Seasonal Variation of Plaque Psoriasis in Relation to Individualized MED-Adjusted Ultraviolet Exposure: A...

Background: Patient-perceived seasonality of psoriasis is frequently reported, yet the independent contribution of objectively quantified, individualized ultraviolet (UV) exposure remains insufficiently characterized. We evaluated seasonal variation in plaque psoriasis and its association with geocoded, phototype-adjusted ambient antipsoriatic radiant exposures (ARE) using mixed-effects modeling. Methods: This cross-sectional study included 119 adults with plaque psoriasis (476 seasonal observations). Participants rated seasonal disease courses using a 7-point scale. Ambient ARE was geocoded to residential postal codes and quantified as a behaviorally weighted dose normalized to individual minimal erythema dose (MED). Mixed-effects logistic regression models, adjusted for relevant confounders, estimated associations with seasonal improvement and worsening. Results: Seasonality was reported by 89.9% of participants (p < 0.001). Summer was the most favorable season, whereas winter was the most detrimental. The highest ARE quartile was independently associated with increased odds of improvement (OR 4.65, 95% CI 2.04-10.58, p < 0.001) and reduced odds of worsening (OR 0.16, 95% CI 0.08-0.33, p < 0.001). Crucially, continuous quadratic modeling revealed a significant inverted U-shaped relationship between UV exposure and improvement, with an estimated turning point of 3.85 (95% CI 1.88-5.82, p < 0.001) for the declared daily ARE (UV_decl) normalized by MED. Beyond this threshold, the probability of improvement attenuated. The protective effect against seasonal worsening remained linear. Conclusions: Psoriasis seasonality demonstrates a robust exposure-response association relationship with ambient UV. The estimated turning point (UV_decl/MED = 3.85) within our modeled exposure metric is exploratory and hypothesis-generating. It suggests an association where moderate UV exposure correlates with patient-perceived benefits, but these diminish at very high levels. This threshold requires external prospective validation before being considered a clinically actionable recommendation.