FDG-PET and ASL MRI identify largely overlapping hypermetabolic and hyperperfusion changes in limbic autoimmune encephalitis.

BACKGROUND: Arterial spin labeling (ASL) MRI has been anecdotally used to assess brain perfusion in autoimmune encephalitis (AE) and its relationship with [18F]FDG-PET dysmetabolism has been scarcely investigated.Considering the physiological coupling of metabolism and perfusion, we aimed to evaluate the degree of correspondence between ASL-MRI and [18F]FDG-PET in AE. METHODS: A retrospective cohort of five patients underwent ASL-MRI and [18F]FDG-PET during the acute stage and at follow-up. We assessed the presence of regions with hypermetabolism on [18F]FDG-PET and hyperperfusion on ASL-MRI and evaluated concordance and spatial overlap of these metrics. Clinical assessment scale in AE and modified Rankin Scale were obtained at baseline and follow-up. RESULTS: In two patients [18F]FDG-PET and ASL-MRI were unremarkable; in three patients there were anatomically overlapping areas of hypermetabolism and hyperperfusion (average DICE similarity coefficient 0.358). Following immunotherapy, metabolic and perfusion changes consistently demonstrated a progressive normalization, aligning with clinical improvement. CONCLUSIONS: We identified suboptimal anatomical correspondence of abnormalities assessed with [18F]FDG-PET and ASL-MRI. Hyperperfusion and hypermetabolism might reflect differently AE-related pathophysiological correlates, but they both demonstrate ability to monitor disease activity. ASL-MRI is a promising marker of disease activity in AE and a favorable alternative to [18F]FDG-PET due to its cost-effectiveness, safety, and wide availability.

High-resolution ultrasound and superb microvascular imaging findings in a case of post-COVID-19 vaccine parsonage-turner syndrome with multiple nerves involvement.

Parsonage-Turner syndrome (PTS) is an idiopathic condition that may be triggered by vaccination against SARS-CoV-2. High-resolution ultrasound can support the diagnosis and monitoring of PTS patients by demonstrating specific nerve abnormalities. The recently implemented superb microvascular imaging technology can help stratify the prognosis of these patients, with the potential to contribute to the clinical management of PTS.

Post-SARS-CoV-2 infection and post-vaccine-related neurological complications share clinical features and the same positivity to anti-ACE2 antibodies.

Introduction: A potential overlap in symptoms between post-acute COVID-19 syndrome and post-COVID-19 vaccination syndrome has been noted. We report a paired description of patients presenting with similar manifestations involving the central (CNS) or peripheral nervous system (PNS) following SARS-CoV-2 infection or vaccination, suggesting that both may have triggered similar immune-mediated neurological disorders in the presence of anti-idiotype antibodies directed against the ACE2 protein. Patients and methods: Four patients exhibited overlapping neurological manifestations following SARS-CoV-2 infection or vaccination: radiculitis, Guillain-Barré syndrome, and MRI-negative myelitis, respectively, sharing positivity for anti-ACE2 antibodies. Autoantibodies against AQP-4, MOG, GlyR, GAD, and amphiphysin, onconeural antibodies for CNS syndromes, and anti-ganglioside antibodies for PNS syndromes tested negative in all patients. Discussion: Anti-idiotype antibodies against ACE2 have been detected in patients who recovered from COVID-19 infection, and it has been hypothesized that such antibodies may mediate adverse events following SARS-CoV-2 infection or vaccination, resulting in the activation of the immune system against cells expressing ACE2, such as neurons. Our data reveal clinically overlapping syndromes triggered by SARS-CoV-2 infection or vaccination, sharing positivity for anti-ACE2 antibodies. Their presence, in the absence of other classic autoimmune markers of CNS or PNS involvement, suggests that they might play an active role in the context of an aberrant immune response. Conclusion: Anti-idiotype antibodies directed against ACE2 may be triggered by both SARS-CoV-2 infection and vaccination, possibly contributing to neurological autoimmune manifestations. Their pathogenic role, however, remains to be demonstrated in large-scale, more structured studies.

Relapsing meningitis and limbic encephalitis in anti-AQP4-Ab-associated neuromyelitis optica spectrum disorder.

OBJECTIVES: neuromyelitis optica spectrum disorder (NMOSD) is a rare autoimmune disease mainly affecting optic nerves and the spinal cord. Due to the potentially irreversible tissue damage, prevention of relapses is of utmost importance. METHODS: We describe the atypical clinical course and pathology results of a patient with anti-aquaporin-4 antibody (anti-AQP4-Ab)-associated NMOSD who developed aseptic meningitis followed by limbic-encephalitis-like presentation with extensive brain lesions upon treatment with rituximab and tocilizumab. RESULTS: The patient developed subacute cognitive decline with magnetic resonance imaging (MRI) evidence of extensive brain white matter lesions. In the hypothesis of an opportunistic brain infection, she underwent brain biopsy of the temporal pole. Pathology results revealed typical NMOSD findings with complement activation, supporting the hypothesis of an atypical presentation of anti-AQP-Ab-associated NMOSD. Accordingly, treatment with the complement-targeting drug eculizumab was started, leading to a dramatic clinical and MRI improvement. DISCUSSION: aseptic meningitis and limbic encephalitis could represent a rare phenotype of anti-AQP4-Ab-associated NMOSD.

Brain lesion microstructure in neuromyelitis optica spectrum disorder and myelin oligodendrocyte glycoprotein disease.

BACKGROUND AND PURPOSE: Neuromyelitis optica spectrum disorder (NMOSD) and myelin oligodendrocyte glycoprotein antibody-associated disease (MOGAD) diagnosis are based on the presence of serological and magnetic resonance imaging (MRI) biomarkers. Diffusion tensor imaging (DTI), neurites orientation dispersion and density imaging (NODDI), and the Spherical Mean Technique (SMT) may be helpful to provide a microstructural characterization of the different types of white matter lesions and give an insight about their different pathological mechanisms. The aim of the study was to characterize microstructural differences between brain typical lesions (TLs) and nontypical lesions (nTLs). METHODS: A total of 17 NMOSD and MOGAD patients [9 Aquaporin4 (AQP4) + NMO, 2 seronegative-NMO, 6 MOGAD] underwent MRI scans on a 3 Tesla MAGNETON PRISMA. Diffusion parameters (fractional anisotropy; mean diffusivity [MD]; intracellular volume fraction [ICVF]; extra-neurite transverse diffusivity; and extra-neurite MD; neurite signal fraction) were obtained using DTI, NODDI, and SMT. Microstructural parameters within lesions were compared through a generalized linear model using age, sex, and total lesion volume as covariates. RESULTS: In NMOSD/MOGAD whole cohort (total lesions = 477), TLs showed increased MD and decreased ICVF compared to nTLs (p < .05), indicating higher inflammation and axonal loss. Similar results were found also in the AQP4 + NMO subgroup (decreased ICVF, p < .05). Furthermore, in NMOSD/MOGAD whole cohort and in AQP4 + NMO subgroup, TLs showed a trend toward higher EXRATRANS than nTLs, suggesting a more severe degree of demyelination within TLs. CONCLUSIONS: TLs and nTLs in NMOSD/MOGAD showed different diffusion MRI-derived microstructural features, with TLs showing a more severe degree of inflammation and fiber disruption with respect to nTLs.

Prospective open-label trial with rituximab in patients with chronic inflammatory demyelinating polyradiculoneuropathy not responding to conventional immune therapies.

BACKGROUND: To evaluate the efficacy of rituximab in chronic inflammatory demyelinating polyradiculoneuropathy (CIDP) patients not responding to conventional immune therapies. METHODS: An open-label, prospective exploratory study was conducted with intravenous rituximab on 17 CIDP patients who had not responded to at least two first-line therapies. The primary endpoint was to determine the proportion of patients who showed improvement 6 months after rituximab therapy. The percentage of responders to rituximab, along with a 95% CI, was reported and compared with the 30% response rate after other immunosuppressive drugs previously documented in the literature. RESULTS: 13 of the 17 treated patients (76.5%) showed improvement at 6 months (95% CI 50.1 to 93.2). Among the 14 patients who completed the 12-month follow-up (2 were lost to follow-up after showing improvement at months 8 and 10, and 1 deteriorated at 6 months), 13 (92.9%) demonstrated improvement at 12 months (95% CI 66.1 to 99.8). Nerve conduction parameters improved by at least 20% in two nerves in 6 out of 15 (40%) patients at 6 months and in 7 out of 13 (53.9%) at 12 months. None of the treated patients withdrew from the study due to side effects. There was a significant reduction of circulating CD19+ cells 15 days, 2, 6 and 12 months after treatment. CONCLUSION: Rituximab seems to be a safe therapy in most patients with CIDP not responding to conventional immune therapies. The high percentage of patients who improved in this study suggests a possible positive effect of rituximab which is worth investigating in future randomised controlled clinical trials. TRIAL REGISTRATION NUMBER: NCT05877040.

Autoimmune encephalitis: what the radiologist needs to know.

Autoimmune encephalitis is a relatively novel nosological entity characterized by an immune-mediated damage of the central nervous system. While originally described as a paraneoplastic inflammatory phenomenon affecting limbic structures, numerous instances of non-paraneoplastic pathogenesis, as well as extra-limbic involvement, have been characterized. Given the wide spectrum of insidious clinical presentations ranging from cognitive impairment to psychiatric symptoms or seizures, it is crucial to raise awareness about this disease category. In fact, an early diagnosis can be dramatically beneficial for the prognosis both to achieve an early therapeutic intervention and to detect a potential underlying malignancy. In this scenario, the radiologist can be the first to pose the hypothesis of autoimmune encephalitis and refer the patient to a comprehensive diagnostic work-up - including clinical, serological, and neurophysiological assessments.In this article, we illustrate the main radiological characteristics of autoimmune encephalitis and its subtypes, including the typical limbic presentation, the features of extra-limbic involvement, and also peculiar imaging findings. In addition, we review the most relevant alternative diagnoses that should be considered, ranging from other encephalitides to neoplasms, vascular conditions, and post-seizure alterations. Finally, we discuss the most appropriate imaging diagnostic work-up, also proposing a suggested MRI protocol.

Assessment of diagnostic criteria for multifocal motor neuropathy in patients included in the Italian database.

BACKGROUND AND PURPOSE: This study aimed to assess the diagnostic criteria, ancillary investigations and treatment response using real-life data in multifocal motor neuropathy (MMN) patients. METHODS: Clinical and laboratory data were collected from 110 patients enrolled in the Italian MMN database through a structured questionnaire. Twenty-six patients were excluded due to the unavailability of nerve conduction studies or the presence of clinical signs and symptoms and electrodiagnostic abnormalities inconsistent with the MMN diagnosis. Analyses were conducted on 73 patients with a confirmed MMN diagnosis and 11 patients who did not meet the diagnostic criteria. RESULTS: The European Federation of Neurological Societies/Peripheral Nerve Society (EFNS/PNS) diagnostic criteria were variably applied. AUTHOR: When applying the American Association of Electrodiagnostic Medicine criteria, an additional 17% of patients fulfilled the criteria for probable/definite diagnosis whilst a further 9.5% missed the diagnosis. In 17% of the patients only compound muscle action potential amplitude, but not area, was measured and subsequently recorded in the database by the treating physician. Additional investigations, including anti-GM1 immunoglobulin M antibodies, cerebrospinal fluid analysis, nerve ultrasound and magnetic resonance imaging, supported the diagnosis in 46%-83% of the patients. Anti-GM1 immunoglobulin M antibodies and nerve ultrasound demonstrated the highest sensitivity. Additional tests were frequently performed outside the EFNS/PNS guideline recommendations. CONCLUSIONS: This study provides insights into the real-world diagnostic and management strategies for MMN, highlighting the challenges in applying diagnostic criteria.

Impact of 2021 European Academy of Neurology/Peripheral Nerve Society diagnostic criteria on diagnosis and therapy of chronic inflammatory demyelinating polyradiculoneuropathy variants.

BACKGROUND AND PURPOSE: There are different criteria for the diagnosis of different variants of chronic inflammatory demyelinating polyradiculoneuropathy (CIDP). The 2021 European Academy of Neurology/Peripheral Nerve Society (EAN/PNS) guidelines provide specific clinical criteria for each CIDP variant even if their therapeutical impact has not been investigated. METHODS: We applied the clinical criteria for CIDP variants of the 2021 EAN/PNS guidelines to 369 patients included in the Italian CIDP database who fulfilled the 2021 EAN/PNS electrodiagnostic criteria for CIDP. RESULTS: According to the 2021 EAN/PNS clinical criteria, 245 patients achieved a clinical diagnosis of typical CIDP or CIDP variant (66%). We identified 106 patients with typical CIDP (29%), 62 distal CIDP (17%), 28 multifocal or focal CIDP (7%), four sensory CIDP (1%), 27 sensory-predominant CIDP (7%), 10 motor CIDP (3%), and eight motor-predominant CIDP (2%). Patients with multifocal, distal, and sensory CIDP had milder impairment and symptoms. Patients with multifocal CIDP had less frequently reduced conduction velocity and prolonged F-wave latency and had lower levels of cerebrospinal fluid protein. Patients with distal CIDP more frequently had reduced distal compound muscle action potentials. Patients with motor CIDP did not improve after steroid therapy, whereas those with motor-predominant CIDP did. None of the patients with sensory CIDP responded to steroids, whereas most of those with sensory-predominant CIDP did. CONCLUSIONS: The 2021 EAN/PNS criteria for CIDP allow a better characterization of CIDP variants, permitting their distinction from typical CIDP and more appropriate treatment for patients.

Patient With Rigidity and Fasciculations: Steroid-Responsive Presentation of Anti-IgLON5 Disease

An 82-year-old man presented with 2-year lasting widespread muscular fasciculations, cramps, and limb stiffness, with spontaneous movements in the right lower limb, unsteady gait (Video 1), and falls. Neurophysiologic studies disclosed signs of neuromuscular hyperexcitability. CSF analysis showed high tau protein concentration (543 pg/mL; reference values, <404) and unique-to-CSF oligoclonal bands. Serum and CSF anti-IgLON5 antibodies were positive (Figure 1). He carried the anti-IgLON5 disease-associated HLA-DRB1*10:01 allele.1 Brain MRI, thoracoabdominal CT, whole-body FDG-PET, and video-polysomnography were unremarkable. No sleep disturbances, bulbar symptoms, parkinsonism, or dementia were detected. Intravenous methylprednisolone (500 mg/d for 5 days), followed by oral benzodiazepines, prompted rapid functional recovery, with limb stiffness and gait improvement (Video 1), which persisted at 6-month follow-up. Anti-IgLON5 disease has progressive course and protean clinical presentations,2 representative, in our patient, for overlapping signs and symptoms of neuromuscular hyperexcitability and rigidity. Identification of rare phenotypes is important because prompt recognition and treatment can improve prognosis.

COVID-19-associated serum and cerebrospinal fluid cytokines in post- versus para-infectious SARS-CoV-2-related Guillain-Barré syndrome.

INTRODUCTION: Guillain-Barré syndrome associated with Coronavirus-2-related severe acute respiratory syndrome (COV-GBS) occurs as para- or post-infectious forms, depending on the timing of disease onset. In these two forms, we aimed to compare the cerebrospinal fluid (CSF) and serum proinflammatory cytokine profiles to evaluate differences that could possibly have co-pathogenic relevance. MATERIALS AND METHODS: We studied a retrospective cohort of 26 patients with either post-COV-GBS (n = 15), with disease onset occurring > 7 days after SARS-CoV-2 infection, or para-COV-GBS (n = 11), with disease onset 7 days or less. TNF-α, IL-6, and IL-8 were measured in the serum with SimplePlex™ Ella™ immunoassay. In addition to the para-/post-COV-GBS patients, serum levels of these cytokines were determined in those with non-COVID-associated-GBS (NC-GBS; n = 43), paucisymptomatic SARS-CoV-2 infection without GBS (COVID, n = 20), and in healthy volunteers (HV; n = 12). CSF cytokine levels were measured in patients with para-/post-COV-GBS, in those with NC-GBS (n = 29), or with Alzheimer's disease (AD; n = 24). RESULTS: Serum/CSF cytokine levels did not differ in para- vs post-COV-GBS. We found that SARS-CoV-2 infection raises the serum levels of TNF-α, IL-6, and IL-8, as well as an increase of IL-6 (in serum and CSF) and IL-8 (in CSF) in either NC-GBS or COV-GBS than controls. CSF and serum cytokine levels resulted independent one with another. CONCLUSIONS: The change of cytokines linked to SARS-CoV-2 in COV-GBS appears to be driven by viral infection, although it has unique characteristics in GBS as such and does not account for cases with para- or post-infectious onset.

Incident anti-LGI1 autoimmune encephalitis during dementia with Lewy bodies: when Occam razor is a double-edged sword.

In Dementia with Lewy bodies (DLB), rapid cognitive decline and seizures seldom complicate the typical clinical course. Nevertheless, concurrent, treatable conditions may be responsible. We report a case of DLB with superimposed anti-LGI1 encephalitis, emphasizing the importance of thorough diagnostic reasoning beyond the simplest explanation amid distinct clinical cues.

An aggressive form of MOGAD treated with aHSCT: A case report.

BACKGROUND: Although myelin-oligodendrocyte-glycoprotein (MOG)-antibody-associated disease (MOGAD) has been considered a more favorable demyelinating central nervous system disorder, recent data evidence that some patients might experience severe relapses and high disability. Actual treatment-options are acquired mostly from anti-aquaporin-4-antibody-positive neuromyelitis optica spectrum disorder and rely on clinical experience. Therefore, treatment of aggressive forms of MOGAD can be challenging. OBJECTIVES AND METHODS: To describe a patient with an aggressive MOGAD treated with autologous hematopoietic stem cell transplantation (aHSCT). RESULTS: A 56-year-old man was diagnosed with MOGAD in 2017 because of right optic-neuritis and anti-MOG-antibody positivity. In the following 2 years, he experienced two optic neuritis with good recovery after high-dose steroid. At the end of 2019, he presented sensory and motor impairment at lower limbs with evidence of several spinal, longitudinally extended, tumefactive inflammatory lesions. Despite sequential treatment with rituximab and tocilizumab alongside high-dose steroid, intravenous immunoglobulins and plasma-exchange, he experienced several clinical relapses and exhibited persistent magnetic resonance activity. He was finally addressed to intense immunosuppression with myeloablative conditioning regimen followed by autologous hematopoietic stem cell transplantation (aHSCT). After 2 years follow-up, he is free from disease-activity. CONCLUSIONS: In a patient affected by aggressive, treatment-refractory MOGAD, aHSCT resulted as safe and was able to suppress disease-activity for over 2 years.

High-resolution ultrasound and magnetic resonance imaging of ulnar nerve neuropathy in the distal Guyon tunnel.

OBJECTIVE: The aim of the present study is to describe the ultrasound (US) and magnetic resonance imaging (MRI) findings in patients with neuropathies affecting the deep (DB) and superficial (SB) branches of the Ulnar nerve (UN) and to investigate the potential role of imaging modalities in the diagnostic workup of these conditions. MATERIALS AND METHODS: We screened our institutional imaging database to identify patients with a diagnosis of UN mononeuropathy, and among them, we reviewed the cases where US disclosed pathological findings affecting the UN terminal divisions. In this latter subgroup, we retrieved available data on MRI and electrodiagnostic tests performed by the patients during the diagnostic workup. All the patients were evaluated with US machines equipped with 17-5-MHz, 18-4-MHz, 24-8-MHz, or 22-8-MHz probes. MRI exams were performed on a 3-T unit equipped with a 64-channel head RF coil. RESULTS: Among 166 patients with UN mononeuropathy, we retrieved 15 patients (9%) for which US detected pathological findings affecting the UN terminal divisions, consisting of 7 cases of DB neuropathy, 4 cases of SB neuropathy, and 4 cases of combined neuropathy involving both nerves. Seven (46.7%) patients were submitted to MRI to integrate US findings. Among patients with SB and DB neuropathies, imaging allowed the identification of 7 traumatic nerve injuries, 2 nerve tumors, and 6 entrapment neuropathies, including 4 cases of nerve compression by a ganglion cyst. CONCLUSION: High-resolution US and MRI are accurate modalities for the investigation of patients with SB/DB neuropathy, can provide critical information on the cause of nerve damage, and guide therapeutic decisions. CRITICAL RELEVANCE STATEMENT: High-resolution US and MRI are accurate modalities for the investigation of patients with superficial/deep branch of the ulnar nerve neuropathy. In the proper setting, US may be regarded as a first-line approach in patients with suspected neuropathies affecting these small branches. KEY POINTS: • Neuropathies affecting the distal ulnar nerve often require multimodal investigations. • US and MRI can provide detailed morphological information about the terminal branches of the ulnar nerve. • US may be considered as a first-line approach in suspected distal ulnar nerve neuropathies.

Asynchronous combined central and peripheral demyelination (CCPD) in a girl with anti-MOG positivity: A case report and review of the literature.

The occurrence of combined central and peripheral demyelination (CCPD) is rare, data are limited to small case and cohort studies, mainly concerning adults. In few patients positivity to anti MOG antibody is reported, thus widening the spectrum of anti-MOG associated disorders (MOGAD). We describe a 7-year-old girl with optic neuritis followed 8 years later by peripheral demyelination, with fluctuating anti-MOG antibody positivity at cell-based assay. From the review of the literature, MOGAD-CCPD appear very rare in childhood, especially with asynchronous course. Clinicians should keep this possibility in mind to better define diagnosis in atypical demyelination syndromes, with therapeutical implications.

Comparing the Impact of COVID-19 on Vaccinated and Unvaccinated Patients Affected by Myasthenia Gravis.

We evaluated 13 patients affected by myasthenia gravis (MG) who had coronavirus disease 2019 (COVID-19) before vaccination and 14 myasthenic patients who contracted severe acute respiratory syndrome-coronavirus-2 (SARS-CoV-2) infection after vaccination to evaluate factors related to different COVID-19 outcomes. We compared the two groups' previous stability of MG and the severity of SARS-CoV-2 infection. Vaccinated and non-vaccinated patients were comparable in terms of severity of the previous MG course (mean maximum myasthenia gravis Foundation of America-MGFA-Class III) and during SARS-CoV-2 infection (mean MGFA Class II). In non-vaccinated patients, the hospitalization and severe course percentages were 61.5%, while the mortality reached 30.8%. The hospitalization, severe course, and mortality percentages in vaccinated patients were 7.1%. In deceased, non-vaccinated patients, greater myasthenia severity in the past clinical history, but not at the time of infection, was observed. Similarly, older age at MG onset and at the time of infection correlated with a more severe COVID-19 course in non-vaccinated patients (p = 0.03 and p = 0.04), but not in the group of vaccinated patients. In summary, our data support a protective role of vaccination in myasthenic patients, even if anti-CD20 therapy might be associated with a poor immune response to vaccines.

CSF Findings in Relation to Clinical Characteristics, Subtype, and Disease Course in Patients With Guillain-Barré Syndrome.

BACKGROUND AND OBJECTIVES: To investigate CSF findings in relation to clinical and electrodiagnostic subtypes, severity, and outcome of Guillain-Barré syndrome (GBS) based on 1,500 patients in the International GBS Outcome Study. METHODS: Albuminocytologic dissociation (ACD) was defined as an increased protein level (>0.45 g/L) in the absence of elevated white cell count (<50 cells/µL). We excluded 124 (8%) patients because of other diagnoses, protocol violation, or insufficient data. The CSF was examined in 1,231 patients (89%). RESULTS: In 846 (70%) patients, CSF examination showed ACD, which increased with time from weakness onset: ≤4 days 57%, >4 days 84%. High CSF protein levels were associated with a demyelinating subtype, proximal or global muscle weakness, and a reduced likelihood of being able to run at week 2 (odds ratio [OR] 0.42, 95% CI 0.25-0.70; p = 0.001) and week 4 (OR 0.44, 95% CI 0.27-0.72; p = 0.001). Patients with the Miller Fisher syndrome, distal predominant weakness, and normal or equivocal nerve conduction studies were more likely to have lower CSF protein levels. CSF cell count was <5 cells/µL in 1,005 patients (83%), 5-49 cells/µL in 200 patients (16%), and ≥50 cells/µL in 13 patients (1%). DISCUSSION: ACD is a common finding in GBS, but normal protein levels do not exclude this diagnosis. High CSF protein level is associated with an early severe disease course and a demyelinating subtype. Elevated CSF cell count, rarely ≥50 cells/µL, is compatible with GBS after a thorough exclusion of alternative diagnoses. CLASSIFICATION OF EVIDENCE: This study provides Class IV evidence that CSF ACD (defined by the Brighton Collaboration) is common in patients with GBS.

Unclassified clinical presentations of chronic inflammatory demyelinating polyradiculoneuropathy.

BACKGROUND: To assess the ability of the 2021 European Academy of Neurology/Peripheral Nerve Society (EAN/PNS) clinical criteria for chronic inflammatory demyelinating polyradiculoneuropathy (CIDP) to include within their classification the whole spectrum of clinical heterogeneity of the disease and to define the clinical characteristics of the unclassifiable clinical forms. METHODS: The 2021 EAN/PNS clinical criteria for CIDP were applied to 329 patients fulfilling the electrodiagnostic (and in some cases also the supportive) criteria for the diagnosis of CIDP. Clinical characteristics were reviewed for each patient not strictly fulfilling the clinical criteria ('unclassifiable'). RESULTS: At study inclusion, 124 (37.5%) patients had an unclassifiable clinical presentation, including 110 (89%) with a typical CIDP-like clinical phenotype in whom some segments of the four limbs were unaffected by weakness ('incomplete typical CIDP'), 10 (8%) with a mild distal, symmetric, sensory or sensorimotor polyneuropathy confined to the lower limbs with cranial nerve involvement ('cranial nerve predominant CIDP') and 4 (1%) with a symmetric sensorimotor polyneuropathy limited to the proximal and distal areas of the lower limbs ('paraparetic CIDP'). Eighty-one (65%) patients maintained an unclassifiable presentation during the entire disease follow-up while 13 patients progressed to typical CIDP. Patients with the unclassifiable clinical forms compared with patients with typical CIDP had a milder form of CIDP, while there was no difference in the distribution patterns of demyelination. CONCLUSIONS: A proportion of patients with CIDP do not strictly fulfil the 2021 EAN/PNS clinical criteria for diagnosis. These unclassifiable clinical phenotypes may pose diagnostic challenges and thus deserve more attention in clinical practice and research.