AbstractBackground and purposeIn addition to combined central and peripheral demyelination, other immune diseases could involve both the central nervous system (CNS) and peripheral nervous system (PNS).MethodsTo identify immune‐mediated diseases responsible for symptomatic combined central/peripheral nervous system involvement (ICCPs), we conducted a multicentric retrospective study and assessed clinical, electrophysiological, and radiological features of patients fulfilling our ICCP criteria.ResultsThirty patients (20 males) were included and followed during a median of 79.5 months (interquartile range [IQR] = 43–145). The median age at onset was 51.5 years (IQR = 39–58). Patients were assigned to one of four groups: (i) monophasic disease with concomitant CNS/PNS involvement including anti‐GQ1b syndrome (acute polyradiculoneuropathy + rhombencephalitis,n = 2), checkpoint inhibitor‐related toxicities (acute polyradiculoneuropathy + encephalitis,n = 3), and anti‐glial fibrillary acidic protein astrocytopathy (subacute polyradiculoneuropathy and meningoencephalomyelitis with linear gadolinium enhancements,n = 2); (ii) chronic course with concomitant CNS/PNS involvement including paraneoplastic syndromes (ganglionopathy/peripheral hyperexcitability + limbic encephalitis,n = 4); (iii) chronic course with sequential CNS/PNS involvement including POEMS (polyneuropathy, organomegaly, endocrinopathy, monoclonal protein, skin changes) syndrome (polyradiculoneuropathy + strokes,n = 2), histiocytosis (polyradiculoneuropathy + lepto‐/pachymeningitis,n = 1), and systemic vasculitis (multineuropathy + CNS vasculitis/pachymeningitis,n = 2); and (iv) chronic course with concomitant or sequential CNS/PNS involvement including combined central and peripheral demyelination (polyradiculoneuropathy + CNS demyelinating lesions,n = 10) and connective tissue diseases (ganglionopathy/radiculopathy/multineuropathy + limbic encephalitis/transverse myelitis/stroke,n = 4).ConclusionsWe diagnosed nine ICCPs. The timing of central and peripheral manifestations and the disease course help determine the underlying immune disease. When antibody against neuroglial antigen is identified, CNS and PNS involvement is systematically concomitant, suggesting a common CNS/PNS antigen and a simultaneous disruption of blood–nerve and blood–brain barriers.