1. Epidemiology and aetiology
Asthma is a common, chronic inflammatory disease of the airways affecting about 5.8 per cent of the UK population. The incidence is highest in pre-school age children and adults aged 40 to 60. Asthma causes around 1,400 deaths per year.
Childhood-onset asthma is more likely to be extrinsic (triggered by an allergen) and adult-onset to be intrinsic. Genetics has a strong influence in the aetiology of asthma. A number of different genes have been implicated.
Asthma occurs as a result of the interaction between environmental stimuli, which have more profound effects if exposure occurs at certain stages of immunological development in people with a genetic predisposition.
The importance of environmental factors is illustrated by the rising prevalence rates of asthma in immigrants from countries with low rates of asthma, such as sub-Saharan Africa, to countries with high rates, such as northern Europe. The effect of environmental factors on genetically-predisposed patients is marked pre-natally, in the first two years of life, and in the early teenage years.
Allergic asthma is characterised by an allergic response resulting from susceptible individuals being exposed to specific triggers. Atopic asthma is a type of allergic asthma that involves an IgE-mediated reaction.
The pathology of asthma involves three components: oversensitive bronchial epithelia, exposure to allergens or other triggers, and the resulting inflammatory response that causes acute airway inflammation and secondary bronchospasm. It is the bronchospasm that causes the symptoms and signs of asthma and that results in increased airflow obstruction.
The characteristic feature of asthma is that the whole process is reversible, either naturally or in response to therapy.
Common asthma triggers
- House-dust mites.
- Animal danders (cats, dogs, horses).
- Aeroallergens (pollens, moulds).
- Perfumes, air fresheners and similar products.
- Passive exposure to tobacco smoke.
- Cigarette smoking.
- Maternal smoking.
- Viral upper respiratory infection.
- Cold air.
- Aspirin and NSAIDs.
There is no single symptom, sign or test that is diagnostic of asthma.
Asthma is a clinical syndrome resulting in variable presentations and severity from a variety of different mechanisms.
The characteristic feature is reversible airway narrowing, which is a result of airway smooth muscle constriction, mucosal oedema and increased mucus production, which are in turn results of the sensitive bronchial mucosa and hyper-reactive bronchial muscles.
Cardinal symptoms of asthma
The pathophysiology underlying the disease results in the four cardinal symptoms of asthma - cough, wheeze, chest tightness and dyspnoea.
Symptoms can vary in their severity and some symptoms may be more marked than others.
Symptoms usually present and resolve over the course of hours to days, usually in response to exposure to triggers or infections of the respiratory tract. In practice it is difficult to identify allergens accurately, skin prick tests can produce false positives and false negatives.
When well, there are not usually any signs of asthma, but during an acute episode, the patient may have tachypnoea, tachycardia and widespread wheezing throughout both lung fields on chest auscultation.
In infants, the only or main symptom is usually a nocturnal cough that persists two to three weeks after a minor URTI or in the absence of any other symptoms.
Peak expiratory flow
In adults and children over the age of eight years, the peak expiratory flow (PEF) will fall to levels below 80 per cent of predicted or best ever and will usually return to normal with recovery from the episode.
Children aged eight years or younger can rarely reliably use a PEF meter.
Patients with more severe asthma may never attain their predicted PEF levels, even when well or when receiving maximum therapy.
The diagnosis of asthma in patients who are able to use PEF meters can be made on the basis of the PEF falling to below 80 per cent predicted or best ever with symptoms, and restoration of the PEF when well again.
History and X-ray
In those patients unable to use a PEF meter, the diagnosis is made on the basis of the history (asthma is more likely if there is a personal history of atopy or a strong history of atopic features in first-degree relatives), and response to therapy.
A chest X-ray is rarely helpful in making the diagnosis, except to exclude other possible diagnoses such as heart failure.
Refer to a respiratory specialist if there is diagnostic uncertainty.
The aims of asthma management are to recognise the disease, abolish all symptoms, restore best-possible lung function and allow the patient to lead a normal life untroubled by the disease or its management. Treatment is aimed at preventing exacerbations and protecting the lungs.
Allergen avoidance seems logical but even if allergens or triggers can be identified, it is seldom easy to avoid them completely.
Avoidance of most pollens and dust is difficult, expensive, time-consuming and not often effective. However, avoidance of cigarette smoke should always be encouraged.
Annual influenza vaccination is recommended in the GMS quality framework for all patients with asthma.
There is excellent evidence of its efficacy in preventing life-threatening exacerbations of asthma, especially in children.
The mainstay of management is medication. All patients with asthma should have access to an inhaled short-acting bronchodilator, but its regular or frequent use should be seen as a marker of poorly controlled asthma.
Asthma is a chronic inflammatory condition so it makes sense for nearly all patients with asthma to take regular inhaled steroids, which are extremely effective and safe at the recommended dosages.
Patients whose asthma is still poorly controlled despite good adherence and inhaler technique should switch to a regular combined inhaled steroid plus long-acting bronchodilator.
Children aged eight or under, or strongly atopic patients of all ages, may benefit more from regular oral leukotriene antagonists in addition to their inhaled steroids.
The majority of patients will be controlled using these options, but the remaining 5 to 10 per cent may need higher-dose inhaled steroids with regular long-acting inhaled bronchodilators or oral long-acting bronchodilators or oral theophyllines .
Patients with stable asthma can cautiously step down therapy slowly.
Always refer patients for a specialist opinion before starting long-term oral steroids. Acute exacerbations should be managed by aggressive use of frequent high-dose inhaled bronchodilators and, often, short courses of oral steroids.
Excellent guidelines on asthma management are available from the British Thoracic Society and the Scottish Intercollegiate Guidelines Network at: www.sign.ac.uk/guidelines/fulltext/63/index.html.