Clinical and therapeutic challenges of smouldering multiple sclerosis.

Introduction: Assessment of the clinical course, neuroimaging and histopathological changes suggests that multiple sclerosis (MS) should not be defined merely as a focal inflammatory disease of the central nervous system (CNS) because the essence of the disease is due to a diffuse, 'smouldering', pathophysiological process.

State of the art: Progression independent of relapse activity (PIRA) is the clinical indicator of smouldering MS. Multiple pathomechanical factors determining smouldering MS have been identified, i.e. continuous activation of microglia, which is the source of smouldering inflammation and the failure of remyelination in MS.

Clinical implications: Our paper presents new neuroimaging markers, including paramagnetic rim lesions (PRLs) and slowly expanding lesions (SELs), potential methods for clinical evaluation and promising therapeutic options, i.e. Bruton's tyrosine kinase inhibitors that prevent PIRA in smouldering MS. With the duration of MS, the efficacy of the current immunomodulatory treatment is reduced, and its effect is insufficient to control smouldering MS.

Future directions: Innovative insights into the pathophysiology and clinical course warrant the need for a holistic approach to MS. The efforts of clinicians should be aimed at indicating subtle neurological deficits in physical performance and cognitive functioning to characterise the disease progression in its early stages. Undoubtedly, a new era for MS is coming in which new resonance markers will be used together with clinical methods to assess smouldering MS, and the treatment will include combination therapy with consideration of drugs that reduce relapse rates and therapy aimed at inhibiting disease progression.