The vulva may be more prone to irritation due to its unique skin characteristics. Water retention is a major function of the stratum corneum. Transepidermal water loss (TEWL), an objective measure of the efficiency and integrity of the stratum corneum, is higher in vulvar skin versus forearm skin, as measured by an evaporimeter (Servo Med Ep 1; Servo Med, Stockholm, Sweden). The mean TEWL in the vulva is 1.42 x 103, whereas in the forearm it is 8.68 x 102, suggesting that the vulvar stratum corneum is more permeable to water. In addition, the vulvar stratum corneum is thinner, measuring 0.02 pm in thickness, whereas the forearm stratum corneum measures 11.2 pm in thickness (3). Higher TEWL in the vulvar skin may mean that it is a less complete barrier and, thus, more prone to the effects of irritants. Skin surface water loss— which may be influenced by occlusion and eccrine sweating, as is vulvar skin/ skin occlusion and skin/garment occlusion—is more variable and subject to more increases (or "bursts") in the vulvar skin versus the forearm skin. This is another potential complicating factor for vulvar skin irritation studies (4).
Given the location, pigmentation, and atypical structure of vulvar skin, objective TEWL measurement appears to offer advantages as an indicator of irritant susceptibility versus that of clinical evaluation by visual screening (5). In an attempt to remove the occlusion factor from the vulvar TEWL, researchers used a desiccation chamber to absorb water evaporation from the vulvar and forearm surfaces (6). By drying the vulvar skin, the TEWL and capacitance (a measure of skin hydration) as measured by a capacitometer decreased (Corneometer CM 820 PC®; Courage and Khazaka, Cologne, FRG). The differences in TEWL and capacitance between dried vulvar and forearm skin were diminished but not eliminated. This finding suggests that occlusion alone is not the answer to vulvar skin's higher intrinsic TEWL; there are likely biochemical differences in the stratum corneum of the vulva and forearm.
Although the vulva has a higher baseline TEWL compared to the forearm, tape stripping the skin (to create compromised skin) produces a less intense response in the vulva. This could be due to the fact that the vulva has less of a physiologic barrier function than the forearm, and stripping the vulvar skin does not compromise its stratum corneum as much as the more competent forearm skin. After tape stripping, the vulvar skin capacitance was lower than that of the forearm, reflective of faster healing (7), possibly because of the vulva's inherent increased blood flow (8) and higher epidermal cell turnover rate. Another difference between forearm and vulvar skin is that the vulvar skin has higher extensibility without a comparable increase in the elastic fiber network and retraction, which potentially could allow for the physiological changes needed in childbirth (9). The vulva shows highly effective repair mechanisms during postpartum recovery. The characteristics that allow for such repair may also predispose the vulvar skin to endogenous and exogenous irritation.
Because of the vulva's inherently higher capacitance, or stratum corneum hydration, it has a higher friction coefficient, m, as measured by the Newcastle Friction Meter (Design Unit, Department of Mechanical Engineering, Newcastle University, Newcastle-upon-Tyne, U.K.). The vulvar skin friction coefficient is 0.66 + 0.03 and that of the forearm skin is 0.48 + 0.01 (10). The higher vulvar skin friction coefficient leads to a higher mechanical friction trauma by contact with garment, skin occlusion, sexual activity, and moisture occlusion from incontinence (11) (which increases the moisture, resulting in a higher friction coefficient) (10). This is yet another physiological reason for the vulvar skin's tendency to become irritated.
The vulva's increased TEWL means that its stratum corneum is not as an effective a barrier against water loss as compared to forearm skin; this, in turn, allows for increased absorption of certain compounds. Hydrocortisone 7.7% applied topically penetrated vulvar skin versus 1.3% percutaneous penetrated in forearm skin. Although the labia majora are the embryologic analog of the scrotum, this structure does not approach scrotal permeability at 42% penetration of hydrocortisone (12). Nonetheless, vulvar skin has more than a seven-fold increase in permeability compared to the forearm skin, which could make the vulva more prone to respond adversely to irritants. The vulva showed more irritation response than the forearm when exposed to the chemical irritants, 17% benzalkonium chloride and 20% maleic acid. Using a visual erythema scale on carefully ring-marginated labia majora skin onto which the irritants were applied, maleic acid caused a positive irritation response in 76% of vulvar skin versus 62% of the forearm skin. Benzalkonium chloride caused positive irritation response in 57% of vulvar skin versus 9% of forearm skin. Rapid onset of burning and stinging sensations upon application of the chemical irritants occurred on the vulvar skin but not on the forearm skin (1). However, the vulva is not more susceptible to all irritants. Sodium lauryl sulfate (SLS), an anionic surfactant, did not cause increased reactivity in the vulvar skin compared to the forearm skin, as demonstrated by visual scoring and a lack of a statistically significant increase in TEWL (13). A recent 4-day skin patch test study of menses and venous blood showed that the vulvar skin was less prone to irritation, as assessed by a four-point erythema scale, as compared to the forearm skin (14).
The vulvar skin has higher hydration (higher capacitance), higher TEWL, and a less-efficient water barrier than nonvulvar skin. Because of these characteristics, polar irritants, such as maleic acid and benzalkonium chloride, can better penetrate hydrated vulvar skin. Normal, nonvulvar skin has less hydrated, more lipophilic stratum corneum; thus, polar compounds do not cross as easily. Irritants such as SLS have lipophilic tails, thus, they do not cross the vulvar skin as easily as they do the forearm skin, which is more permeable to small, lipophilic compounds (15). This helps explain the vulvar skin's varied irritant responses to different chemicals.
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