Background: Neuromyelitis Optica Spectrum Disorder (NMOSD) is an autoimmune neurological disorder that is characterized by optic neuritis and longitudinally-extended transverse myelitis lesions in spinal segments. Magnetic Resonance Imaging (MRI) findings are part of the diagnostic process in NMOSD patients, and abnormal lesion patterns may cause deviation from a correct diagnosis. 
Clinical Presentation and Intervention: A 43-year-old female patient with abrupt cognitive loss, motor dysfunction and tumefactive spread of the demyelinating lesions in her brain presented to the Neurology Clinic of Kashani Hospital. Anti Aquaporine 4(AQP-4) antibody was observed, and the patient responded well to the NMOSD treatment. 
Conclusion: NMOSD can be considered a differential diagnosis during an episode of Fulminant Demyelinating Disorder (FDD) with the tumefactive spread of the lesions.