The Aim of the study is to improve neurological impairment and function in patients with early diagnosis of various PNS associated with well-characterized onconeural antibodies anti-Hu, anti-Yo, anti-CV2/CRMP5. The primary endpoint of the study is the percentage of patients with neurological improvement after 3 months of immunotherapy with IVIg. "Success" is defined by ≥ 1 point lower score in the modified Rankin Scale (mRS) after treatment compared to baseline. This is a prospective, open-label trial explores the efficacy of drugs approved for treatment of immune-mediated neurological disorders given early in the evolution of PNS associated with well-characterized onconeural antibodies. A total number of 17 patients will be treated. This is a multicenter study carried under the auspices of the French National Reference Center on PNS.