AQP4-IgG-positive myelitis occurring in a patient with small cell lung cancer: a case report.

Background: Acute myelopathy in patients with small cell lung cancer (SCLC) is often attributed to metastatic spinal cord involvement. However, non-compressive inflammatory conditions, particularly aquaporin-4 immunoglobulin G (AQP4-IgG)-positive neuromyelitis optica spectrum disorder (NMOSD), can produce similar clinical and radiologic features, creating substantial diagnostic uncertainty. Case presentation: A male aged 65 years with extensive-stage SCLC presented with acute urinary retention followed by rapidly progressive paraplegia. Spinal magnetic resonance imaging (MRI) revealed longitudinally extensive intramedullary T2-hyperintense lesions affecting the cervical and thoracic spinal cord, with patchy contrast enhancement and no evidence of vertebral destruction, epidural mass, or spinal cord compression. Cerebrospinal fluid (CSF) examination demonstrated inflammatory abnormalities without malignant cells, and infectious causes were excluded. Serum and CSF testing were strongly positive for AQP4-IgG, establishing a diagnosis of AQP4-IgG-positive NMOSD. The patient received high-dose intravenous methylprednisolone followed by adjunctive intravenous immunoglobulin (IVIG). Plasma exchange was not performed because of advanced malignancy and concerns regarding procedural risks. Satralizumab was initiated during corticosteroid tapering for maintenance immunotherapy. Neurological function subsequently stabilized, and partial recovery of independence in activities of daily living was achieved. Conclusions: This case highlights that AQP4-IgG-positive myelitis can occur in patients with SCLC. In patients presenting with non-compressive longitudinally extensive transverse myelitis (LETM), particularly when imaging shows no evidence of compression and CSF cytology is negative for malignant cells, prompt AQP4-IgG testing is critical to prevent misdiagnosis and enable early initiation of appropriate immunotherapy.