Introduction. There is no single, commonly accepted, standard definition of secondary progressive multiple sclerosis (SPMS), an absence that poses a challenge for clinicians.

State of the art. SPMS is characterised by inflammation, neurodegeneration and disease progression with the presence or absence of relapses. No biochemical or radiological biomarkers are currently available to indicate the precise secondary progressive course in individual patients. The retrospective approach to identifying SPMS patients raises many difficulties, especially in terms of determining the time point of progression. Currently, the most precise diagnosis of SPMS is based on the definition proposed by Lorscheider et al., where SPMS is defined as a disability progression by 1 step on the Expanded Disability Status Scale (EDSS) in patients with EDSS ≤ 5.5 or of 0.5 EDSS steps in patients with EDSS ≥ 6 in the absence of a relapse, a minimum EDSS score of 4 and pyramidal functional system (FS) score of 2, and confirmed progression over ≥ 3 months, including confirmation within the leading FS.

Clinical implications. The need to establish criteria for the diagnosis of SPMS is currently of crucial importance due to emerging treatment opportunities including siponimod, a sphingosine 1-phosphate (S1P) receptor modulator selective for S1P1 and S1P5 receptors. It is reasonable to introduce drugs at the earliest possible stage of lesion progression to reduce inflammation a nd t o p rotect t he c entral n ervous s ystem ( CNS) a gainst i rreversible n eurodegeneration.

Future directions. Further studies with prospective, multicentre and long term follow-up design are needed to provide better insights into SP course in MS patients. This should be supported by radiological, biochemical and pathological evaluations to help establish reliable and sensitive biomarkers to guide clinical practice.