Key messages ^{(Level of evidence III-2)}

Wheezing in infancy and childhood is not a single disorder and is just as likely to be due to causes other than asthma.

In more than half of the children who wheeze within their first three years, the wheezing is transient and does not increase the risk of later asthma.

Wheezing that persists and is likely to develop into asthma can have an early or late onset.

Children with persistent wheezing are more likely than those with transient wheezing to have features of atopy and reduced lung function by the age of six years.

The earlier asthma symptoms such as persistent wheezing appear, the more severe the disease in later life and the greater the damage to the airways.

There is a clear association between parental asthma or atopy, persistent wheezing and asthma.

Exposure to tobacco smoke significantly increases the risk of wheezing in the first three years of life but not in later childhood.

Maternal smoking during pregnancy is a risk factor for transient wheezing and affects lung development.

There is a strong association between atopy, persistent wheezing and asthma, but the role of allergic sensitisation in the development of asthma is not yet clear.

Environmental conditions that increase the rate of bacterial and viral infections are risk factors for transient wheezing but may protect against persistent wheezing and asthma.

Children with frequent simple colds and other common childhood infections in infancy are less likely to develop persistent wheezing in later childhood.

Asthma and wheezing — is prevention possible?

Asthma is a complex, heterogeneous disease involving both the airways and the immune system. Inflammatory responses, usually associated with atopy, lead to respiratory symptoms such as airway hyper-responsiveness, wheeze, cough and breathlessness.

Generally, asthma is identified by the presence of these symptoms, together with features of atopy or a family history of atopy or asthma, and impaired lung function.¹ However, asthma is not easy to define and diagnose, especially in the first years of life. Wheeze, cough and breathlessness are common in young children and can all be symptoms of conditions other than asthma.¹

The prevalence of asthma, atopy and wheezing is on the rise all over the world.1 Intense research efforts are underway to identify what might be causing this increase – and to understand more about the early natural history of asthma. There is increasing evidence that asthma originates in the first years of life and involves a complex interaction between genetic and environmental factors.²

Many studies are investigating the relationship between wheezing and asthma to see whether wheezing can be used as a predictor of asthma. So far there has been considerable progress in defining different wheezing disorders in childhood and identifying factors predictive of persistent wheeze and asthma. However, on current evidence it is not yet possible to develop interventions to address likely causative factors.

Primary preventive measures target a population that is still healthy but at risk of a disease, mostly using a range of interventions. However, it remains uncertain whether manipulating factors that may be associated with asthma can reduce its prevalence or delay onset.

Quality of the evidence

This paper is based on review of the recent literature as outlined below. Most evidence in this area is from observational cohort studies, and would therefore be ascribed a level of evidence of III-2.

Sources of evidence

Observational, prospective, birth cohort or other longitudinal studies, either of whole population or of high risk groups, that identify factors associated with persistent wheezing and subsequent development of asthma.

Reviews and other studies exploring the aetiology and prevention of wheezing and asthma.

While there is increasing interest in preventing asthma in those genetically at risk,³ there remain a number of limitations to the available evidence.

Limitations of existing evidence

There are difficulties in assessing lung function in children aged under five using standard tests such as spirometry.
The fact that many of the studies are so recent means that there has been limited time for follow-up.
Many studies focus on infants at high risk of atopy or asthma (usually on the basis of family history) so findings cannot always be applied to the general population.