Demyelinating, Infectious, and Autoimmune Disorders: MRI Pattern and Antibody
The lesion shape often answers the question
Central nervous system demyelination questions turn on lesion distribution, longitudinal extent, and antibody status.
Multiple sclerosis requires dissemination in space (lesions in at least two of four regions: periventricular, cortical or juxtacortical, infratentorial, spinal cord) and dissemination in time. Typical lesions are ovoid, perpendicular to the ventricles, and short in the spinal cord. CSF-specific oligoclonal bands can substitute for dissemination in time. Radiologically isolated syndrome describes typical MRI findings without symptoms.
Neuromyelitis optica spectrum disorder is aquaporin-4 antibody mediated, with longitudinally extensive transverse myelitis over three or more segments, severe optic neuritis, area postrema syndrome with intractable hiccups or vomiting, and periependymal lesions. Relapses are severe; acute treatment is high-dose corticosteroids with plasma exchange for poor response, and maintenance therapy uses complement or IL-6 targeted agents. MOG antibody disease often presents with bilateral optic neuritis, ADEM-like illness, or conus involvement.
Infections. Herpes simplex encephalitis involves temporal and inferior frontal lobes with fever, seizures, and behavioral change; start acyclovir before imaging and lumbar puncture return because CSF polymerase chain reaction sensitivity falls after several days. Neurosyphilis should be considered in any meningitis with stroke or uveitis. Progressive multifocal leukoencephalopathy produces nonenhancing white matter lesions in immunocompromised patients without mass effect.
Autoimmune encephalitis. Anti-NMDA receptor encephalitis causes psychiatric symptoms, seizures, dyskinesias, and hypoventilation, often with an ovarian teratoma. LGI1 and CASPR2 antibodies produce limbic encephalitis with faciobrachial dystonic seizures and peripheral hyperexcitability, respectively.
Rule of thumb: spinal cord demyelination over three or more segments means test aquaporin-4, not just start MS therapy.