Neuromyelitis optica (NMO) is characterized by severe optic neuritis and longitudinally extended, transverse myelitis. There have been long controversial whether NMO is a variant of multiple sclerosis (MS) or a different disease. However, since the discovery of an NMO-specific autoantibody to aquaporin 4 (AQP4), a dominant water channel in the central nervous system densely expressed on foot processes of astrocytes, the clinical distinction between NMO and MS has become clear, and now AQP4 antibody status is critically important for neurologists in deciding on treatment strategy. Moreover, pathological studies demonstrated an extensive loss of immunoreactivities to AQP4 and glial fibrillary acidic protein (GFAP) with relative preservation of the staining of myelin basic protein in acute NMO lesions, which is not seen in MS. In fact, the GFAP levels in the cerebrospinal fluid during acute exacerbation of NMO are remarkably elevated, while the values in MS are not different from those in controls. In addition, recent experimental studies conducted in vitro and in vivo have shown that AQP4 antibody is pathogenic. These findings strongly suggest that AQP4 antibody has diagnostic, therapeutic and pathogenetic implications, and that severe astrocytic damage mediated by AQP4 antibody distinguishes NMO from MS.
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