Psoriasis and seasonal variation: A systematic review on reports from Northern and Central Europe—Little overall variation but distinctive subsets with improvement in summer or wintertime

BACKGROUND: Positive influence of the sun on psoriasis is a common assumption in dermatology. Other season‐related factors such as mental health may interfere. However, the role of seasonal effects on psoriasis needs to be clarified. This review aims to systematically analyze the literature on seasonal variation on psoriasis with emphasis on Northern and Central Europe representing temperate climate conditions. MATERIALS AND METHODS: Enrolled literature was identified through PubMed, EMBASE, and BIOSIS. An additional manual search of old reports before the introduction of efficient modern therapies, which can interfere with the spontaneous disease, was performed. RESULTS: Thirteen studies were enrolled. About 50% of psoriasis patients were stable and showed no seasonal difference between seasons. Approximately 30% improved in summer, and 20% performed better in winter, some with marked summer worsening. European results matched international reports from different continents and hemispheres with climate extremes. The psychological effects could not be ruled out. CONCLUSION: About 50% of psoriasis patients experience a season‐independent disease, however, with a subset of patients who do better in summer. Others again do better in winter, with a few of these having marked worsening in warm periods. Individual season‐related activity records should be paid proper attention to when considering light therapy or climatotherapy as a treatment.