What causes cutaneous xerosis?

Several factors, both internal and external, can disrupt the skin's barrier function and cause xerosis. One of the main ones is age. Indeed, as we age, the synthesis of lipids in the stratum corneum, which ensures the trapping of water in the inner layers of the skin, decreases significantly. Thus, as their production declines, the skin becomes more vulnerable to dryness. At the same time, the synthesis of sebum by the sebaceous glands slows down. This lipid mixture is part of the natural composition of the hydrolipidic film, a protective veil located on the surface of the epidermis. Therefore, a decrease in sebum production results in a weakening of the hydrolipidic film and the skin barrier.

In young skin, the water content of the stratum corneum is approximately 15 to 20%. In mature skin, this percentage can drop below 10%.

In addition to the passing of time, environmental factors can influence dry skin. Exposure to cold and dry air, typical of winter periods or air-conditioned environments, accelerates transepidermal water loss (TEWL). Indeed, cold air reduces the skin's ability to retain moisture by disrupting the structure of lipids in the stratum corneum, hence an increase in TEWL. Wind, by drying the skin surface, weakens the hydrolipidic film, which has similar repercussions. Finally, exposure to the sun should not be overlooked as UV rays cause degradation of the lipids in the stratum corneum, thereby increasing skin permeability.

Less commonly, skin xerosis can be due to the use of unsuitable skincare products disrupting the skin barrier. Products containing harsh surfactants, such as sulfates, particularly the sodium lauryl sulfate (SLS) and the sodium laureth sulfate (SLES), can damage the skin by removing the hydrolipidic film present on its surface, thereby increasing TEWL. These compounds are mainly found in cleansing products, such as soaps or cleansing gels. Certain treatments, like isotretinoin, prescribed for acne, but also anticancer drugs, certain diuretics or even medications for prostate disorders, can also severely dry out the skin and lead to xerosis.

Finally, xerosis can be the result of a disruption in the skin microbiota. To clarify, the skin microbiota refers to the collection of microorganisms living on the skin's surface, including bacteria, fungi, yeasts, and more. These organisms constantly interact with each other and the immune system, maintaining the integrity of the skin barrier. When an imbalance in the microbiota occurs, whether due to the invasion of external microorganisms, the proliferation of species naturally present on the skin, or their reduction, an inflammatory response can be triggered.

Indeed, it has recently been highlighted that the proportion of Staphylococcus aureus, bacteria naturally present in the skin flora, drastically increases during outbreaks of eczema. Furthermore, researchers believe that the lipophilic yeast Malassezia is linked to atopic dermatitis and contributes to exacerbating skin inflammation. However, chronic inflammation can alter the structure of lipids, accelerate water loss, and compromise the integrity of the skin barrier, thus promoting the onset of xerosis. A modification of the skin microbiota can therefore be considered an important factor in the development and worsening of xerosis.