DO CHILDREN GROW OUT OF ASTHMA?

by Dr Anne H Thomson MD, FRCP, Consultant Paediatrician (Respiratory Disease), John Radcliffe Hospital, Oxford

It is commonly stated that children grow out of asthma. The author reviews the evidence and identifies factors influencing prognosis. She stresses the importance of preventing these children from starting to smoke.

Introduction

Parents of asthmatic children often ask whether their child will grow out of their asthma. This question is difficult to answer, as we cannot predict the future. Another piece of parental folklore (which I have not seen in medical texts) concerns seven-year cycles. This claims that if a child is not better by the age of 7, he/she may be better by the age of 14. What is the evidence behind these medical myths?

Wheezing in pre-school children

In a pre-school child, the most common history is intermittent attacks of wheeze, apparently precipitated by upper respiratory tract infections, with complete remission between attacks. A smaller proportion of parents give a history of exercise or nocturnal symptoms between attacks, and there may be other precipitating factors.

Interpretation of the literature on wheezing in pre-school children is made difficult by the lack of a real definition of asthma in this group, and the number of synonyms which are used (e.g. wheezy bronchitis, spastic bronchitis, asthmatic bronchitis, wheeze-associated respiratory illness and, particularly in the USA, bronchiolitis). This variety of terms may have stemmed from a reluctance to label a young child as asthmatic, particularly as many of these children follow a different clinical pattern from older wheezing children. An excellent review (1) summarises the position.

Under diagnosis of asthma

Asthma is both under diagnosed and under treated in children, (2) and using terms such as wheezy bronchitis contributes to this by implying that infection, rather than bronchial responsiveness, is the problem. Other studies (3-5) have shown that children with wheezy bronchitis cannot be separated from those with asthma, in terms of atopic markers, but seem to form part of a continuous spectrum. This has led to the term "asthma" being used to describe all wheezing illness, including that apparently precipitated by viral infection. Lumping cases together in this way may have improved treatment of bronchospasm in young children, but it does not alter the fact that there seems to be two different populations. (1)

Clinical studies

Many young children with viral precipitated episodic wheeze improve. One study of 80 children with wheezy bronchitis who were followed up for 12 years (6) found that over half the children had stopped wheezing by the age of 3, with 15 more children having stopped wheezing by the age of 11. Only 22 children continued to wheeze after 12 years, and symptoms in 21 of these children had clearly improved. The presence or development of allergy increased the risk of persistent asthma. (6)

A national cohort birth study of 11,465 babies who were followed up at 5 and 10 years (87% follow-up rate at 10 years), supports a good prognosis for episodic wheeze in pre-school children. (7) 2,345 children had at least one wheezing attack before the age of 5 years. 1,869 of these children (80%) were free from wheeze for at least 12 months by 10 years old.

Children with more wheezing attacks between the ages of 1 and 4 were more likely to have asthma when 10 years old. However, 67% of 702 children who had more than four attacks of wheeze under 4 years old were free of wheeze at the age of 10 years. This study also found that children who were reported to have wheeze with bronchitis in the first 5 years were significantly more likely to be symptom-free at the age of 10 years than those who had been labelled asthmatic (80% versus 50%).

There is therefore evidence that pre-school children with episodic wheeze in response to, or in association with, virus infection improve with age. This seems less likely in children who present with, or develop, atopic symptoms. Some children will therefore grow out of asthma by the age of 7.

School age and puberty

There have been a number of follow-up studies of school-age children with asthma. A review of 23 retrospective studies showed that 47% of 6,737 patients with childhood asthma were reported to be asymptomatic by the end of puberty, and a further 37% had shown a marked improvement. (8)

A careful prospective study of 38 children with chronic perennial asthma examined regularly through childhood and puberty found that improvement in asthma occurred in some children before the onset of puberty, but that the rate of improvement increased greatly during puberty. (9) Improvement before puberty was most likely in patients with mild asthma. Overall, 42% of the group were symptom-free at the end of puberty (17% had been severe asthmatics, and 54% had had moderate-to-mild asthma). Of 17 children observed for one to five years after stage 5 (fully mature) puberty, six continued to improve, seven were unchanged, and three relapsed.

Childhood through to adult life

An early, important general practice study in East London followed a group of 267 children under 12 years old for 20 years. (10) After 20 years, half the cohort had mild or no symptoms, but the others were symptomatic and had had time off work or been hospitalised for their disease. 21% had chronic unremitting asthma, and 27% had experienced at least three years free from symptoms, but had subsequently relapsed. The average age at relapse was 18 years. A family or personal history of atopic disease and severe asthma at onset predisposed to chronic asthma 20 years later.

A series of studies following a cohort of children from Melbourne provide the most useful long-term, follow-up data. These children were selected at 7 years old from a community-based age cohort, and have been followed up at 14, 21, and 28 years old. The only selection criterion was a history of wheeze, and asthma severity was assessed by frequency of wheezing episodes. However, children who had wheezed in early life, but who had stopped by 7 years old, may have been missed by this study. (4, 11-13)

Over half of the children who wheezed infrequently at 14 years old had not wheezed for more than three years when they reached 21 years old. Of those patients with frequent wheezing in childhood, 40% had less frequent wheeze during the next seven years, but 25% had more frequent wheezing. Almost half of the patients with persistent wheeze at 14 years old had less frequent wheeze at 21 years old; only 4% had stopped completely.

Seventy per cent of patients with frequent and persistent wheeze at 21 years old said that their asthma had started when they were less than three years old. Many of these asthmatics had some evidence of chest deformity.

The 28-year, follow-up study(13) found that about 50% of patients had not changed (26% had worsened and 24% had improved). Wheeze recurred in 31% of those who were wheeze-free at 21 years old, and many of these had only had trivial wheeze during childhood.

Relapse or latency

Relapse following years of quiescence, reported by some long-term studies, is an important finding. Several studies have showed that asymptomatic children with asthma have persisting pulmonary function abnormalities, (14-16) with evidence of hyperinflation and small airways obstruction. In the Melbourne studies, patients who had not wheezed for three years or more at the 21- and 28-year follow-ups had normal lung function, but those who had wheezed in the three years before examination had abnormal lung function.

Studies which follow people for less than ten years, or do not continue after puberty, could therefore give a false impression of the natural history of childhood asthma. Levison's remark that it is the paediatrician who is outgrown rather than the asthma, (17) is significant. Long-term prospective studies should therefore be extended to examine both later effects and the relationship between childhood asthma and the development of chronic obstructive airways disease. (18)

People who wheeze in childhood continue to be at risk, given the appropriate stimulus in later life. We therefore need to consider preventative measures. There is no firm evidence that normalising lung function improves the prognosis in childhood asthma. Non-specific airway hyper-reactivity is found in patients who are clinically free of asthma, but have had asthma in the past. (19) However, optimal treatment will decrease bronchial responsiveness in adults. (20)

Cigarette smoking

A more important and immediate preventive action may be to stop people with a history of asthma from starting to smoke. Smoking has been shown to be associated with rapid decline in small airways function in adults with asthma. (21) In the Melbourne study, 40% of the patients with asthma (of any severity) were smokers at 28 years old. Evidence linking passive smoking and respiratory morbidity in childhood is steadily accumulating.

Conclusion

The course of childhood asthma is variable, and the prognosis can only be estimated for populations rather than for individual children. The incidence of wheeze, particularly with viral infections in pre-school children, is very high, and many of these children will cease wheezing as they reach school age. The improvement in asthma during childhood is greatly accelerated at puberty. In addition, management of asthma gets easier as the child gets older, which makes symptoms less troublesome for the child and his or her family. Some factors increase the likelihood that their asthma will persist.

When I am asked whether a child will grow out of asthma, I normally answer that it usually improves during childhood and at puberty. Some asthmatic children stop wheezing completely, but all children who have had asthma may relapse, given the right circumstances. It is therefore very important that they do not smoke.

Practical points

• The most common history in a pre-school child is intermittent attacks of wheeze precipitated by upper respiratory tract infections.
• Many children with viral precipitated episodic wheeze improve. The prognosis is poorer if the child develops an allergy.
• Improvement in asthma occurs before the onset of puberty, but the rate of improvement increases greatly during puberty.
• Relapse is more common in people with a personal or family history of atopy, or severe asthma at the onset.
• Non-specific airway hyper-reactivity is found in patients who are clinically free of asthma, but have had asthma in the past.
• Patients who wheeze in childhood are likely to remain at risk, given the appropriate stimulus.
• People with a history of asthma should be advised not to smoke, as smoking has been shown to be associated with a rapid decline in small airways function in adults with asthma.

References

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