Paraneoplastic Neurological Syndromes are rare autoimmune complications linked to the presence of systemic cancer. Despite their autoimmune origin, they have historically shown little response to immunotherapy. The reason for this failure is probably related to too late administration of immunotherapy, at a stage where the inflammation has already disappeared and irreversible neuronal loss has occurred. The protocol focuses on patients with anti-Hu antibody sensory neuronopathy. This single arm trial consists of an early immunotherapy combining Intravenous Immunoglobulin (IVIG) for 3 months, cyclophosphamide and methylprednisolone for 6 months, at the rate of 1 cycle per month. The percentage of patients with clinical improvement will be evaluated (ONLS) at 3 months. The tolerance of the treatment will also be evaluated as well as other functional scales at 3 and 6 months.

Inclusion Criteria:

• Age ≥ 18 years old
• "Possible" sensory neuronopathy according to the criteria of Camdessanché et al. [2] with ONLS score ≥ 1
• Dominant picture of sensory ataxia (damage to the central nervous system and/or the neuromuscular junction is allowed, provided that it has a minor impact on the patient's disability)
• Positive anti-Hu antibodies in blood and/or cerebrospinal fluid
• Outpatient (modified Rankin Score (mRS) 2 or 3)
• Onset of neurological symptoms less than 3 months ago
• Free, informed, written and signed consent
• Affiliation to a social security or beneficiary scheme (except AME)

Exclusion Criteria:

• Known hypersensitivity to one of the treatments under study, to their metabolites, or to one of the excipients
• Absolute contraindications to IVIg: selective IgA deficiency, known thrombophilia, patients suffering from type I or II hyperprolinemia, hypersensitivity to human immunoglobulins
• Absolute contraindications to cyclophosphamide: vaccination against yellow fever in the 3 months preceding inclusion, acute urinary infection, pre-existing hemorrhagic cystitis, urinary tract obstruction, acute bone marrow failure
• Contraindication to methylprednisolone: live vaccines, or live attenuated vaccines within 3 months, infectious state or evolving virus (hepatitis, herpes, chickenpox, shingles)
• More than two courses of IVIg administered within 3 months before recruitment
• Other concomitant immunotherapy
• Other cause of immunosuppression (acquired or congenital)
• Treatment with checkpoint inhibitors in progress or completed less than 3 months previously
• Woman or man without effective contraception
• Pregnant or breastfeeding woman
• History of psychiatric or general illnesses that may contraindicate treatment
• Patients unable to complete the follow-up required by the study
• Patients under guardianship or curatorship
• Patient deprived of liberty by a judicial or administrative decision

The procedures carried out with the French data privacy authority (CNIL, Commission nationale de l'informatique et des libertés) do not provide for the transmission of the database, nor do the information and consent documents signed by the patients. Consultation by the editorial board or interested researchers of individual participant data that underlie the results reported in the article after deidentification may nevertheless be considered, subject to prior determination of the terms and conditions of such consultation and in respect for compliance with the applicable regulations.

Beginning 3 months and ending 3 years following article publication. Requests out of these time frame can also be submitted to the sponsor

Researchers who provide a methodologically sound proposal.