Relapse incidence in women and men throughout the course of multiple sclerosis: An MSBase cohort study

Introduction: Only one large retrospective cohort study and several smaller analyses examined predictors of relapse incidence in MS. Sex, age and MS duration were suggested as determinants of relapse activity. While in relapsing-remitting MS women are overrepresented in the ratio of 3:1 to men, in primary progressive disease both sexes are represented equally. A lower probability of relapse in men could be the reason for this change, with primary progressive (PP) MS representing the “extreme” of low relapse activity. Aims: To evaluate effect of sex on the incidence of MS relapses. To assess the hypothesis that the female-to-male ratio increases gradually with relapse activity and that PPMS represents a non-relapsing extreme along this continuum. To directly compare effects of age and MS duration on relapse incidence. Methods: Annualised relapse rates were calculated using the MSBase registry. Patients with incomplete data or less than one year of follow-up were excluded. Patients with PPMS were only included in the sex ratio analysis. Relapse incidences over 40 years of MS duration or up to 70 years of age were compared between females and males using Andersen-Gill and Poisson models. Female-to-male ratios stratified by annual relapse count were evaluated across disease duration and patient age and compared between relapse-onset and PPMS. All models were adjusted for therapy and pregnancy. Results: Among 11,570 eligible patients with relapse-onset MS (82,552 patient-years), 48,362 relapses were recorded. Relapse frequency was 17.7% higher in females compared to males. Within the initial five years, the female-to-male ratio increased from 2.3:1 to 3.3:1 in patients with 0 to >=4 relapses per year, respectively. The magnitude of this sex effect increased at longer MS duration and older age. However, the female-to-male ratio in patients with relapse-onset MS and zero relapses in any given year was double that of the patients with PPMS. Patient age was a more important determinant of decline in relapse incidence than disease duration. Conclusions: Females are predisposed to higher relapse activity than males. However, this sex-related effect does not explain the markedly lower female-to-male ratio in PPMS. Decline in relapse activity over time is more closely related to patient age than MS duration. This information helps us better understand the effects of sex and time on relapse incidence and define PPMS as an entity distinct from the relapse-onset MS.