Neuromyelitis optica spectrum disorder: pathophysiological approach

Aim: To review the main pathological findings of Neuromyelitis Optica Spectrum Disorder (NMOSD) associated with the presence of autoantibodies to aquaporin-4 (AQP4) as well as the mechanisms of astrocyte dysfunction and demyelination. Methods: An comprehensive search of the literature in the field was carried out using the database of The National Center for Biotechnology Information from . Systematic searches were performed until July 2022. Results: NMOSD is an inflammatory and demyelinating disease of the central nervous system mainly in the areas of the optic nerves and spinal cord, thus explaining mostly the clinical findings. Other areas affected in NMOSD are the brainstem, hypothalamus, and periventricular regions. Relapses in NMOSD are generally severe and patients only partially recover. NMOSD includes clinical conditions where autoantibodies to aquaporin-4 (AQP4-IgG) of astrocytes are detected as well as similar clinical conditions where such antibodies are not detected. AQP4 are channel-forming integral membrane proteins of which AQ4 isoforms are able to aggregate in supramolecular assemblies termed orthogonal arrays of particles (OAP) and are essential in the regulation of water homeostasis and the adequate modulation of neuronal activity and circuitry. AQP4 assembly in orthogonal arrays of particles is essential for AQP4-IgG pathogenicity since AQP4 autoantibodies bind to OAPs with higher affinity than for AQP4 tetramers. NMOSD has a complex background with prominent roles for genes encoding cytokines and cytokine receptors. AQP4 autoantibodies activate the complement-mediated inflammatory demyelination and the ensuing damage to AQP4 water channels, leading to water influx, necrosis and axonal loss. Conclusions: NMOSD as an astrocytopathy is a nosological entity different from multiple sclerosis with its own serological marker: immunoglobulin G-type autoantibodies against the AQP4 protein which elicits a complement-dependent cytotoxicity and neuroinflammation. Some patients with typical manifestations of NMSOD are AQP4 seronegative and myelin oligodendrocyte glycoprotein positive. Thus, the detection of autoantibodies against AQP4 or other autoantibodies is crucial for the correct treatment of the disease and immunosuppressant therapy is the first choice.