gms | German Medical Science

Phase II of the International Study of Asthma and Allergies in Childhood (ISAAC) indicated similarly high differences in the prevalence of asthma symptoms in children worldwide using standardized questionnaires as have first been described during the ISAAC Phase I [Ref. 1], [Ref. 2]. However, the main aim of Phase II of ISAAC was to investigate these differences further using physiological parameters and assessment of potential determinants of the observed differences in informative populations [Ref. 3]. The aim of the present analysis was to investigate the contribution of atopic sensitization towards the occurence of asthma.

Study population and Methods: Random samples (n=1000) of school children (ages 9-11 years) were studied in 28 centres (20 countries) that reflect almost the full range of prevalence variation observed in Phase I. Study instruments included standardized questionnaires with detailed questions on the occurrence and severity of asthma symptoms, the clinical management and a broad range of potential risk factors. In addition, standardized protocols were applied for skin prick testing. Six commonly occurring aeroallergens were tested: house dust mite (Dermatophagoides pteronyssinus and D.farinae), cat, the outdoor mould Alternaria tenuis, a mixture of commonly occurring grass pollen and a mixture of commonly occurring tree pollen. A reaction was considered as positive if the mean diametre was ≥3mm. Two centres performed the skin prick test in stratified subsamples of about 100 children with and 100 children without wheeze in the past year. The statistical analyses took the stratified sampling scheme into account.

The prevalence of wheeze during the last 12 months ranged from 3% to 22%. The prevalence was particularly high in NewZealand and the United Kingdom and lowest in China and Albania. The prevalence of atopic wheeze (wheeze plus a positive skin prick test to any of the six tested allergens) ranged from less than 1% to 14%. Atopic wheeze was least frequent in Ghana and India and most frequent in NewZealand and Spain. Nonatopic wheeze was lowest in China (0.5 -2%) and highest in the UK (10%). Among children with wheeze during the last year there was substantial variation in the proportion with a positive skin prick test (4 to 70%). The odds ratios for the association between wheeze in the past year and a positive skin prick test varied from 0.9 in Turkey to more than 10 in Estonia and one centre in China. The attributable risk among the exposed children, i.e. the children having a positive skin prick test, ranged from low values of 0% in Turkey and Greece and 16% in Georgia to high values of 81% in the Netherlands and up to 98% in China. The population attributable risk ranged from low values of 0% in Turkey and Greece and 2-3% in India, Ghana and Ecuador to high values of 59% in the Netherlands and 93% in China.

Although atopic sensitisation can explain a substantial proportion of the worldwide variation in wheeze in the past year, the study also demonstrates marked differences in the prevalence of non-atopic wheeze. This translates into very different attributables risks both among the exposed as well as in the total population. The reason for this are high differences in the association of wheeze and atopy on the individual level, as well as in the prevalence of atopic sensitization. Atopy seems to play a minor role outside of Western Europe, NewZealand and China. Overall, the role of atopy in the manifestation of asthma symptoms may have been overestimated in the past.