This extrinsic type is the commonest and occurs in patients who develop allergy to inhaled antigenic substances. They are also frequently atopic showing positive responses to skin prick testing with the same antigens. The hypersensitivity reaction in the lung (and skin) is of the immediate type (type 1) involving IgE-mediated mast cell activation. Allergen avoidance is particularly relevant to managing this type of asthma.
Some patients exhibit wheeze and breathlessness in the absence of an obvious allergen or atopy. They are considered to have intrinsic asthma and because of a lack of an identifiable allergen, allergen avoidance has no place in their management.
Some patients develop wheeze that regularly follows within a few minutes of exercise. A similar response occurs following the inhalation of cold air since the common mechanism appears to be airway drying. Inhalation of a p2-adrenoceptor agonist, sodium cromoglicate (see below) or one of the newer leukotriene receptor antagonists (see below) prior to either challenge prevents broncho-constriction.
Asthma associated with chronic obstructive pulmonary disease
A number of patients who have persistent airflow obstruction exhibit considerable variation in airways resistance and hence in their benefit from bronchodilators drugs for asthma. It is important to recognise the coexistence of asthma with chronic obstructive pulmonary disease in some patients, and to assess their responses to bronchodilators or glucocorticoids over a period of time (as formal tests of respiratory function may not reliably predict clinical response in this setting).
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