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The Hardening Phenomenon in Irritant Contact Dermatitis: Cosmetic Implications
By: Howard I. Maibach, MD, University of California School of Medicine, San Francisco and Shannon A. Watkins, MD, Yale New Haven Hospital
Posted: October 30, 2009, from the November 2009 issue of Cosmetics & Toiletries.
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Recent evidence suggests that components of the immune system play an integral role in eliciting ICD.3 The skin’s barrier function may also play a role in ICD since patients with altered barrier function, such as those afflicted by atopic dermatitis or challenged by filaggrin null mutations, are more prone to develop ICD.4, 6
Basics of Hardening/Accommodation
As noted, the course of ICD is variable but when chronic ICD is avoided and subsides in spite of continued exposure to the irritant, this condition is referred to as hardening (see Figure 1).
This term, also known as accommodation, has been defined as the adaptation of skin to the cause of irritant contact dermatitis. Its existence has been known for centuries, though its precise mechanisms require elucidation. Some have stated that this process begins with an irritative phase that largely resolves in spite of continued contact with the irritant.2 Following is a summary of theories presented in the literature thus far that attempt to explain the phenomenon of hardening (see Table 1).
Changes in Morphology and Barrier Function
One theory to account for the decreased irritant reactivity of hardened skin is an altered barrier function. This theory holds that morphologic changes in skin structure and composition decrease the irritant’s access to skin. Accommodated skin has a relatively thicker layer of stratum granulosum, compared with normal skin.2, 7 Animal models also have shown a thickened and slightly hyperkeratotic stratum corneum layer with an epidermis 3X thicker and sebaceous glands larger than normal skin.7 Thus, the wider diffusion barrier resulting from these changes could make it more difficult for the irritant to penetrate skin; however, quantitative data is necessary to prove and quantify this theory definitively.
Altered lipid composition of the stratum corneum may also contribute to the decreased irritant response in hardened skin. Heinemann et al. showed that irritant-induced hardening was accompanied by a significant increase of ceramide 1 and therefore proposed that ceramide 1 plays a key protective role against irritation.8