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

Type:Uv phototherapy Time:2017-11-07 17:00:16

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.

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