Case Report: Neuromyelitis optica spectrum disorder associated with anti-argonaute antibodies presenting with subacute combined degeneration-like features.

Aquaporin-4 antibodies (AQP4-Abs) are a key diagnostic biomarker of neuromyelitis optica spectrum disorder (NMOSD), and anti-Argonaute antibodies (AGO-Abs) have recently been reported in a range of autoimmune neurological conditions, although their clinical significance remains undetermined. In this report, we describe a 36-year-old woman who initially presented with sensory disturbances and mild vitamin B12 deficiency who was initially diagnosed with subacute combined degeneration. Vitamin supplementation partially improved her symptoms but her condition subsequently deteriorated, and she developed paraparesis, ascending sensory loss, and urinary incontinence. Magnetic resonance imaging revealed longitudinally extensive transverse myelitis with gadolinium enhancement. Serum and cerebrospinal fluid analyses were positive for AQP4-Abs and AGO-Abs, supporting the diagnosis of NMOSD with concomitant presence of AGO antibodies. Electrophysiological studies showed asymmetric axonal sensorimotor polyneuropathy, suggesting peripheral nervous system involvement contributing to early diagnostic uncertainty. The patient received immunotherapy including high-dose corticosteroids, intravenous immunoglobulin, and plasma exchange, after which marked neurological improvement was observed. Longitudinal multimodal assessments incorporating antibody titers, expanded disability status scale scores, somatosensory-evoked potentials, and neuroimaging were performed during follow-up. This case highlights the heterogeneous clinical presentation of NMOSD, the potential coexistence of AGO antibodies, and the possibility that serological changes may not parallel clinical recovery. Comprehensive antibody evaluation and multimodal monitoring may assist clinical decision-making in atypical neuroimmunological presentations.