Application of definitions for conversion to secondary progressive MS in a Danish nationwide population.

Background: The number of patients with relapsing remitting multiple sclerosis (RRMS) who convert to secondary progressive (SP) MS is uncertain, and with emerging treatment options for SPMS, it is important to identify RRMS patients in transition to the SP phase. The objective of the present study was to characterize clinical parameters and use of disease modifying therapies in patients diagnosed with SPMS and RRMS patients already entered the SP phase by use of the Danish Multiple Sclerosis Registry (DMSR).

Methods: We used a cross-sectional design, including all living patients with MS as of June 30, 2020 from DMSR. First, we applied the MSBase definition of SPMS on all RRMS patients. Second, we applied the slightly modified inclusion criteria from the EXPAND clinical trial on patients with clinically confirmed SPMS and patients with RRMS fulfilling the MSBase definition of SPMS to identify SPMS patients recently progressed who may benefit from treatment with disease modifying therapy. We compared clinical characteristics and disease-modifying therapy use in the different patient groups.

Results: Among patients with clinically confirmed SPMS, application of a slightly modified EXPAND trial inclusion criteria for SPMS (m-EXPAND) captured patients who had converted to SPMS more recently and who had relapsed and initiated high-efficacy treatment more frequently. Moreover, our RRMS patients fulfilling the "SPMS"-criteria according to MSBase and recently progression according to m-EXPAND had similar characteristics and remarkably resembled the SPMS population in the EXPAND trial.

Conclusion: Our results indicate that data-driven diagnostic definitions might help identify RRMS patients at risk for SPMS and we highlight the challenges and reluctance in diagnosing SPMS in clinical practice.