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Background
Neuromyelitis Optica Spectrum Disorder (NMOSD) is autoimmune demyelinating disease of the central nervous system. Development of serum autoantibodies (Ab) against the astrocyte aquaporin-4 (AQP4) water channels plays a key role in pathophysiology. Serum immunoglobulin G (IgG) Ab target the AQP4 water channels and binding of AQP4-Ab to astrocyte triggers classical complement cascade, which leads to demyelination and neuronal loss. Higher prevalence and greater severity amongst those diagnosed with other autoimmune conditions, such as Sjogren’s Syndrome and systemic lupus erythematosus have been seen. International Panel for NMO diagnosis (IPND) in 2015 defined current criteria for diagnosis and stratification based on AQP4-IgG.

Educational Goals / Teaching Points
Knowledge of AQP4 distribution and pattern of involvement can provide diagnostic clues in seronegative patients. Magnetic resonance (MR) imaging has become an important tool for diagnosis particularly for recognition of seronegative and patients with unspecified serologic status. Distinguishing NMOSD from other demyelinating diseases including multiple sclerosis, antimyelin oligodendrocyte glycoprotein-associated myelitis, and acute disseminated encephalomyelitis is essential because treatment approaches are different and timely diagnosis affects long term outcomes. Familiarize the audience with MR imaging features of NMOSD and differentiating findings from other demyelinating diseases.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
Brain areas expressing AQP4 include subpial and peri ependymal areas along the ventricles, brainstem (including the area postrema), optic chiasm, diencephalon (hypothalamus and thalami), and corpus callosum. It is important to know AQP4 distribution as it can provide diagnostic clues in seronegative patients. Optic nerve typically involves the posterior optic pathway including chiasm and optic tract. Spinal cord lesions in NMOSD involve three or more contiguous vertebral segments and appear as bright spots on T2WI, which are dark on T1WI with ring enhancement pattern resulting in a lens-shaped appearance.

Conclusion
With the discovery of AQP4-Ab in 2004, diagnostic criteria have evolved. International Panel for NMO diagnosis criteria were developed in 2015 and is currently used to diagnose and stratify NMOSD based on AQP4 ab serology status. Magnetic resonance (MR) imaging has become an important tool for diagnosis particularly for recognition of seronegative and patients with unspecified serologic status. Here we provide background and showcase the spectrum of imaging findings of NMOSD.