Interictal burden, not ictal burden, drives work productivity impairment in headache disorders: insights from a Japanese cross-sectional study

Headache disorders cause work productivity and activity impairment (WPAI). There are two distinct types of burden caused by headache disorders: interictal burden measured by Migraine Interictal Burden Scale-4 (MIBS-4) and ictal burden measured by Headache Impact Test-6 (HIT-6). However, the impact of interictal burden on WPAI remains unclear. This study aimed to investigate whether MIBS-4 score (interictal burden) is associated with WPAI among individuals with headache disorders, in contrast to HIT-6 score (ictal burden). We conducted a school-based online survey of students’ parents in Tsubame City, Japan, in 2024. The questionnaire included age, sex, headache characteristics, MIBS-4 and HIT-6 scores, and overall work productivity impairment (OWPI) assessed using the WPAI questionnaire. A structural equation model (SEM) evaluated the effects of MIBS-4 and HIT-6 scores on OWPI. The headache diagnosis was solely based on the questionnaire. Among 5,227 households, 21.6% (1,127) responded, and 678 responses from parents with headache disorders were analyzed (median age 43, IQR: 39–47 years; 92.9% female). Median MIBS-4 score was 4 (2–6), HIT-6 score 58 (53–64), and OWPI 20.0% (10.0% – 30.0%). Only MIBS-4 score was significantly associated with OWPI (standardized coefficients [β] = 0.23, 95% CI: [0.14–0.33]), whereas HIT-6 score was not (β = 0.06, [-0.03–0.16]). Younger age, female sex, longer duration of headache, more monthly headache days, moderate/severe pain, presence of nausea/vomiting, photophobia, phonophobia, and osmophobia were directly associated with MIBS-4 score, and these clinical characteristics indirectly associated OWPI mediated via MIBS-4 score, whereas HIT-6 was not. OWPI was associated with MIBS-4 score (interictal burden), not HIT-6 score (ictal burden). Additionally, clinical characteristics are indirectly associated with OWPI mediated via MIBS-4 score. The stronger link between MIBS-4 and WPAI may be due to the persistent nature of psychological symptoms during the interictal period, which imposes a daily burden. The gender imbalance in this study may limit the generalizability of the findings to male workers, suggesting that workplace interventions may need to consider gender-specific burdens. Nevertheless, to improve OWPI, greater emphasis should be placed on addressing interictal burden.