Severe Anti-CV2/CRMP5, Anti-Hu, and Anti-SOX1 Antibody-Positive Paraneoplastic Neurological Syndrome Associated With Tumor Recurrence During Atezolizumab Therapy

We report a case of an 80-year-old man with small-cell lung cancer who developed paraneoplastic neurological syndrome (PNS) after four years of immune checkpoint inhibitor (ICI) therapy (atezolizumab). The patient presented with bilateral optic neuropathy, parkinsonism, and truncal ataxia. MRI revealed lesions in the basal ganglia and medial temporal lobes. Serological testing revealed a marked elevation of tumor markers, along with the presence of anti-CV2/CRMP5, anti-Hu, and anti-SOX1 antibodies. PNS onset may be primarily triggered by tumor recurrence, with long-term ICI exposure further amplifying the underlying immune response.