Background: Differentiating neuromyelitis optica spectrum disorder (NMOSD) from its mimics is crucial to avoid misdiagnosis, especially in the absence of aquaporin-4-IgG. While multiple sclerosis (MS) and myelin oligodendrocyte glycoprotein-IgG associated disease (MOGAD) represent major and well-defined differential diagnoses, non-demyelinating NMOSD mimics remain poorly characterized. Methods: We conducted a systematic review on Pubmed/Medline to identify reports of patients with non-demyelinating disorders that mimicked or were misdiagnosed as NMOSD. Three novel cases seen at the authors' institutions were also included. The characteristics of NMOSD mimics were analysed and red flags associated with misdiagnosis identified. Results: A total of sixty-eight patients were included; 35 (52%) were female. Median age at symptoms onset was 44 years (range, 1-78). Fifty-six (82%) patients did not fulfil the 2015 NMOSD diagnostic criteria. The clinical syndromes misinterpreted for NMOSD were myelopathy (41%), myelopathy+optic neuropathy (41%), optic neuropathy (6%), or other (12%). Alternative etiologies included genetic/metabolic disorders, neoplasms, infections, vascular disorders, spondylosis, and other immune-mediated disorders. Common red flags associated with misdiagnosis were lack of CSF pleocytosis (57%), lack of response to immunotherapy (55%), progressive disease course (54%), and lack of MRI gadolinium enhancement (31%). Aquaporin-4-IgG positivity was detected in five patients by enzyme-linked immunosorbent assay (n=2), cell-based assay (n=2: serum, 1; CSF, 1), and non-specified assay (n=1). Conclusions: The spectrum of NMOSD mimics is broad. Misdiagnosis frequently results from incorrect application of diagnostic criteria, in patients with multiple identifiable red flags. False aquaporin-4-IgG positivity, generally from nonspecific testing assays, may rarely contribute to misdiagnosis.
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