, mucus is secreted in moderate amounts that are easily cleared and don’t cause any respiratory distress.
In an asthmatic, there is hypersecretion of thick mucus that can form a mucus plug which can cause airway obstruction.
C-Inflammatory oedema and vasodilatation
Steps of an asthmatic response of the airways:
Early Asthmatic Response (EAR)*:
In asthmatics, inhalation of an allergen causes an early response characterized by smooth muscle contraction and increased mucus production. This response in 15 minutes, reaches its peak in about 30 minutes and resolves within 90-180 minutes.
What happens is that when an asthmatic inhales an allergen, it binds to specific antibodies on the surface of Mast cells. These are a type of cells of the immune system that plays a key role in the inflammatory response. Mast cells contain granules rich in an inflammatory mediator called histamine.
When an allergen binds to specific antibodies on the surface of mast cells , it causes Degranulation of the granules and release of histamine from the mast cells. This histamine is the cause of the smooth muscle contraction and increased mucus production in the EAR*.
Note:- There are other inflammatory mediators released form mast cells namely prostaglandin D2 and leukotriene C4 but histamine is the main inflammatory mediator in the causes of asthma.
Remember:Causes of asthma
Asthmatic Inhales Allergen – Then
Allergen Binds to Abs On the surface Of mast cells – Then
Mast cells release Histamine and other mediators – Then
Histamine causes Smooth muscle contraction and increased mucus production.
Late Asthmatic Response*:
The EAR* is followed by a partial recovery period over the next 1 to 2 hours and then by a further progressive deterioration of respiratory function that reaches its maximum between 6 to 12 hours. A full recovery occurs by 24 to 48 hours.
During the LAR*, there is a striking infiltration of the airways by inflammatory cells namely eosinophils, lymphocytes, monocytes, macrophages and neutrophils.
(Remember factor 3 of the airway obstruction) – in Causes of Asthma
LAR* is considered to be the primary mechanism responsible for Bronchial HyperResponsiveness(BHR*).
The BHR* is an exaggerated bronchoconstriction of smooth muscles and airway narrowing after inhalation of allergens (as we mentioned in the 1st article, allergens are normally harmless stimuli to normal individuals. They only cause bronchial asthma in patients with hyperresponsive airways(asthmatics).
In EAR*, Contraction of the bronchial smooth muscles(bronchoconstriction) is the important factor of airway obstruction.
In LAR*, inflammatory oedema and Vasodilatation are the important factors of airway obstruction.
To Sum up – Causes of Asthma:
An asthmatic with BHR* inhales an allergen – Then
After 15 minutes EAR* starts reaching its maximum in 30 min (smooth muscle
contraction and increased mucus)This is due to release of histamine mainly,
prostaglandin D2 and leukotriene C4 – Then
After 90-180 min EAR* resolves (Partial recovery period) – Then
After 6-12 hours LAR* starts(Progressive deterioration of respiratory function)
This is due recruitment of inflammatory cells (eosinophils,basophils, neutrophils,etc) by Prostaglandin D2 and leukotriene C4 (Inflammatory Oedema and vasodilatation) – Then
After 24 to 48 hours – Full Recovery