Introduction
Asthma is a chronic lung condition in which there is chronic inflammation of the airways, and hypersensitivity of the airways. Symptoms include wheeze, cough, chest tightness and SOB (dyspnoea). It is often worse at night.
- The immune response is CD4 mediated, and the lungs will show an eosinophil infiltrate
It is characterised by
airflow obstruction which is varied over time, and reversible. Technically, asthma exists where the obstruction is reversible by >15%, and
COPD exists where it is reversible by <15%.
- Many patients will fall into a grey area near the boundary of these two diseases. These patients may typically be in their 30s and early 40s, and have a history of smoking, but as their airway obstruction is reversible, then they may technically be given a diagnosis of asthma. In actual fact, they are more likely to have early stage COPD. However, the diagnosis is not that significant because the treatment is very similar. Despite the fact COPD is ‘not reversible’, patients with COPD often get symptomatic relief from inhalers (although the only way to improve prognosis is to stop smoking, and give LTOT).
- In the UK about 2000 patients per year die from asthma.
Three main characteristics
- Airflow limitation – this is usually reversible, either spontaneously, or with treatment
- Airway hyper-responsiveness – this occurs to a wide range of stimuli
- Inflammation of the bronchi – with infiltration by eosinophils, T cells and mast cells. there is associated plasma exudate, oedema, smooth muscle hypertrophy, mucus plugging and epithelial damage.
The disease often ‘flares up’ with viral infections – which often cause a loud wheeze.
Epidemiology
- Increasing in incidence, particularly in Western countries
- 10-20% of those in the 2nd decade of life are affected. This is where prevalence is at its highest
- In children, boys are more likely to be affected
- After puberty, girls are more likely to be affected
- 50% of those who have childhood asthma, but then ‘grow out of it’, will relapse in adulthood
Aetiology
The disease can either be intrinsic (aka cryptogenic); where no causatory factor can be found, or extrinsic, where there is a definite external cause.
- Intrinsic – this often starts in middle age, and is sometimes called late onset asthma. No trigger can be identified.
- Extrinsic – this usually occurs in atopic individuals who have positive skin prick test results. This type of asthma 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 huors-days after exposure.
Atopy
This is a term used to describe people who often have
allergies / asthma / hayfever, and where:
The trait runs in families (i.e. genetic component)
- The ADAM33 gene is associated with airway hyper-responsiveness, and airway remodelling
- The PHF11 gene is associated with increased IgE production
The individuals often have skin reactions to common allergens
The individuals have IgE antibodies to many common allergens – these antibodies are present in 30-40% of the UK population, and there is a strong correlation between the levels of IgE and the severity of asthma and airway hyper-responsiveness
The development of atopy – the hygiene hypothesis – this is a theory that states that growing up in a ‘clean’ environment in the early years of life can cause atopy. If you grow up in a ‘dirty’ environment, and are exposed to various bacterial, fungal and viral proteins, this is thought to help ‘direct’ your immune system away from recognising inert particles as allergens. It is almost as if your immune system has to attack something, and in the absence of actual pathogens, it just goes for whatever it can find.
