Asthma is a common disorder of breathing characterised by widespread narrowing of smaller airways within the lung. In the UK the prevalence among children in the 5–12 age group is around 10 per cent, with up to twice the number of boys aﬀected as girls. Among adults, however, the sex incidence becomes about equal. The main symptom is shortness of breath. A major feature of asthma is the reversibility of the airway-narrowing and, consequently, of the breathlessness. This variability in the obstruction may occur spontaneously or in response to treatment.
Cause Asthma runs in families, so that parents with asthma have a strong risk of having children with asthma, or with other atopic (see ATOPY) illnesses such as HAY FEVER or eczema (see DERMATITIS). There is therefore a great deal of interest in the genetic basis of the condition. Several GENES seem to be associated with the condition of atopy, in which subjects have a predisposition to form ANTIBODIES of the IgE class against allergens (see ALLERGEN) they encounter – especially inhaled allergens.
The allergic response in the lining of the airway leads to an inﬂammatory reaction. Many cells are involved in this inﬂammatory process, including lymphocytes, eosinophils, neutrophils and mast cells. The cells are attracted and controlled by a complex system of inﬂammatory mediators. The inﬂamed airway-wall produced in this process is then sensitive to further allergic stimuli or to non-speciﬁc challenges such as dust, smoke or drying from the increased respiration during exercise. Recognition of this inﬂammation has concentrated attention on anti-inﬂammatory aspects of treatment.
Continued inﬂammation with poor control of asthma can result in permanent damage to the airway-wall such that reversibility is reduced and airway-narrowing becomes permanent. Appropriate anti-inﬂammatory therapy may help to prevent this damage.
Many allergens can be important triggers of asthma. House-dust mite, grass pollen and animal dander are the commonest problems. Occupational factors such as grain dusts, hard-metals fumes and chemicals in the plastic and paint industry are important in some adults. Viral infections are another common trigger, especially in young children.
The prevalence of asthma appears to be on the increase in most countries. Several factors have been linked to this increase; most important may be the vulnerability of the immature immune system (see IMMUNITY) in infants. High exposure to allergens such as house-dust mite early in life may prime the immune system, while reduced exposure to common viral infections may delay the maturation of the immune system. In addition, maternal smoking in pregnancy and infancy increases the risk.
Clinical course The major symptoms of asthma are breathlessness and cough. Occasionally cough may be the only symptom, especially in children, where night-time cough may be mistaken for recurrent infection and treated inappropriately with antibiotics.
The onset of asthma is usually in childhood, but it may begin at any age. In childhood, boys are aﬀected more often than girls but by adulthood the sex incidence is equal. Children who have mild asthma are more likely to grow out of the condition as they go through their teenaged years, although symptoms may recur later.
The degree of airway-narrowing, and its change with time and treatment, can be monitored by measuring the peak expiratory ﬂow with a simple monitor at home – a peak-ﬂow meter. The typical pattern shows the peak ﬂow to be lowest in the early morning and this ‘morning dipping’ is often associated with disturbance of sleep.
Acute exacerbations of asthma may be provoked by infections or allergic stimuli. If they do not respond quickly and fully to medication, expert help should be sought urgently since oxygen and higher doses of drugs will be necessary to control the attack. In a severe attack the breathing rate and the pulse rate rise and the chest sounds wheezy. The peak-ﬂow rate of air into the lungs falls. Patients may be unable to talk in full sentences without catching their breath, and the reduced oxygen in the blood in very severe attacks may produce the blue colour of CYANOSIS in the lips and tongue. Such acute attacks can be very frightening for the patient and family.
Some cases of chronic asthma are included in the internationally agreed description CHRONIC OBSTRUCTIVE PULMONARY DISEASE (COPD) – a chronic, slowly progressive disorder characterised by obstruction of the airﬂow persisting over several months.
Treatment The ﬁrst important consideration in the treatment of asthma is avoidance of precipitating factors. When this is a speciﬁc animal or occupational exposure, this may be possible; it is however more diﬃcult for house-dust mite or pollens. Exercise-induced asthma should be treated adequately rather than avoiding exercise.
Desensitisation injections using small quantities of speciﬁc allergens are used widely in some countries, but rarely in the UK as they are considered to have limited value since most asthma is precipitated by many stimuli and controlled adequately with simple treatment.
There are two groups of main drugs for the treatment of asthma. The ﬁrst are the bronchodilators which relax the smooth muscle in the wall of the airways, increase their diameter and relieve breathlessness. The most useful agents are the beta adrenergic agonists (see ADRENERGIC RECEPTORS) such as salbutamol and terbutaline. They are best given by inhalation into the airways since this reduces the general side-eﬀects from oral use. These drugs are usually given to reverse airway-narrowing or to prevent its onset on exercise. However, longer-acting inhaled beta agonists such as salmeterol and formoterol or the theophyllines given in tablet form can be used regularly as prevention. The beta agonists can cause TREMOR and PALPITATION in some patients.
The second group of drugs are the antiinﬂammatory agents that act to reduce inﬂammation of the airway. The main agents in this group are the CORTICOSTEROIDS. They must be taken regularly, even when symptoms are absent. Given by inhalation they have few side-eﬀects. In acute attacks, short courses of oral steroids are used; in very severe disease regular oral steroids may be needed. Other drugs have a role in suppressing inﬂammation: sodium cromoglycate has been available for some years and is generally less eﬀective than inhaled steroids. Newer agents directed at speciﬁc steps in the inﬂammatory pathway, such as leukotriene receptor-antagonists, are alternative agents.
Treatment guidelines have been produced by various national and international bodies, such as the British Thoracic Society. Most have set out treatment in steps according to severity, with objectives for asthma control based on symptoms and peak ﬂow. Patients should have a management plan that sets out their regular treatment and their appropriate response to changes in their condition.
Advice and support for research into asthma is provided by the National Asthma Campaign.
Prognosis Asthma is diagnosed in 15–20 per cent of all pre-school children in the developed world. Yet by the age of 15 it is estimated that fewer than 5 per cent still have symptoms. A study in 2003 reported on a follow-up of persons born in 1972–3 who developed asthma and still had problems at the age of nine. By the time these persons were aged 26, 27 per cent were still having problems; around half of that number had never been free from the illness and the other half had apparently lost it for a few years but it had returned.