Objective To explore the occurrence and characteristics of aggressive multiple sclerosis (AMS) in adult-onset multiple sclerosis (MS) patients.

Methods Prospectively collected data (1980–2009) from British Columbia, Canada, were retrospectively analysed. AMS was defined in three different ways (AMS1, 2 and 3): ‘AMS1’—confirmed Expanded Disability Status Scale (EDSS) ≥6 within 5 years of MS onset; ‘AMS2’—confirmed EDSS ≥6 by age 40; and ‘AMS3’—secondary progressive MS within 3 years of a relapsing-onset course. Three respective ‘non-aggressive’ MS comparison cohorts were selected. Patients’ characteristics were compared between aggressive and non-aggressive cohorts using multivariable logistic regression, with findings expressed as adjusted OR (AOR) and 95% CI.

Results Application of the three definitions to the source population of 5891 patients resulted in 235/4285 (5.5%) patients fulfilling criteria for AMS1 (59.6% were female; 74.5% had relapsing-onset MS), 388/2762 (14.0%) for AMS2 (65.2% were female; 92.8% had relapsing-onset MS) and 195/4918 (4.0%) patients for AMS3 (61.0% were female). Compared to the respective control cohorts, those with AMS were more likely to be male (AOR=1.5, 95% CI 1.1 to 2.0 (AMS1); 1.6, 95% CI 1.3 to 2.1 (AMS2); 1.8, 95% CI 1.3 to 2.4 (AMS3)), older at MS symptom onset (AOR=1.1; 95% CI 1.1 to 1.1 (AMS1 and AMS3)) and have primary progressive MS (AOR=2.3, 95% CI 1.6 to 3.3 (AMS1); 2.7, 95% CI 1.7 to 4.4 (AMS2)).

Conclusions AMS was identified in 4–14% of patients, depending on the definition used. Although there was a relative preponderance of men and primary progressive MS presenting with AMS, the majority of patients were still women and those with relapsing-onset MS.