Читать книгу Ridley's The Vulva - Группа авторов - Страница 118

The barrier function of skin is dependent on the degree of hydration, the presence of a stratum corneum, and occlusive factors. Vulval keratinised skin differs from that elsewhere in hydration, permeability, and friction, but there is no significant difference between pre‐and post‐menopausal age groups. There is little information on variations in these functions on the mucosal surface of the inner vulva, but it has been assumed that this will be similar to that of the oral mucosa, where there is a considerable increase in permeability.

Transepidermal water loss (TEWL) is the rate at which water diffuses across the stratum corneum and is an indicator of hydration and therefore of barrier function. TEWL varies with the number of cell layers in the epidermis, which in turn varies at different body sites. This number is lowest in the genital area [7], and it has been shown that water diffuses faster across the stratum corneum of the labium majus than the forearm [8,9], therefore increasing hydration. The occlusive environment may also play a part in this.

The coefficient of friction is higher on the vulva [9], making it more prone to mechanical damage. Friction can also be affected by occlusion, obesity, immobility, and use of sanitary wear. The vulva is less susceptible to irritants than forearm skin [10]. However, urinary ammonia levels will increase the pH and activate faecal enzymes, which will contribute to irritant features in patients with urinary incontinence [11].

The vulva is more permeable to hydrocortisone than the forearm [12], but this does not fall after the menopause as it does on the forearm. The difference in permeability and hydration will influence the effect and absorption of topically applied preparations [12,13], but it also has a practical application in cases of suspected contact irritancy and allergy. Routine patch testing may not be sufficient [14], and additional tests may be needed to allow for the effects of friction and epidermal morphology [15]. These physiological changes mainly studied in Caucasian women have also been confirmed in Asians [16].