Intra- and interindividual attack frequency variability of chronic cluster headache

Background: The lack of knowledge about the intra- and interindividual attack frequency variability in chronic cluster headache complicates power and sample size calculations for baseline periods of trials, and consensus on their most optimal duration. Methods: We analyzed the 12-week baseline of the ICON trial (occipital nerve stimulation in medically intractable chronic cluster headache) for: (i) weekly vs. instantaneous recording of attack frequency; (ii) intra-individual and seasonal variability of attack frequency; and (iii) the smallest number of weeks to obtain a reliable estimate of baseline attack frequency. Results: Weekly median (14.4 [8.2–24.0]) and instantaneous (14.2 [8.0–24.5]) attack frequency recordings were similar (p = 0.20; Bland-Altman plot). Median weekly attack frequency was 15.3 (range 4.2–140) and highest during spring (p = 0.001) compared to the other seasons. Relative attack frequency variability decreased with increasing attack frequency (p = 0.010). We tabulated the weekly attack frequency estimation accuracies compared to, and the associated deviations from, the 12-week gold standard for different lengths of the observation period. Conclusion: Weekly retrospective attack frequency recording is as good as instantaneous recording and more convenient. Attack frequency is highest in spring. Participants with ≥3 daily attacks show less attack frequency variability than those with <3 daily attacks. An optimal balance between 90% accuracy and feasibility is achieved at a baseline period of seven weeks. The ICON trial is registered in ClinicalTrials.gov under number NCT01151631.