Development of cutaneous tolerance to ultraviolet B during ultraviolet B phototherapy for psoriasis

Abstract

During a schedule of multiple exposures to ultraviolet B radiation (UVB, 280-320 nm), skin develops a reduced sensitivity, variously called tolerance, photoadaptation, accommodation or acclimatization. In this study we have investigated the development of tolerance in the normal skin of a group of psoriatic patients during the course of UVB therapy. Tolerance was assessed by phototests carried out on non-lesional skin as frequently as possible throughout the treatment. Maximum tolerance was developed by the group of individuals most sensitive to UVB, which was twice that of the least sensitive group. The minimal perceptible erythema dose (MPE) increased rapidly in the first 2 weeks (220% per week) and reached a plateau by the eighth week of 800% above the baseline MPE dose. For the more sensitive patients there was a further increase in sensitivity (decrease in MPE dose) after the ninth week of continuous treatment. Tolerance to UVB also involves pigmentation in the first few weeks, but in these patients there was no evidence of hyperpigmentation by the end of treatment. While epidermal hyperplasia is most likely to play a leading role in the development of tolerance to UV, there is no reason to expect this protection to decrease under conditions of continuous exposure. Thus, accommodation to ultraviolet radiation (UVR) is not a monotonically increasing process but appears to alter the accepted reactions of human skin to UVR.

Photoadaptation to ultraviolet (UV) radiationin vivo: photoproducts in epidermal cells following UVB therapy for psoriasis

“…Darker skin type, as expected, was protective in this study, but tanning, a significant photoprotective mechanism of human skin, has been shown in another study to be responsible for only 10% of the photoprotection in human volunteers after 4 weeks of cumulative doses of UVB 26 . Photoadaptive responses of human skin have also been measured using the minimal perceptible dose 27 …”

Photoadaptation to ultraviolet (UV) radiation in vivo: photoproducts in epidermal cells following UVB therapy for psoriasis

“…26 Photoadaptive responses of human skin have also been measured using the minimal perceptible dose. 27 The lack of association between challenge doses of UVB and levels of adducts in this study could be explained by narrow ranges of UVB doses, variable tanning and epidermal thickening responses among individuals as well as potential experimental errors in the measurement of DNA photoproducts. However, the fact that TTT and TTC dimers moved in the same direction within individuals suggests that experimental errors are unlikely.…”

Skin surface topography grading is a valid measure of skin photoaging

“…While dermal elastosis is described as a distinct consequence of UVR exposure (10) and hence an accurate measure of photoaging, it does not necessarily follow that it is the most appropriate standard against which to validate skin surface topography grades. Other histological changes associated with skin photoaging include thickening of the stratum corneum (11, 12), stratum granulosum (13) and epidermis (10, 14), oedematous changes to the epidermis (15), and a line of collagen fibril bundles running parallel to the skin surface in the upper papillary dermis, termed the Grenz zone (15). The present study was carried out in a community setting in Queensland, Australia, in order to assess the validity of skin surface topography grading as a measure of photoaging.…”