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

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. 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. 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. 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. 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.