Using the nationwide Danish Multiple Sclerosis Registry we aimed to describe the population with seondary progressive multiple sclerosis(SPMS) disease course.

A population-based characterization of persons with SPMS has been lacking.

Beside the description of the clinician defined SPMS population, we aimed to define a population not coded as SPMS but fulfilling the following criteria at index date 19. June, 2019: Expanded Disability Status Scale (EDSS) score of 3.0–6.5 and 6 month confirmed EDSS worsening ≥1 if EDSS <6 and ≥0,5 if EDSS  ≥6,0 within 2 years before index date.

Of the 14619 persons with MS, 9043 (61.9%) had RRMS, 2236 (15.3 %)  SPMS, 1251 (8.5 %) PPMS and 2089 (14.3 %) were unclassified.

The number of adults with SPMS was 2236 (66.4 % women). Of the population with relapsing MS 299 (71.5 % women) persons fulfilled the criteria for worsening (worsening cohort).

The median age of the SPMS population was 60.3 years (59.3 years for men and 60.8 years for women) and of the worsening cohort  48.2 years (47.6 years for men, 48.4 years for women). Median EDSS was 6.0 (SPMS) and 4.0 (worsening cohort).

The median disease duration at SPMS conversion was 15.5 years, while 11.2 years in the worsening group. Relapses were recorded within 2 years in 5.1 % of the SPMS group and in 30.4 % in the worsening cohort. The majority (70.0 %) with SPMS population did not received disease modifying therapy (DMT) at index date, 6.9 % were on interferons, 7.4 % on other moderate efficacy and 15.4 % on high efficacy DMT. The distribution in the worsening cohort was: 13.3 % on no DMT, 5.0 % on interferons, and 31.4 % on other moderate efficacy and 50.1 % on high-efficacy DMT.