Background The severity of polymorphic light eruption (PLE) is highly variable. The results of studies of the prevalence, pathogenesis, provocation and treatment of PLE may be highly dependent on the severity of disease in the patients studied.
Objectives To produce a simple, valid and reproducible method to assess the severity of PLE.
Patients and methods Eighty patients were asked about the PLE they had experienced during the preceding 12 months, using a standardized interview comprising 16 questions. The answer to each question received a score. A PLE Severity Index (PLESI) was formulated, consisting of 10 questions, with a possible total score of 2–100. The internal consistency of the PLESI (the extent to which the responses to different questions correlated with each other) was assessed by reliability analysis, using Cronbach's method. Twenty patients were re-interviewed 7–27 days later to assess the repeatability of the PLESI. The ease of provocation of PLE by exposure at 24-h intervals to solar-simulated radiation was assessed on a five-point scale in nine of the 80 subjects (the EOPSSR score).
Results The value of Cronbach's α for the PLESI was 0·77. The distribution of the PLESI was consistent with a normal distribution, with a mean value of 52·7 and standard deviation of 19·4. It had a coefficient of repeatability of 20·1. The PLESI was positively correlated with EOPSSR (rs =0·69, P = 0·039) and the number of years since onset of PLE (rs = 0·25, P = 0·03). There was no association between the PLESI and the duration of persistence of the eruption after ceasing sun exposure (rs = 0·12, P = 0·30), the development of tolerance as summer progressed (rs = − 0·14, P = 0·39), gender (P = 0·50) or skin type (P = 0·87).
Conclusions This study has (i) validated the concept that a single score can reflect disease severity in PLE by showing that the principal characteristics of the condition, including, for example, the extent of anatomical distribution and the ease of provocation of the eruption, correlate with each other; (ii) formulated the PLESI, which is a simple, valid and reproducible way of assessing disease severity; we suggest it could be used worldwide to determine the severity of PLE among patients enrolled in future PLE research; (iii) shown that the ease with which the eruption is provoked by solar-simulated radiation correlates with the severity of the condition; and (iv) shown that the duration of persistence of the eruption after sun exposure does not correlate with the severity of the condition.