Update on pathology of central nervous system inflammatory demyelinating diseases.

Multiple sclerosis (MS) is by far the most common central nervous system inflammatory demyelinating disease (CNS-IDD). It is diagnosed according to detailed criteria based on clinical definitions, magnetic resonance imaging (MRI) and cerebrospinal fluid findings. However, in rare instances, atypical syndromes associated with CNS demyelination, such as unusual MRI findings or poor response to standard treatment, may eventually necessitate a CNS biopsy with neuropathological examination. Pathology remains the gold standard in the differentiation of atypical CNS-IDDs, the recognition of which is essential for establishing the correct prognosis and optimal therapy. However, one must bear in mind that between different CNS-IDDs there are still overlapping features, even in the pathology. In this review, we compare and highlight contrasts within a spectrum of CNS-IDDs from the neuropathological perspective. We characterise pathological hallmarks of active vs chronic multiple sclerosis. Also, we define differences in the pathology of MS, acute disseminated encephalomyelitis (ADEM), aquaporin 4-IgG positive (AQP4-IgG+) neuromyelitis optica spectrum disorder (NMOsd), and myelin oligodendrocyte glycoprotein antibody-associated disease (MOGAD). Detailed description of the particular CNS-IDD pathology is crucial on an individual patient level (when clinically justified in atypical cases) but also from a broader perspective i.e. to advance our understanding of the complex disease mechanisms. Recent immunobiological and pathological discoveries have led to the description of novel inflammatory CNS disorders that were previously classified as rare MS variants, such as NMOsd and MOGAD. Multiple sclerosis remains an umbrella diagnosis, as there is profound heterogeneity between patients. Advances in neuropathology research are likely to disentangle and define further CNS-IDDs that used to be categorised as multiple sclerosis.