Chronic inflammatory condition of the airways that causes episodic exacerbations of bronchoconstriction

Aetiology

• Intrinsic: often starts in middle age, no trigger can be identified
• Extrinsic: usually occurs in atopic individuals who have positive skin prick test results
  • Causes 90% of childhood cases, and 50% of adults with chronic asthma
  • It is often accompanied by eczema
  • Non-atopic individuals can develop asthma in later life via sensitisation to e.g. occupational agents, aspirin, or as a result of taking β-blockers for hypertension or angina
  • Extrinsic asthma involves a type I hypersensitivity reaction to inhaled allergens (there is also a delayed phase reaction, type IV hypersensitivity which occurs hours-days after exposure

Pathophysiology

Characteristics

Classically, asthma has three characteristics:

• Airflow limitation, which is usually reversible spontaneously or with treatment
• Airway hyper-responsiveness to a wide range of stimuli (see later)
• Bronchial inflammation with T lymphocytes, mast cells, eosinophils with associated plasma exudation, oedema, smooth muscle hypertrophy, matrix deposition, mucus plugging and epithelial damage
  • Curschmann spirals can be seen on histology, which are where shed epithelium becomes whorled mucous plugs

Control of bronchial calibre

Bronchial calibre is controlled by a balance between:

• The sympathetic nervous system which causes bronchodilation and decreases mucous secretion via β2-adrenoceptors
• The parasympathetic nervous system which causes bronchoconstriction and increases mucus secretion via M3-receptors

Classification

• Atopic asthma: atopy is a term used to describe a group of disorders, including asthma and hayfever, that appeared to run in families, have positive skin-prick tests to common inhalant allergens and have circulating allergen-specific antibodies
  • Elevated serum IgE levels are linked to airway hyper-responsiveness and the prevalence of asthma
  • Pathophysiology involves a strong TH2 response to an antigen - mast cells result in the initial asthma attack, and the accumulation of eosinophils result in the late phase
• Non-atopic asthma: asthma which does not appear to be immunologically mediated, it is associated with recurrent viral respiratory tract infection
  • Bronchoconstriction due to airway hyper-responsiveness, not as much due to inflammation and leukocyte infiltration

Typical triggers in atopic asthma

• Infection
• Night time/early morning