73
Photodermatoses
Photo Facts
• More than 95% of UVR that reaches the earth’s surface is UVA.
• UVA, but not UVB, can penetrate through glass windows.
• The penetration of UVR into the skin is wavelength-dependent (Fig. 73.1).
Fig. 73.1 Depth of penetration of different wavelengths of UV light into human skin. Depth of penetration varies greatly with the thickness of the different skin layers and their composition (e.g. melanin content). The beginning of the wedge-shaped portion of the penetration symbol represents a decrease to approximately one-third of the incident energy density, and the tip of the symbol represents a decrease to approximately 1%. Figure not drawn to scale.
• Principles and formulations of sunscreens are discussed in the Appendix.
Cutaneous Effects of UVR Exposure: Acute
• Typically, a UVB-induced sunburn appears within 30 minutes to 8 hours of sufficient exposure, peaks at 12–24 hours, and diminishes over hours to days with desquamation.
• Tanning occurs as a biphasic response to UVR and is wavelength-dependent.
– ‘Delayed tanning’ usually results from UVB exposure and peaks about 3 days after sun exposure.
Cutaneous Effects of UVR Exposure: Chronic
• The visible long-term effects of UVR include photoaging and photocarcinogenesis.
• Cutaneous signs of photoaging are pictured in Fig. 73.2 and also include solar lentigines, sunburn lentigines, and ephelides; in more darkly pigmented skin, small seborrheic keratoses, melasma, and dyspigmentation may be seen.
Fig. 73.2 Cutaneous signs of significant photoaging. A, B Solar elastosis of the cheek and arm with the characteristic yellow discoloration and thickening of the skin. C Cutis rhomboidalis nuchae with deep furrowing of the posterior neck. D Poikiloderma of Civatte with sparing of the anterior neck. E Multiple open comedones of the malar region in Favre–Racouchot syndrome. F Brown discoloration of the dorsal hand due to hemosiderin deposits from recurrent solar purpura. G Fragility of atrophic photodamaged skin of the shin. H Erosive pustular dermatosis of the bald scalp. I Photoaging in a 93-year-old African-American woman, demonstrating fine wrinkling and numerous tiny seborrheic keratoses. A, Courtesy, Henry Lim, MD; B, Courtesy, Jean L. Bolognia, MD. E, I, Courtesy, Henry Lim, MD; F, G, Courtesy, Jean L. Bolognia, MD.
• Chronic UVR is responsible for development of actinic keratoses, squamous cell carcinoma, basal cell carcinoma, and cutaneous melanoma (see Chapters 88 and 93).
Photodermatoses
• The photodermatoses are classified into four broad categories (Table 73.1) and are discussed in Tables 73.2 (Figs. 73.3–73.10), Table 73.3 (Fig. 73.11), and Table 73.5 (Fig. 73.12).
Table 73.2
Clinical features of the idiopathic, probably immunologically based, photodermatoses and photoaggravated dermatoses.
