Is there a seasonal pattern in giant cell arteritis? Revisiting the evidence in a large monocentric cohort over 30 years

BACKGROUND: Despite perceived clinical clustering, the evidence for GCA seasonality is conflicting and reliance on diagnosis or biopsy dates rather than symptom onset may have masked genuine temporal patterns. OBJECTIVES: To determine whether GCA exhibits a seasonal pattern of symptom onset, and to contextualise any month/season effects against long-term trends and clinical subgroups. METHODS: We analysed a monocentric cohort from Berlin-Buch (Germany, 1994-2023). Consecutive patients with clinically confirmed GCA and a documented symptom-onset date were included (n = 1149). We tested uniformity across meteorological seasons and months via exact multinomial tests and fitted negative-binomial models with b-splines for long-term trends, month-length offsets, and alternative seasonal specifications (month-by-month reference, season factor, cosinor with one/two cycles). Subgroups were defined by sex and GCA phenotype (cranial, large-vessel, mixed). Ultrasound was the primary diagnostic modality for confirming GCA. RESULTS: Seasonal distribution was balanced across seasons (winter 27.0%; spring 25.4%; summer 24.0%; autumn 23.6%; p = 0.36). Monthly distribution departed from uniformity (p = 0.002), with significantly higher incidence in January vs February (+88%), December vs February (+80%), and January vs September (+67%) with similar incidence rates across the rest of the year. Subgroup analyses revealed no consistent seasonality by sex or phenotype. CONCLUSION: Across nearly three decades, we found no convincing evidence for broad meteorological seasonality in GCA symptom onset, although short-term monthly variation, including a relative increase in December-January, was observed.