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Spinal vascular malformations are rare causes of progressive myelopathy, often leading to misdiagnosis as inflammatory disorders like neuromyelitis optica spectrum disorder (NMOSD). We present the case of a man in his late 30s who developed a 9-month history of progressive low back pain, ascending paraesthesia and worsening paraparesis. Initial MRI showed longitudinally extensive transverse myelitis (LETM), and he was treated for seronegative NMOSD with steroids and plasmapheresis despite negative AQP4-IgG and MOG-IgG. As his condition didn't improve, a spinal dural arteriovenous fistula (SDAVF) was suspected and confirmed via digital subtraction angiography (DSA). Following successful endovascular embolisation, the patient's motor strength improved substantially and he regained the ability to ambulate with a cane. This case highlights that while double-negative (DN) NMOSD is a challenging clinical syndrome, the identification of mimics like SDAVF is of higher diagnostic priority to prevent the morbidity associated with inappropriate long-term immunosuppression.