Bronchial asthma: mechanism and its classification

 Bronchial Asthma: Mechanism and its classification 



Bronchial asthma is a chronic inflammatory disease of airway of lungs. It develops due to hyperresponsiveness of airways due to various stimuli which result in narrowing of air passage. The episode of airflow obstruction occurs with bronchospasm which is often reversible, and is associated with episodes of breathlessness, wheezing, chest tightness and coughing particularly at night and early morning and gets worse with exercise.
The disease is often lifelong, dudh symptoms and bronchospasm are intermittent, the Airway inflammation persist even during asymptomatic period. The attack may last for minut 2 hours and even days and in between the attacks the patient is normal. 
The disease is more prevalent in children than adult and in urban areas than in rural. However it can occur at any age, but in majority of cases it starts before the age of 10 years. It is more common in boys up to the age of adolescence, but in adults male female ratio is same.

Etiology 

The ideological factors of asthma are broadly divided into two-
1. Inducing factors
2.  Trigger factors

 Inducing factors

These factors make a person more susceptible to develop asthma. These are-
1.  Genetic factor- A  distinct gene for atopy on chromosome 11q has been identified. Gene at chromosome 5q has been linked to immunoglobulin IgE responsible for AHR (Airway hyper responsiveness) other genes related to interleukin 4,5,6,10,13,17A TNF-alpha are also associated with pathogenesis of asthma.
2.  Tobacco smoking- exposure of tobacco smoke tobacco smoking during pregnancy and infancy.
3. Viral infections- Rhino-syncytial virus infection in early life.
4. Obesity- BMI more than 30 kg per metre square predisposes to asthma.
5.Allergens- Exposure to allergens in infancy.

 Trigger factors

These factors provoke the symptoms and susceptible person. These are-
1. Allergens- inhalation of allergens in the most immediate Trigger factor for asthma. The common allergens are house dust, mite, pollen, mould, animal dander etc.
2. Exercise- Vigorous exercise lead to asthma.
3. Stress- Stress is one of the most common psychological factor for triggering asthma.
4. Environment- environmental factors are the most important triggering factors like air pollution, extreme hot or cold weather, strong wind,  traffic pollution cooking gas fumes, passive inhalation of tobacco smoke, paint, sprays etc.
5. Occupation - exposure to Chemicals or substances related to occupation can Trigger asthma like dyes, chrome, nickel, wood dust (sawdust),cotton dust, cement, asbestos etc.
6. Infections- upper respiratory infections are triggering factors for asthma. Viral infections are most triggering example due to rhinovirus, respiratory syncytial virus, Para influenza virus. Bacterial infections with Mycoplasma and Chlamydia can Trigger asthma.
7. Food- Some food allergens can Trigger asthma like milk, egg, wheat, colour additives, food preservatives, sea foods etc.
8. Drug- some medicines can Trigger asthma like beta-blockers, cholinergic drugs, Aspirin, NSAIDs
9. Gastro-oesophageal Reflux can Trigger asthma. 

Mechanism of initiation of asthma

Vinyl allergens is inhaled by asthmatic patient, obstruction of airway starts within minutes and show effect within 20 to 30 minutes. The inhaled allergens when comes in contact with the mast cells on the mucosa of airways they release histamine, arachidonic acid derivatives like leukoteriene's and Prostaglandin-F. This is called early phase reaction. In this case there is direct action of mast cell derived mediators, causing bronchospasm. In late phase reaction, airway obstruction occurs after 6 to 10 hours. It is inflammatory in nature.
As a result of chronic inflammation of airways there is increased contract ability of surrounding smooth muscles which leads to narrowing of airways producing wheezing. The narrowing is reversible.

 Clinical features-



  1. Intermittent reversible attack of dyspnoea (breathlessness).
  2.  Wheezing
  3. Cough
  4. Chest tightness
  5. Symptoms get worse during night and early morning.
  6. Attacks may occur in late night "Nocturnal asthma"
  7.  Disrupted sleep
  8. Increased production of mucus, which is thick, and difficult to expectorate.
  9. Between the two  attacks(episodes)patient is symptom free. 
  10. Patient may have other associated atopic conditions like allergic rhinitis, excimer or urticaria.
  11. During acute Attack patient may be restless, anxious, agitated, orthopnoeic,  tachypnoeic. Breathing through pursed lips with prolonged expiration. Wheezing is audible and cyanosis may develop. 

Physical examination -

  • Hyper resonant chest on percussion. 
  • On auscultation bilateral wheeze is heard,  wheezing is prominent sign of asthma. 
  • In wild cases wheezing may be detected only on forced expiration. 
  • In severe exacerbation of asthma,  wheezing may be absent.  Other signs og airflow obstruction like increase in respiratory rate,  flaring of alae nasi,  use of ascessory muscles for respiration and pulsus paradoxus are seen. 

Classification of Asthma

On the basis of etiology asthma is classified unto 2 types-
1. Extrinsic (allergic or atopic) 
2. Intrinsic (Idiopathic or non-atopic cryptogenic) 

Extrinsic Asthma-

This is IgE mediated.  This is the most common type of asthma which begins in early life.  Patient has family history and often may suffer from other allergic diseases like rhinitis, urticaria, atopic dermatitis(eczema). Attack may be seasonal and precipitated by aeroallergens like pollens, grass weed,  animal danders, house dust, moulds etc. Occupational asthma may be stimulated due to gases, fumes, chemical dusts etc.  There is increased level of serum IgE and skin test is positive for allergens.

Intrinsic Asthma

There is no family or personal history of allergy or atopic disease. Serum IgE levels are normal Skin test is normal.  The symptoms are developed after upper respiratory tract infection by virus. This usually starts late in life(adult). Perennial symptoms are common. 

Life threatening features:-

  • Central cynosis
  • Exhaustion, confusion, reduced concious level. 
  • Bradycardia
  • Silent chest
  • Can't speak 
  • Unrecordable peak flow
  • Pulse>120/min
  • Pulsus paradoxus

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