Skin barrier function changes

FIGURE 19.3 Measurement of changes in skin barrier function may be a relevant surrogate parameter for the prevalence of eczema. 

When used under standardized conditions [112], the Comeometer is able to provide highly reproducible values and to detect very small changes in the moisture content of the stratum comeum. An excellent correlation is obtained between the visual scoring of skin dryness induced by surfactants in a soap chamber test [67] and readings from the Cor-neometer. Such a correlation between the two parameters could be obtained only when measurements were performed after partial recovery of the skin barrier function (TEWL should be below 10 g/h/m ). 

Skin resurfacing risks and complications are identical whether they are from peels, lasers, or dermabrasion. Most of these complications such as hypertrophic scarring, prolonged erythema [over 3 months), and pigmentary changes are related to wound depth and not specifically to the modality used. Other complications, such as infections and contact dermatitis occur due to the impaired skin barrier function. 

Epidermis Complete removal of the dermis may be achieved by several mechanical, thermal, and chemical techniques. Most commonly, the epidermal-dermal junction is split by heating the skin to 60 C for 30-120 s [83, 84], Pitman et al. [85] could show that such a treatment does not impair the barrier function. The use of ethylene diamine tetraacetic acid, sodium bromide, or ammonia fumes has also been reported [80, 83, 86], It may, however, be suspected that the use of sufficiently strong acids or bases may change the buffer capacity of skin, which would especially influence the penetration behavior of ionizable drugs. 

All the above-mentioned changes in lipid composition and organization in diseased and dry skin likely contribute to an impaired stratum corneum barrier function and increased susceptibility to dry skin. However, as previously indicated, abnormalities in the process of envelope formation may also strongly influence the stratum corneum barrier integrity. Therefore, more information is required to elucidate the precise mechanisms by which stratum corneum structure and function are altered. 

Seasonal changes affect the condition of normal skin and may trigger various cutaneous disorders.28,29 In common dermatitis, a decline in barrier function often parallels the increased severity of clinical symptomatology. All these conditions tend to worsen during the winter season when humidity is low.30,31 Abundant indirect evidence indicates that decreased humidity precipitates these disorders, whereas, in contrast, increased skin hydration appears to ameliorate these conditions. The mechanisms by which alterations in relative humidity might influence cutaneous function and induce cutaneous pathology are poorly understood. 

Chronic inflammation causes premature aging of the skin, as observed in patients with atopic dermatitis. The constant inflammatory process leads to decreased function of the skin barrier, accompanied by loss of skin moisture. Presumably, the skin of such patients contains decreased levels of HA. Alternatively, the HA may reflect that found in chronological aging, with a change in the ability to take on water of hydration with enhanced association with tissue structures and loss of extractabil-ity. Demonstration of such changes and the precise histolocalization of this decreased HA deposition would be of intrinsic interest, a study that has not been performed yet. 

Burr and Burr reported in 1929 a new deficiency disease produced by the rigid exclusion of fat from the diet. 1 Rodents fed a fat-free diet showed reduced growth and reproductive failure, accompanied by two prominent changes in the skin, that is, increased scaliness and impaired barrier function.1,2 Reversal of the features of deficiency by administration of linoleic acid (LA), led to the concept of essential fatty acids (EFA) that cannot be synthesized by the higher animals.2 Similarities between the clinical features of EFA deficiency and atopic dermatitis led Hansen in 1937 to discover low blood levels of unsaturated fat in atopic children,3 and he later reported that EFA-deficient infants developed an eczematous rash, which responded to LA supplements.4 Several studies had previously examined a range of dietary oil supplements in atopic dermatitis,5-8 with generally reported benefit. 

Changes in barrier function due to skin disease generally result either from alteration of the lipid/protein composition of the stratum comeum or from abnormal epidermal differentiation (e.g. in psoriasis). As far as transdermal bioavailability is concerned, however, patches intended for systemic therapy are labelled for application only at normal skin sites, free from dermatologic pathology. 

Dermal absorption, the process by which a substance is transported across the skin and taken up into the living tissue of the body (USEPA, 1992), is a complex process. The skin is a multilayered biomembrane with particular absorption characteristics. It is a dynamic, living tissue and as such its absorption parameters are susceptible to constant changes. Upon contact with the skin, a portion of the substance can penetrate into the non-viable stratum comeum and may subsequently reach the viable epidermis, the dermis and, ultimately, the vascular network. During the absorption process, the compound may be subject to biotransformafion (Noonan and Wester, 1989). The stratum comeum provides the skin its greatest barrier function against hydrophilic compounds, whereas the viable epidermis is most resistant to highly lipophilic compounds (Flynn, 1985). 

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