• asthma epidemiology

‘Asthma’ is now recognised as an umbrella term that includes a heterogeneous group of phenotypes whose aetiology and prognosis vary. This heterogeneity is even more significant in asthma diagnosed before 5 years, using the symptom of wheeze as the predominant criterion, even though not all wheeze is asthma. Therefore, wheezing, which is common in preschool children, is often incorrectly labelled as asthma. Two pragmatic clinical phenotypes1 with distinct risk factors2 3 have been used to describe early life wheeze and assist in determining asthma in early childhood: episodic viral wheeze (EVW) and multitrigger wheeze (MTW). Prognosis is good in recurrent mild EVW although remission is uncommon in atopic MTW and children usually go on to develop asthma. Care should be taken when using these categories, as individual children can change category over time.1 Cohort studies have also described phenotypes of early childhood wheeze by using changing ‘asthma’ symptoms with age and determining distinct patterns in underlying groups (using latent class analysis and latent class growth analysis). These include investigations on cohorts using either symptoms of wheeze (the Melbourne Atopy Cohort Study (MACS),4 5 the Avon Longitudinal Study of Parents and Children (ALSPAC),6 the Prevention and Incidence of Asthma and Mite Allergy (PIAMA)7 (Columbia Center for Children’s Environmental Health)8) or symptoms of wheeze and cough (the Leicestershire cohort9). The aetiology and prognosis of each of these phenotypes have been found to vary significantly with many early life factors including gender, atopy, eczema, parental allergic disease, breast feeding, exposure to tobacco smoke, siblings, exposure to pets, day care attendance, respiratory tract infections and body mass index. Risk factors associated with each phenotype, however, show similar findings across all these analyses independent of populations …