Side Shift of Attacks in Cluster Headache: A Prospective Single-center Study

Purpose: Although strict unilaterality is a characteristic of cluster headache (CH), side shift of attacks has been reported. We aimed to assess the prevalence and patterns of side shifts, as well as their correlations with clinical characteristics and treatment response in CH patients. Methods: We prospectively recruited and followed up CH patients at a university hospital. Patients with two or more lifetime CH bouts were interviewed about their side shift history using a structured questionnaire. The demographics and disease characteristics were collected at baseline, and the treatment response at 2- to 4-week follow-up examinations was compared between patients with versus without side shifts. Results: Out of 124 CH patients, 26 (21.0%) experienced side shifts. Sixteen (61.5%) experienced shifts between bouts, 13 (50.0%) within a bout, and four (15.4%) within an attack, with none (0%) reporting bilateral pain during an attack. Among patients who experienced shifts between bouts, six (37.5%) reported a single shift during the entire disease course, while 10 (62.5%) reported multiple shifts between bouts. The demographics, characteristics, and treatment response did not significantly differ according to the history of side shift. Conclusion: In our study, the prevalence and pattern of side shifts were comparable to the results from earlier studies. The presence of side shifts did not show significant association with a specific clinical profile and their incidence did not impact the treatment response. These findings suggest that side-shifting CH is not a distinct entity or migraine variant, but rather within the spectrum of CH.