The conventional classification of multiple sclerosis (MS) into relapsing–remitting, secondary progressive, and primary progressive phenotypes has long guided diagnosis, prognosis, and therapeutic decision-making. However, accumulating evidence indicates that disability accumulation frequently occurs independently of clinical relapses, challenging relapse-centric and phenotype-based models of disease evolution. The concept of progression independent of relapse activity (PIRA) has emerged as a clinically relevant framework capturing this phenomenon across MS phenotypes. In this state-of-the-art narrative review, we propose a spectrum-based reinterpretation of MS, integrating PIRA with concepts of smouldering-associated worsening and neurologic reserve. We highlight the heterogeneity of relapse-independent worsening, distinguishing transient from persistent PIRA, and discuss how ageing-related decline in compensatory capacity contributes to the clinical unmasking of progression over time. Within this framework, secondary progressive MS is redefined as the clinically recognizable accumulation of persistent relapse-independent worsening, while primary progressive MS is conceptualized as early predominance of clinically manifest progression due to limited reserve rather than a distinct disease entity. Finally, we examine diagnostic and therapeutic implications of a spectrum-based model in the contemporary era, emphasizing the limitations of relapse-centric treatment strategies and unmet needs in addressing progression-related biology. By reframing MS as a dynamic continuum shaped by the interaction between ongoing pathology and evolving neurologic reserve, this review aims to support earlier recognition of clinically meaningful progression and to inform more biology-aware approaches to disease monitoring and therapy.
Vasilev et al. (2026) studied this question.