[Characteristics of allergic asthma]

The frequency of extrinsic (or allergic) asthma varies with age and sex. An inborn tendency to atopy, shown by a positive Phadiatop test or by skin tests positive to air-borne allergens, is found in 50% of asthmatic patients. The arguments in favour of the allergic origin of asthma are primarily clinical: same time and place of symptoms in the typical form, frequent bouts of acute rhinitis, history of familial or personal atopy. The responsible allergen is often easily identified by questioning, and its nature can be confirmed by positive skin tests. The allergens most frequently involved are air-borne allergens, notably house dust mites furs or feathers of domesticated animals, and pollens. Compared with intrinsic asthma, extrinsic asthma has the following features: it begins at an earlier age, it is less severe, it seldom evolves towards chronic ventilatory impairment and, by definition, it implies that an allergen is responsible for the onset or worsening of the symptoms.