A rare presentation of CASPR2-associated Morvan syndrome overlapping with GM1-positive AMSAN: a case report.

Background: Morvan syndrome is a rare autoimmune disorder characterized by peripheral nerve hyperexcitability with autonomic and central nervous system involvement, most commonly associated with antibodies against contactin-associated protein-like 2 (CASPR2). Acute motor and sensory axonal neuropathy (AMSAN) is an axonal variant of Guillain-Barré syndrome linked to anti-ganglioside antibodies and often manifests as severe limb weakness. Their concurrent presentation is unusual and raises the possibility of shared immune targets within peripheral nerve microdomains. Case presentation: A 70-year-old man presented with a relapsing course of progressive lower-limb weakness accompanied by widespread muscle twitching, severe insomnia with nocturnal hyperarousal, and refractory constipation. He had a prior episode diagnosed as AMSAN that improved after immunotherapy but relapsed four months after treatment was discontinued. Neurological examination demonstrated bilateral lower-limb weakness with reduced tendon reflexes. Moreover, electrophysiological studies confirmed diffuse multifocal peripheral nerve injury with superimposed peripheral nerve hyperexcitability. In addition, immunologic testing revealed serum anti-GM1 antibodies and anti-CASPR2 IgG in both serum and cerebrospinal fluid. Collectively, these findings supported a diagnosis of recurrent AMSAN coexisting with CASPR2-associated Morvan syndrome. Combined immunotherapy with corticosteroids and intravenous immunoglobulin, alongside symptomatic management, resulted in marked clinical improvement. Conclusion: This case report describes a rare overlap of relapsing AMSAN and Morvan syndrome. This antibody-defined coexistence is hypothesis-generating and may reflect synergistic immune injury involving nodal and paranodal regions. This case underscores the importance of recognizing overlapping phenotypes to guide diagnostic profiling and immunomodulatory therapy.