Table 1

Some postulated factors associated with two common types of wheezing syndrome, at different ages

Risk or causal factor		Typical period in childhood		Typical example^*	Comment
Risk or causal factor		Early (0-3 years)	Later (> 5 years)	Typical example^*	Comment
Disturbance in lung growth	+	−	B	Maternal smoking implicated6 7
Increased susceptibility to respiratory virus infections	++	+	B>A	Breastfeeding protective⁸ Possibly related to environmental pollution9 10 Associated with low birth weight11
Excessive airway secretions	++	±	B>A
Acute airway inflammation
(A) Virus	++	+	A and B	12
(B) Allergen	±	++	A
Chronic airway inflammation	?	++	A	13 14
Smooth muscle constriction (β₂responsiveness)	+	++	A>B	Possibly different pathways at different ages
Atopy	−	+	A
Bronchial responsiveness	Unrelated to symptoms or atopy (B)7 15-17	Related to symptoms and atopy (A)4 18-20	A and B	Familial element present in both types20 22

Environmental and host factors will determine actual age and period over which these factors operate. The ‘asthma’ phenotype is dependent on the precise combination of factors present in an individual.
Key: ^* syndromes A and B are described in the text and are meant only to represent possible varieties of wheezing disease, and not to suggest that these are the only varieties or that they are necessarily mutually exclusive.