Inside Out or Outside In: Does Atopic Dermatitis Disrupt Barrier Function or Does Disruption of Barrier Function Trigger Atopic Dermatitis?

Prevention of AD onset using systemic interventions has also been investigated. Probiotics have been suggested as a means to modify the gut microbiota and reduce systemic and mucosal inflammation. Lactobacillus reuteri taken prenatally by pregnant women and by newborns has shown mild benefit in preventing some forms of AD.29 Although they are not approved by the US Food and Drug Administration for this indication, systemic interventions for moderate-to-severe AD such as methotrexate and cyclosporine certainly have shown benefit in managing ongoing illness and breaking the cycle of disease.³⁰ The efficacy of these agents points to the role of systemic inflammation in ongoing AD activity. Moreover, the inside-out hypothesis recently has led to the proliferation of promising new therapeutic agents in the pipeline to treat the systemic Th2 inflammation that occurs in severe AD (eg, anti–IL-4/13 receptor antibody, anti–IL-13 antibodies, and biologics targeting IL-12/23, IL-22, and IL-31 receptors).³¹

Final Thoughts

Atopic dermatitis is a multifactorial disease associated with barrier disruption and intense systemic inflammation. It is likely that both the inside-out and outside-in hypotheses hold true in different subsets of AD patients. It is clear that some individuals are born with filaggrin defects that sufficiently trigger systemic inflammation, resulting in AD. On the other hand, there are clearly some individuals with inflammatory dysregulation that results in systemic inflammation and secondary barrier disruption. Until we can determine the genomic triggering or promoting event in each individual patient, large-scale introduction of active prevention and severity reduction strategies may not be realistic. In the meantime, we can approach AD in childhood from the inside out, through appropriate treatment of systemic inflammation of AD, and from the outside in, with treatment and prevention via emollient use in newborns.