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In the absence of pathognomonic clinical features or a definitive laboratory test, multiple sclerosis (MS) remains ultimately a diagnosis of exclusion. Accurate diagnosis is increasingly important with available
disease modifying therapy. Unfortunately the rate of misdiagnosis remains around 5%-10%, indicating that 1 in 20 patients thought to have MS has, instead, a condition resembling MS. In this review we describe conditions that may be confused with MS because they can present as lesions disseminated in time, space, or both. Conditions often confused with MS may be inflammatory (systemic lupus erythematosus, Sjögren’s syndrome, vasculitis, sarcoidosis, Behçet’s
disease), infectious (
Lyme disease, syphilis, progressive multifocal leukoencephalopathy, HTLV-1 infection, herpes zoster), genetic (lysosomal disorders, adrenoleukodystrophy, mitochondrial disorders, CADASIL), metabolic (vitamin B12 deficiency), neoplastic (CNS lymphoma) and spinal (degenerative and vascular malformations) diseases. The key to the accurate diagnosis of MS is vigilance for atypical features, suggesting the possibility of an alternative diagnosis.