Aetiology

Genetics

There is good evidence to suggest that genetic factors are important in predisposition to atopic eczema. Twin studies have shown a much higher concordance for monozygotic (85%) than for dizygotic twins (21%),15 although no single gene has yet emerged as a consistent marker for atopic eczema. There may be several, and it is possible that the tendency to atopic eczema might be inherited independently from atopy.

Environment

There are several general and specific clues that point strongly to a role of the environment in disease expression.16 It is difficult to explain the large increase in the prevalence of atopic eczema over the past 30 years in terms of genetics.9 It has been shown that atopic eczema is more frequent in wealthy families.17 It is unclear whether this positive social class gradient reflects exposure to indoor allergens or whether it reflects a whole constellation of other factors associated with social "development". Other studies have shown an inverse association between the prevalence of eczema and family size.18 This observation led to the "hygiene hypothesis" - that children in larger families were "protected" from expressing atopy because of frequent exposure to infections.19 Some evidence for this "protective" effect of infections on atopic eczema has been shown in relation to measles infection.20

Migrant studies also point strongly to the role of environmental factors in atopic eczema. For example, 14-9% of black Caribbean children living in London develop atopic eczema (according to the UK diagnostic criteria) compared with only 5-6% of similar children living in Kingston, Jamaica.21

Further work has suggested that the tendency to atopy may be programmed at birth and could be related to factors such as maternal age.22 The observation that many cases of atopic eczema improve spontaneously around puberty is also difficult to explain in genetic terms alone.2 Specific environmental risk factors for expression of eczema are still not fully elucidated. Allergic factors such as exposure to house-dust mite may be important, but non-allergic factors such as exposure to irritants, bacteria and hard water may also be important.23

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