Ensuring adequate skin moisture is often a neglected aspect of providing optimal treatment for lower extremity skin conditions. With this in mind, this author discusses evolving paradigms of skin structure and function, reviews key ingredients of topical moisturizers and emphasizes patient education on OTC moisturizer use.
The skin barrier is the body’s first line of defense to external influences. When it is disrupted, the skin barrier can become susceptible to allergens, irritants and infection. There is a prominent emphasis on skin barrier repair at dermatology conferences, especially when faculty are discussing eczema and psoriasis. Inflammatory skin disorders, such as the aforementioned conditions and plantar xerosis, tinea pedis and acne, necessitate addressing the compromised skin barrier in addition to treating the underlying skin issue in order to have both short- and long-term treatment success.
Accordingly, let us take a closer look at key principles of skin barrier repair and how one can incorporate these considerations into treatment plans for lower extremity skin conditions he or she sees daily in the office.
Evolving Perspectives On Skin Structure And Function
In medical school, one learns a basic three-layer model of the skin: epidermis, dermis and subcutaneous layer. Generally, the skin keeps foreign materials out but retains a level of moisture. This simplistic model served us during board exams and basic patient care. However, the progression of research has reemphasized the essential role of the epidermis, especially the stratum corneum, in various skin disorders. The stratum corneum, a critical aspect of the skin barrier, is a key player beyond just being a fortress to keep foreign invaders out. The stratum corneum plays a role in what we call the skin microbiome and the brick and mortar models. In podiatric medicine, this means viewing the stratum corneum as more than simply a place where hyperkeratotic lesions form and surgical incisions take place. These newer models serve as a paradigm shift in how we manage our patients.
Before discussing skin barrier compromise and repair, we must consider what occurs in healthy skin. Let us begin with epidermal cell turnover. In a 28-day cycle, the epidermal cell, first called a basal cell, travels from the basal layer of the epidermis to the stratum corneum where it becomes a corneocyte and later sheds from the skin surface.
There is no cell division past the basal layer and as the cell moves upward in the epidermis, it loses its nuclei. In the spinous layer, cells become more polyhedral
in shape and express proteins called desmosomes.1 Ultimately, the desmosomes connect the cells together. The cells in the granular layer, now considered keratinocytes after moving beyond the basal layer, contain keratohyalin and lamellar granules.1 The lamellar granules carry lipid products that construct the intercellular lipid content of the stratum corneum.1 As the cells transition from the granular layer to the stratum corneum, the cells extrude those keratohyalin granules to become corneocytes. One of the proteins in the keratohyalin granules is profilaggrin, which degrades into filaggrin (filament aggregating protein). Filaggrin binds to the keratin protein in the cells until it is time for the corneocyte to further degrade. When it is time for filaggrin to break down, it forms urocanic acid, which is crucial in ultraviolet (UV) protection, and other amino acids essential to the maintenance of epidermal moisture.1
Many of us think of the corneocytes in the stratum corneum as physically inert cells that release into the air at the end of 28 days. Research over the years proves that the stratum corneum is a dynamic layer that is instrumental in maintaining skin health.1 The brick and mortar model consists of “bricks” that are the corneocytes connected by desmosomes and “mortar,” which consist of the natural moisturizing factor and intercellular lamellar lipids extruded from the lamellar granules. To maintain moisture homeostasis, the natural moisturizing factor attracts and binds water to keep the stratum corneum hydrated. The intercellular lipids consist of cholesterol, fatty acids and ceramides.1 Ceramide, a sphingolipid has hydrophobic and hydrophilic moieties that allow the lipids to stack and bind water, ultimately keeping the stratum corneum hydrated and flexible.
How Does The Skin Maintain Moisture?
As the corneocyte’s transit comes to an end near the surface of the epidermis, the amount of water content or lack thereof determines its corneocyte shedding from the stratum corneum. In dry or windy weather, there is a decrease in the water content in the cell, which activates the proteins to degrade filaggrin. This increases the natural moisturizing factor to maintain moisture homeostasis.1 Finally, enzymes break down the desmosomes in between the corneocytes in order for the cells to shed. As with any chemical reaction involving proteins, this enzymatic process can only occur when the water content is just right. In dry conditions, the desmosomes do not break down, which leads to a buildup of corneocytes that appear clinically as xerotic and hyperkeratotic skin. As podiatric physicians, we see this manifestation on the lower extremities daily.
The process of losing water from the skin is called transepidermal water loss. In a healthy epidermis, the intercellular lipids prevent water loss to the external world while the natural moisturizing factor maintains the water content in the corneocytes, thus preventing them from desquamating too soon.1
In addition to the moisture homeostasis that is constantly maintained in the stratum corneum, pH is also a key player in skin barrier function. Ranging between 4.5 and 5.5, the skin is naturally acidic and is often referred to as “the acid mantle.”2 Stratum corneum pH influences corneocyte desquamation, defense from microorganisms and initiation of inflammation.2 When urocanic acid, a breakdown product of filaggrin, degrades, its breakdown products help to maintain the acidic pH of the epidermis.2 The skin also has a mutually beneficial relationship with the surface microflora, which helps to maintain the acidic nature and provide another layer of host defense against other microorganisms.3 This natural microbiome provides a “roof ” to the aforementioned brick and mortar model. All of these factors are part of an ornate biochemical process to maintain the integrity and moisture of the skin.
When the pH becomes more alkaline from disruption in its water and chemical content, an increase in the growth of Staphylococcus and Candida can occur, and displace the resident microflora.1 However, Trichophyton rubrum, the most common dermatophyte to cause tinea pedis and onychomycosis in the world, takes advantage of the acidic pH of the skin by first growing well in an acidic environment and subsequently shifting the pH to alkaline, which shuts down many of the stratum corneum’s key enzymes.3 By raising the pH through ammonia production, T. rubrum does not allow the skin’s innate defense mechanisms to function and causes havoc in the tissue, which we see clinically as tinea pedis and/or onychomycosis.
When all of these biochemical entities work in concert, the healthy stratum corneum desquamates in the appropriate length of time and has the proper level of hydration, lipids and acidic pH to maintain moisture balance and antimicrobial defense.
Cleansers (soaps), weather (wind, dryness, sun), medical conditions (such as Sjogren’s syndrome), medications (isotretinoin) and advancing age affect this delicate balance of the chemical reactions necessary to maintain the best moisture, lipid and pH levels.1 What helps to repair the insults inflicted on the stratum corneum? A moisturizer.
A moisturizer restores barrier function to the epidermis, creates a protective film, increases hydration and improves the skin surface visually.4 The moisturizer does this through multiple avenues as it creates an occlusive film, humectants to pull water from the dermis into the epidermis and emollients to provide plasticity.4
What You Should Know About Occlusives, Humectants And Emollients
The triple action of a topical moisturizer (occlusive, humectant and emollient) provides the foundation to repair the skin barrier.4 Besides water and the moisturizer itself, additional ingredients in a topical moisturizer may include: UV protection (sun protection factor (SPF)); colloidal oatmeal as with the Aveeno® (Johnson & Johnson) products; antioxidants for anti-aging (vitamin C, vitamin A, niacinamide); and physiologic lipids (ceramides).5-7 These additional ingredients help create unique and elegant formulations tailored to various skin concerns.5-7
As a practitioner, you have a choice of over-the-counter (OTC) preparations and prescription “device” topical products engineered to repair the skin barrier to recommend to your patients. Moisturizers found in both OTC products and those dispensed from the pharmacy are so much more elegant and useful than what most doctors and patients consider and should become a mainstay of your practice if they are not already. These moisturizers are necessary to use in addition to the topical medications such as a topical corticosteroid or antifungal one may use to treat the underlying skin disorder. However, you and the patient must be aware of the ingredients on the labels of products geared to treat the skin issue in question. One should also refrain from using a fragranced moisturizer, especially when it comes to managing inflamed skin as this could cause irritation.
Ingredients such as petrolatum and mineral oil cause occlusion and decrease transepidermal water loss. These components create a hydrophobic barrier or film to retain the appropriate level of moisture. Waxes like carnauba, beeswax and lanolin also act as occlusive agents. Sterols, fatty acids and phospholipids are occlusive agents that work best when one applies them to recently dampened skin.5-7 Interestingly, petrolatum reduces transepidermal water loss by 99 percent as it creates a barrier which water cannot pass.7 However, its greasiness may be unpleasant and could reduce patient adherence.
Humectants promote water absorption from the dermis to stratum corneum and are another key ingredient in a moisturizer. Examples of humectants are hyaluronic acid, glycerin, gelatin and sorbitol.5-7 Glycerin has a unique characteristic with its “reservoir effect,” a lasting effect on the skin long after the product’s removal, possibly by regulating the water channels in the stratum corneum.7
In addition to occlusives and humectants, the other ingredient needed in a moisturizer is an emollient. An emollient provides a smooth texture to the skin by filling in the spaces of the corneocytes that are in the process of desquamating.5 The second most common ingredient added to moisturizers is dimethicone, a silicone-based agent, which will give a smooth and slippery feel to the skin after application.7 Given these active ingredients and their effect on the skin barrier, you will find most moisturizers list various levels of petrolatum, glycerin and dimethicone as their base with other active agents added to the product.
How To Advise Patients Looking For OTC Moisturizers
Accessibility and cost are factors in our patients obtaining these products. Since moisturizers are imperative to our patients’ success in controlling their skin issue, a product that is readily attainable is pertinent to adherence. It is also important to discuss with the patient the “why” of adding another topical ingredient to a possibly complicated skin regimen that involves prescription pharmaceuticals. Highlighting the ingredients on the label and remembering that they are listed in order of decreasing concentration are also key points to connect with the patient.
Depending on the skin condition in question, the physician may guide the patient to an ingredient or description of what the product should do. For example, the CeraVe line has a subset of psoriasis-geared products that contain urea and salicylic acid 2% to aid in softening the plaques. Aveeno Eczema Therapy Itch Relief Balm (Johnson and Johnson) has colloidal oatmeal and ceramides to comfort dry, irritated skin. Not all moisturizers are created equal and patients should refrain from using fragrance-laden products that are rampant at local malls and pharmacies. Patients may also look for products that have the National Eczema Association or National Psoriasis Foundation seal on their label.
A healthy skin barrier is pertinent for a patient’s success in managing his or her skin condition, whether it is a form of eczema or tinea pedis. A moisturizer that has the ingredients necessary to repair the skin barrier makes all the difference in not only the patient’s quality of life but in the long-term success of reducing the chronicity of the acute flares that made the patient present to the office originally. The stratum corneum is not an inert layer of the epidermis that just desquamates. It is a dynamic layer that not only protects the body but maintains hydration at a constant pace. In your practice, it is essential to add moisturizers to your patient’s treatment regimen in addition to the topical pharmaceutical agent for the underlying inflammatory condition or superficial fungal infection.
Dr. Vlahovic is a Clinical Professor in the Department of Podiatric Medicine at the Temple University School of Podiatric Medicine in Philadelphia.
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