## 5 Symptoms of NMOSD
### Imaging and Diagnosis of NMOSD  
The diagnosis of NMOSD predominantly relies on identifying AQP4-IgG in the patient's serum. According to the international consensus diagnostic criteria of NMOSD, seropositive AQP4 IgG patients exhibit at least one of the clinical characteristics, such as ON or area postrema syndrome and the presence of acute myelitis, brainstem syndrome, acute diencephalic clinical syndrome with lesions, or symptomatic cerebral syndrome with lesions [17]. Diagnostic criteria for seronegative AQP4-IgG patients are stringent with neuroimaging analysis. Additionally, the patients must have one of the previously mentioned seropositive clinical characteristics such as ON and transverse myelitis with area postrema syndrome [17]. The AQP4 seronegative patients were further diagnosed using cell-based assays repeatedly more than two times within 6 to 9 months [59]. Compared with MS, the relapses in NMOSD are more severe; ON induces inflammation, pain, and visual loss. The spinal cord damage triggers inflammation, which causes fatigue, neuropathic pain, bowel and bladder dysfunction [60], alterations in consciousness, respiratory failure, paraplegia, and even death [9]. Diagnosing and differentiating MS and NMOSD is difficult for neurologists in AQP4-IgG seronegative cases. A differential diagnosis method is required to facilitate accurate diagnosis of autoimmune CNS disorders. Imaging the brain, optic nerve, spinal cord, and retina could favour the understanding of the disease progression and necessary measures for treating autoimmune CNS disorders. Magnetic resonance imaging (MRI) and retinal optical coherence tomography (OCT) are non-invasive imaging techniques that play an important role in diagnosing and differentiating MS and NMOSD. OCT helps visualize the retinal neuronal axons, retinal ganglion cells, and neuroaxonal retinal damages [61]. Retinal OCT, brain, spinal cord, and optic nerve MRI methods are useful in differential diagnosis, screening, and monitoring the degree of CNS damage [62]. MRI studies in patients with ON caused by NMOSD and MS revealed that NMO patients showed bilateral optic nerve and chiasm enhancement [63]. [PERSON] et al. developed a live cell and single molecule imaging strategy for visualizing AQP4 diffusion and formation of OAPs by comparing the AQP4 transfected cells and AQP4 knockout mice [64]. Quantum dot-labelled AQP4 proteins were tracked through the reflection of the green fluorescent protein and fluorophore-labelled AQP4 using total internal reflection fluorescence microscopy. Biophysical methods such as photobleaching and fluorescence correlation spectroscopy, the physiology of AQP4 isoforms, I interactions, site of OAP formations, size, and stability can also be studied [64]. The presence of inflammation in the visual pathways and cloud-like and pencil-thin ependymal enhancement have been reported using a contrast-enhanced MRI [65; 66; 67]. An increase in the thickness of the anterior visual pathways and optic T2 signal hypersensitivity during ON is a marker of neuroinflammation [68]. For the past two decades, imaging research has attained peculiar development in structural and functional imaging at subcellular resolution and has contributed a lot to understanding the tissue damage mechanisms, disease progression sites, and degree of pathology, monitoring the disease patterns in individual patients. Although imaging methods have been technically developed, limitations exist, such as a need for standard protocols and imaging techniques to understand and differentiate neuroinflammation in NMOSD and other neuroautoimmune diseases. Clinical MRI examination is important in suspected demyelinating myelitis to prevent misdiagnosis. MRI examinations provide necessary cues for differential diagnosis of different myelopathies and support proper treatment decisions [69]. AQP4+ NMOSD subjects showed massive inflammation, complete spinal cord impairment, and other motor deficits such as sensory loss, para or quadripareresis, and sphincter and erectile dysfunction [70].