Photoprotection in vitiligo and normal skin. A quantitative assessment of the role of stratum corneum, viable epidermis and pigmentation.
Gniadecka M, Wulf HC, Mortensen NN, Poulsen T
Pigmentation, stratum corneum and viable epidermis are considered to be the main factors protecting against ultraviolet radiation. We quantitatively investigated the degree of photoprotection provided by these structures in vitiligo and adjacent normally pigmented skin. In 14 patients 61 MED tests were performed in vitiligo and adjacent normally pigmented skin using a solar simulator. The thickness of stratum corneum and viable epidermis was determined from frozen skin sections, and pigmentation was calculated by measuring skin reflectance at 555 nm and 660 nm. To analyse photoprotection, the UV dose necessary to evoke erythema was regressed against the thickness of stratum corneum and viable epidermis, pigmentation and the erythema grade in the MED test. By analysing regression coefficients we found that stratum corneum was the main photoprotective factor not only in vitiligo but also in normally pigmented skin. The effect of pigmentation in normal skin was slightly less prominent. Stratum corneum was thicker in vitiligo than in normally pigmented skin. However, the photoprotection due to stratum corneum was similar in both groups because significantly less photoprotection was achieved per thickness unit of stratum corneum in vitiligo than in normal skin. Neither in vitiligo nor in normally pigmented skin did the photoprotection depend on viable epidermis. Our data quantitatively document the importance of stratum corneum and pigmentation. Hyperkeratosis in vitiligo offers just as efficient photoprotection as does the normal stratum corneum in pigmented skin.