understanding - stratum corneum (epidermal skin barrier layer)

understanding - TEWL (transepidermal water loss)

cosmetics for sensitive skin

dynamics of sensitive skin - good ingredients to use - etc.

research needed

treatment of sensitive skin - symptoms and useful regimen

Sensitive skin is not an exactly defined complex entity. The most important factors are known, but still a lot of chemical and physiological reactions are not investigated. Sensitive skin has its own dynamics with toxic irritations and immune reactions. The main battlefield of sensitive skin is the stratum corneum, because stinging, itching and tight feeling arises here. Perhaps 10-20% or more of all people around the world have sensitive skin. This has to be considered in the developement of cosmetical and pharmaceutical products.

At present, the definition of sensitive skin is still debated. Contradictory findings have been reported. Some investigators reported that sensitive skin seems to be due either to a constitutional anomaly, or be caused by skin disease, ageing or occupational exposure to irritants.

On the other hand some investigators found no constitutional increase in skin reactivity. It is not well clear if this word sensitive should be applied exclusively to observable hyperreactive susceptibility to such sensations as stinging, itching and tight feeling after the application of a topical product. Undoubtedly very often many reported reactions to cosmetics and toiletries are frequently sensory phenomena with no visible effects. This impossibility to classify this phenomenon between the several skin diseases, led to define the sensitive skin with the definition of status cosmeticus.

However, clinicians and the people working in the personal care industry know that some individuals report symptoms and sometimes show traditional signs of irritation, when certain material are applied to the skin. The reasons for sensitive skin in these persons may be obvious, but often the complaints and the signs of irritation occur in individuals appearing to be normal.

In fact, many cosmetics, free of known compounds of allergic hypersensitivity, produce an unremarkable clinical picture in a certain number of individual persons. Moreover, the same cosmetic products completely harmless for the great part of the consumers, are nonetheless capable of producing a stinging sensation in some others. The describe visible skin reactions of the butterfly area of the face such as erythema, and non inflammatory sequelae, such as dryness and roughness, claiming to have tried from ten to twenty different cosmetics without solving the problems.

The observable hyperreactive response has been found above all among different ethnic groups, in particular Asians more than Causcasians, probably correlated with the greatest exposure to the sun and with the higher relative humidity.

The most important factors of sensitive skin are: 1. Skin dryness 2. Ageing of the skin 3. Atopic skin condition. The strateum corneum is the main battlefield, because stinging, itching and tight feeling arises in the stratum corneum. In constitutional anomalies the decisive barrier between the body and the environment, the basal layer is -at least partly- damaged: infections and other pathological conditions follow. A lot of skin diseases start with artificial damages of the stratum corneum. Irritants are water, syndets(synthetic detergents), soaps, chemicals, desinfectans and the sun.

Normal skin possesses a potent barrier to water loss. The primary location on this barrier is the stratum corneum. Most of the barrier is accounted for by the intercellular lipids, delivered by the lamellar bodies at the level of the stratum granulosum and packed into tight layers. Ceramides are glycolipids that perform a key role in the organisation of the intercellular lipids. Sebum contributes lipids to the skin surface as well with only a minor contribution to barrier function. In addition, filaggrins in the keratohyaline granules is degraded to amino acids, which act as humectants holding water in the epidermis. When transepidermal water loss (TEWL) exceeds the rate of water transport form the dermis, dehydration of the epidermis, especially the stratum corneum, occurs. Excess TEWL can occur under environmental conditions of low humidity and wind. If the barrier is perturbed, either intrinsically of be external factors such as surfactants or solvents, TEWL increases and dryness may occur even in a normal environment.

With ageing, there are several changes that occur that may contribute to dryness. Normal aged skin shows decreased levels of ceramides, the lamellar bodies lack the normal lamellate internal structure on electron microscopy, and the number of keratohyaline granules is decreased, as is amino acid content of the stratum corneum. In dry, aged skin, sebum lipids and free fatty acids are also decreased. Baseline TEWL is normal, but it is easier to disturb the barrier in aged skin than younger skin, and recovery is delayed. Atopic skin is also prone to dryness, with decreased waterbinding capacity and increased TEWL. Like aged skin, the skin of atopic individuals possesses less lipid, especially ceramide. The extrusion of the lamellar bodies into the intercellular space is delayed and incomplete. Baseline TEWL is increased, and water binding capacity is decreased.

Cosmetics for sensitive skin

• Special ingredients must be selected, others deselected
• The raw materials must have very high (pharmaceutical) standards
• Special questions must be answered: how will the vehicle composition influence the absorption? etc.
• Vicious three: fragrance, emulsifier, preservatives
• Cosmetics have to be tested on persons with sensitive skin

Atopic and aged skin both in general seem to be prone to inflammation. This skin is more susceptible to irritants. In fact, tape stripping induces keratinocyte producing of proinflammatory cytokines. Conversely, irritant dermatitis leads to increased TEWL and prolonged periods may be required for complete healing and a return to baseline TEWL and hydration. - Dry skin is found more commonly in patients with atopic dermatitis, as well as in unaffected family members. Similarly, unaffected family members of patients with ichthyosis vulgaris may experience significant dry skin, perhaps reflecting variable expressivity of the ichthyosis. Unaffected obligate gene carriers for ichthyosis vulgaris have been found to have abnormal keratohyaline granules and profilaggrin and filaggrin levels intermediate between normal and that found in affected individuals. Ichthyosis vulgaris can be confirmed in a small percentage of patients diagnosed with atopic dermatitis.

From all these considerations follows the necessity to know and to study the differences in the skin physiology, in order to determine its qualitative and quantitative ability to react to different kind of cosmetic ingredients.

Moreover, more rigorous relationships should be sought between symptoms and provoking factors level for identifying the involved chemical compounds.

Dynamics of sensitive skin

• Vicious circle of reactions: toxic substances and free radicals penetrate through the disrupted skin barrier, that triggers the immune reaction and inflammation, and this is accompanied by further free radicals and endotoxins, which impair cells and cause further destruction of the barrier
• Immunmodulators can stabilize the skin reactivity: omega-3-oils, rape seed oils etc.
• The immune reactions running in the skin are not only the cause but also the result of the hypersensitivity
• Cosmetics and nutritive supplements containing omega-3-oils etc. may allow to change the immune reactions and reduce inflammation

The keys to treatment of sensitive skin are rehydrations and restoration of the normal skin barrier function. Avoidance of contributing environmental factors is critical. Hot water bathing and use of soap must be limited or eliminated: lipidized soaps and nonsoap cleansers may be acceptable. Humidification of the environmental and avoidance of dry heat and wind are helpful. Topical application of moisturizers, especially after bathing, rehydrates the skin and reestablishes a barrier to both water loss and penetration of external irritants. Oil baths result in an increase of skin surface lipids and rehydration comparable to, but no better than moisturizers. The direct hydration of the skin is of short duration and comparable to tap-water bathing. Moisturizers with urea, lactic acid, ammonium lactate, or other alpha-hydroxy acids, which act as humectants, may have an additional benefit. Thus, it is known that water is the only plastifier of the horny layer, and its variation of only 1% can significantly change the skin´s elasticity and permeability also. However, it is possible that people, identified and characterized by a sensitive skin, appear to have impaired mechanisms of percutaneous absorption and, consequently, skin hyperreactivity due to the increased amount of irritant absorbed.

Dermatological research is needed

• To study the differences in the physiology of stratum corneum
• To determine the qualitative and quantitative ability to react to different kinds of pharmaceutical and cosmetic ingredients
• To study the relationship between symptoms and provoking factors´ level for identifying the involved chemical compounds
• To develop an unique treatment concept

In attempt to clearly define, understand and predict all the reactions associated with the sensitive skin, cosmetic preparations has to be designed cautiously, selecting and evaluation in advance, both the vehicles and the active ingredients used. The setting up of an appropiate vehicle for sensitive skin, requires, not only a scrupulous selection of the raw materials, but also a deep knowledge of the remarkable problems linked to the difficulty of releasing the several active principles at cutaneous level, problem only partly foreseeable through mathematical models.

The vehicle composition will influence the absorption process and the relative solubility of the chosen active principles will influence besides other factors the stratum corneum. The formulator´s experience will be of fundamental importance as well as the studies on cosmetic efficacy and tolerability and the usage habits of the final consumer.

Treatment of Sensitive Skin

• Rehydration with masking emollients (synthetic hydrocarbons like mineral oil, fatty alcohols, traditional esters etc.), occlusive agents (petrolatum, dimethicone), skin protectants (glycerin), hygroscopic agents (propylen glycol, urea, alpha hydroxy acids), enhanced desquamation, liquid fluidizing agents (glycerin, maleated soybean oil), enhancers of barrier repair (key-enzymes of cholesterol)
• Restoration of the normal skin barrier function
• Avoidance of contributing environmental factors is critical and not very successful
• Limitations to hot water bathings and use of soap
• Humidification of the environment
• Avoidance of dry heat and wind is helpful
• Topical application of moisturizers (urea, lactic acid, ammonium lactate)
• Oil baths result in an increase of skin surface lipids, but are not better than moisturizers
• The direct hydration of the skin is of short duration
• Remember: 1% more water in the horny layer can significantly change the skin´s elasticity and permeability, because water is the only plastifier of the horny layer.

There a lot of checklists for the symptoms of sensitive skin. The most important points are:

1. Your skin becomes red or splotchy after aggressive cleansing or a hot shower or bath.
2. Your skin reacts to environmental changes, like a drop in temperature.
3. Your skin flushes easily
4. You have fair skin with visible blood vessels
5. You sunburn easily
6. Your skin gets red easily, even when you just touch it
7. You frequently feel stinging sensations after using a cleanser or applying a moisturizer or sunscreen.

There are some simple rules for people with sensitive skin: simplify your skincare routine. If you have truly sensitive skin, cleanse no more than twice a day, exfoliate once a week at most and try not to use too many products. If the products you use contain few ingredients that lessens your chances of irritation. So it is always smart to shop for products with short ingredient lists.