Asthma

Characteristics:

Asthma has 3 characteristics:

1. Reversible airflow limitation
2. Airway hyperresponsiveness
3. Inflammation of the bronchi

Prevalence:

• Affects 15-20% of people in their 20s
• Commoner in the West

Classification:

2 classes:

• Intrinsic or cryptogenic (when no causative agent can be identified)
• Extrinsic (implying a definite external cause)

Extrinsic asthma:

• Occurs most frequently in atopic individuals who show (+ve) skin-prick reactions to common inhaled allergens
• (+ve) skin tests to inhalant allergens are shown in 90% of children and 50% of adults

Intrinsic asthma:

• often starts in middle age
• Many show (+ve) skin tests and on close questioning give a history of respiratory symptoms compatible with childhood asthma

Aetiology and pathogenesis:

There are 2 major factors involved in the development of asthma:

• Atopy and allergy
• Airway hyperreactivity

Atopy and allergy:

• Atopic individuals readily develop antibodies of IgE class against common materials present in 30-40% of the population
• There is a link between serum IgE levels and both the prevelance of asthma and airway responsiveness to histamine or methacholine (cholinergic agonist)

Airway hyperreactivity:

• Demonstrated by asking the patient to inhale gradually increasing doses of histamine or methacholine
• This induces a transient episode of airflow limitation in susceptible individuals (~20% of the population)
• The dose of the agonist (provocation dose) necessary to produce a 20% fall in FEV₁ is known as the PD₂₀FEV₁
• Patients with clinical features of asthma respond to very low doses of methacholine, i.e. they have a low PD₂₀FEV₁ (<11μmol)

Precipitating factors:

• Occupational sensitizers
• Cold air
• Exercise
• Atmospheric pollution, irritant dusts, vapours and fumes
• Emotion
• Drugs (e.g. NSAIDs)

Clinical features:

• Wheezing
• Episodic SOB
• Cough (especially nocturnal)
• Chest ‘tightness’

Investigations:

Lung function tests:

• The diagnosis of asthma is based on the demonstration of a greater than 15% increase in FEV₁ or PEFR following the inhalation of a bronchodilator

Peak flow charts

Exercise tests

Histamine or methacholine bronchial provocations test:

• This test should NOT be performed on individuals who have poor lung function (FEV₁ <1.5L)

Trial of corticosteroids (as for COPD)

CXR:

• Useful in excluding other possible diagnoses

Skin-prick tests

Blood and sputum tests

Management:

The aims of management are as follows:

• To abolish symptoms
• To restore normal or best possible long-term airway function
• To reduce the risk of severe attacks
• To enable normal growth to occur in children
• To minimise absence from school or work

Drug therapy:

Β₂-adrenoceptor agonists:

• E.g. salbutamol 100μg (2 puffs PRN) or salmeterol (50-100μg 1 puff daily)
• Cause relaxation of bronchial smooth muscle
• Very effective at relieving symptoms but does little for the underlying inflammatory nature of the disease
• Only the mildest asthmatics, with intermittent attacks should rely on bronchodilator treatment alone

Anticholinergic bronchodilators:

• E.g. ipratropium bromide 20-40mg TID by inhalation

Inhaled corticosteroids:

• E.g. inhaled beclomethasone (available in doses of 50, 100 and 250μg per puff)
• Unwanted effects of inhaled corticosteroids include:
• Oral candidiasis
• Hoarseness
• Abnormalities of bone formation (with very high steroid doses)
• Growth retardation in children if inhaling >400μg daily

Oral corticosteroids:

• Used for those patients who are not controlled on inhaled corticosteroids
• Keep dose as low as possible to avoid side-effects

Leukotriene receptor antagonists:

• E.g. Montelukast (single tablet daily)
• Improves control when used with an inhaled steroid