Intrathecal positivity of immunoglobulins can be observed also in neuroinflammatory diseases of the CNS other than MS, such as optic neuritis, autoimmune encephalitis, neurosarcoidosis, neuromyelitis optica (NMO), and anti-myelin oligodendrocyte glycoprotein (MOG) antibody disease, and it can be due to several mechanisms including the altered permeability of the blood–brain barrier (BBB), an intrathecal synthesis of immunoglobulins, or a combination of both [13,19,20]. Here, MOG is linked to myeloid sarcoma.