Epidemiology of Asthma by Age at Diagnosis

Asthma is a heterogenous disease which is suggested to differentiate by age at onset according to previous literature. It seems that adult-onset asthma would have weaker response to asthma medication and worse prognosis than asthma which has occurred in childhood, and that the average patient characteristics would be different between them. However, the previous studies considering age at asthma onset are quite scarce, have rarely been based on population data and have usually left out older adults. In them, asthma is also mostly divided only dichotomously to adult and child-onset asthma, although dividing asthma by more various age at onset groups, like younger and older adults, could also have further importance.

Therefore, the aim of this study was to, in an adult population sample, investigate versatilely age at asthma diagnosis and its association with asthma remission, symptom burden, and other common chronic diseases. The aim was also to investigate age-specific prevalence and incidence of asthma.

This study utilized data which were collected by sending a FinEsS respiratory questionnaire to 16 000 subjects aged 20-69 years during year 2016. The subjects were randomly selected by the Finnish Digital and Population Data Services Agency from Helsinki and Seinäjoki-Vaasa area populations, conforming their age and sex distributions. Asthma was categorized by age at diagnosis in two different ways: firstly, to child (0-17 years) and adult-diagnosed (18-69 years) asthma, and secondly, to early (0-11 years), intermediate (12-39 years), and late-diagnosed (40-69 years) asthma. Age at asthma diagnosis was also used as a continuous variable in the analyses.

The main findings of this study were the following. Totally 8199 (51.5%) subjects responded, 879 (11.1%) reported asthma and 842 additionally reported age at asthma diagnosis. Early-diagnosed asthma was reported by 245 (29.1%), intermediate- diagnosed by 358 (42.5%) and late-diagnosed by 239 (28.4%) subjects.

In the Seinäjoki-Vaasa data (responders N=4173), the 10-year age and gender- specific incidence of new asthma diagnosis was highest in 0-9-year-old males and 40- 49-year-old females. Adult-diagnosed asthma (N=271, 63.7%) was the dominant phenotype of asthma in the studied population and became dominant in females by 38 and in males, by 50 years of age as related to child-diagnosed asthma (N=155, 36.3%).

In the whole study data (responders N=8199), late-diagnosed asthma was in remission only in 5% of cases, and as age at asthma diagnosis increased, the probability of being in remission decreased (p<0.001). In early-diagnosed asthma, males were more often in remission than females (36.7% vs. 20.4%, p=0.006). Subjects with adult-diagnosed asthma reported also more asthma symptoms than subjects with child-diagnosed asthma (median 4 vs. 3 symptoms, p<0.001). In multivariable logistic regression analysis including 12 different variables, significant risk factors of attacks of breathlessness in the last 12 months in subjects with physician-diagnosed asthma were female gender, family history of asthma, adult- diagnosed asthma, and allergic rhinitis.

The most common non-respiratory disease in subjects without asthma (18.9%) and with late-diagnosed asthma (42.3%) was hypertension, and in early- (17.5%) and intermediate-diagnosed asthma (21.1%), obesity. Late-diagnosed asthma was significantly more often associated with three or more non-respiratory diseases as opposed to subjects without asthma (p<0.001) or other age at diagnosis groups (p=0.007). The analyses were widely redone by excluding COPD, and it did not affect the significant differences between key variables.

In the light of these results, adult-diagnosed asthma and especially late adult diagnosed asthma is a marked health burden in the general adult population and asthma diagnosed in childhood less common, more prone to be asymptomatic and less associated with multimorbidity. Therefore, both absolutely and relatively speaking, more resources should be allocated to investigation and treatment of adult-diagnosed asthma especially if diagnosed in late adulthood.