Although research on atopic dermatitis has been going on over the last
40 years, only the last 7 years have witnessed considerable growth in
atopic dermatitis epidemiology studies. Epidemiology concerns occurrence
and risk factors of the disease and can be of help in genetic counseling.
To get a precise idea about the risk factors for atopic dermatitis, it
is important to have an accurate estimate about the prevalence of the
disease. In addition, without developing certain diagnostic criteria,
which are standardized, validated and universally applicable, it is not
possible to get accurate data on atopic dermatitis prevalence.
Currently, the epidemiology of atopic dermatitis is based on less than
perfect data about its prevalence. However, it is now estimated that atopic
dermatitis affects 10% to 20% of children worldwide, especially those
less than five years old. The prevalence in US is 17% and worldwide it
is 10% to 25%. The incidence of atopic dermatitis is increasing and along
with the cost of treatment, becoming an increasing burden on society.
It can occur anywhere in the world and in any race.
Epidemiological studies have shown that genetic factors carry the highest
disk factor. In general it is a disease of the rich; it’s incidence
in a family is inversely related to family size i.e. the larger the family,
the less the likelihood of atopic dermatitis. It occurs more among the
migrants and finally, its increasing incidence strongly points towards
the environment as a determining factor of the many clinical expressions
of atopic dermatitis.
Effect of genetics
Numerous studies have confirmed the role of genetic factors in the development
of atopic dermatitis. They have shown that atopic dermatitis is a genetic
disorder based on the interactions of unknown number of genes and environmental
factors. In a family with a child affected by atopic dermatitis, there
is generally a family history of eczema or any of the other atopic diseases;
asthma and rhinitis.
The inheritance pattern is more complex and does not follow the Mendelian
pattern. Parental atopy (parents with allergies) carries the highest risk
factor. An atopic mother is a higher risk factor than an atopic father.
This can be explained by stronger maternal genetic heritability, shared
physical environment of mother and child, or environmental events that
affect the fetus in utero.
In one study of all atopic diseases, a stronger correlation was found
between siblings than between siblings and parents, indicating that environmental
factors, especially during childhood, may be responsible for increased
incidence of atopic dermatitis. The identification of parental genes may
be of great help in studying the pathogenesis of atopic dermatitis.
Atopic dermatitis is often accompanied by asthma and hay fever. A multifactorial
cause for atopic dermatitis has been suggested, with genetic and environmental
factors influencing disease development.
However, genetic studies on atopic dermatitis show, interestingly, that
the suspected gene regions of high risk identified show little overlap
with the suspected gene regions of asthma suggesting a strong possibility
that different sets of genes may be responsible for these two atopic diseases.
Instead, some of the identified regions overlap with suspected regions
for psoriasis, another chronic skin disease. The implication being, those
genes in the skin involved in psoriasis might be playing an important
role in atopic dermatitis pathogenesis. Genetic mutation of the epidermal
barrier protein, called, filaaggrin is one predisposing factor for atopic
dermatitis, but there are many more unkown genes involved.
One important suspected region is on a chromosome, whose genes are involved
in the regulation of epidermal differentiation. Recent genetic studies
have identified a cluster of genes on the outermost layer of the skin
which may be as important as those which modify atopic dermatitis. Identification
of the responsible genes would be of enormous help in diagnosis and treatment
of the disease.
To summarize, atopic dermatitis genetics are complex and appear to involve
several genes, each with a different function. Some relate to immunoglobin
E response, some to the skin barrier function and some to skin inflammation.
However, there are still gaps in our knowledge about gene–environment
interactions, better studies of the natural history of atopic dermatitis,
and better clinical trials that answer questions that are important to
physicians and their patients.
Effect of IgE
Several candidate genes with immunological functions were shown to be
associated with atopic dermatitis in at least one study. Some of them
were shown to influence the response of the immunoglobin E.
IgE is a monomeric immunoglobin. It binds to receptors on mast cells
triggering the release of inflammatory mediators. It normally exists at
levels below those of other immunoglobins. However, in patients with atopic
dermatitis, particularly children, the levels are significantly higher
at 80%. But 10% of patients have normal levels of IgE with no specific
response. These observations show that the IgE response is not a primary
cause, but a phenomenon of atopic dermatitis. A type of IgE, known as
cord blood IgE, is a helpful predictor of atopic dermatitis.