Demyelinating Diseases Differential for MS

Educational scope notice: This is a study note for medical students, not medical advice, diagnosis, or treatment guidance. Clinical management should follow local protocols and current guidelines.

Several demyelinating diseases resemble MS, and confusing them matters because the treatments differ sharply. Some MS therapies can worsen neuromyelitis optica spectrum disorder, so antibody testing for aquaporin-4 and myelin oligodendrocyte glycoprotein is standard before committing to MS disease-modifying therapy, especially with an optico-spinal presentation. The comparison below follows four diseases across the same features so the contrasts stay visible.

The four diseases at a glance

Neuromyelitis optica spectrum disorder

NMOSD is a severe autoimmune disease of the central nervous system once thought to be simultaneous bilateral optic neuritis with transverse myelitis. The 2004 discovery of AQP4-IgG widened it into a spectrum including diencephalic, brainstem, and cerebral lesions. Aquaporin-4 is a water channel concentrated in optic nerve, spinal cord, periventricular regions, hypothalamus, and area postrema, which explains the characteristic sites.

Incidence is roughly 0.05 to 0.40 per 100,000 per year, higher in Asian and African populations. Seropositive patients have more severe attacks, worse outcomes, more relapses, and more frequent coexisting autoimmune disorders than seronegative ones. Oligoclonal bands are uncommon, seen in fewer than 20%, which itself contrasts with MS. Core clinical features are:

• Optic neuritis.
• Acute myelitis.
• Area postrema syndrome (otherwise unexplained hiccups or vomiting).
• Acute brainstem syndrome.
• Symptomatic narcolepsy or diencephalic syndromes.
• Symptomatic cerebral syndromes.

Diagnosis with positive AQP4-IgG needs one core feature with exclusion of alternatives; without antibodies it needs two core features with dissemination in space and additional MRI requirements.

Imaging is distinctive. The cord shows longitudinally extensive transverse myelitis spanning at least 3 contiguous segments, centrally located with more than half the axial area involved, often cervical or thoracic with rostral extension into the brainstem and patchy lens-shaped enhancement. About 14% first present with short lesions, so a short lesion does not exclude NMOSD. Optic nerve involvement is often bilateral, longitudinally extensive over more than half the nerve length, posterior including chiasm, with later atrophy. Brain lesions cluster around ventricles, the ependymal corpus callosum surface, diencephalon, area postrema, and corticospinal tracts, sometimes with cloud-like enhancement.

Myelin oligodendrocyte glycoprotein-associated disease

MOG-IgG appears in a substantial minority of AQP4-seronegative NMOSD-like cases and defines what is now considered a separate disease rather than a form of MS or NMOSD. MOG sits on the outer myelin lamellae, so this is a myelin disorder, unlike AQP4-positive NMOSD, which is primarily astrocytic. The large MOGADOR cohort showed that only about 19% of MOG-IgG-positive patients met NMOSD criteria, confirming it stands apart.

MOGAD affects younger patients with lower female predominance than NMOSD, often follows infectious prodromes (about 60%), and carries a more favorable prognosis despite relapses. Children present with ADEM-like phenotypes; adults present with optico-spinal disease.

• Optic involvement is typically bilateral, anterior, edematous and tortuous with long segments, optic disc swelling, and perioptic sheath enhancement, usually sparing the chiasm.
• Cord lesions are longitudinally extensive and central with an H-sign pattern on axial images and a sagittal T2 line, frequently involve the conus (about 40%), enhance less often, and resolve more completely than in NMOSD or MS.
• Adult brain lesions are few, bilateral, fluffy and poorly demarcated in white matter, cortex, deep grey matter, and pons; children show large ADEM-like brainstem and deep grey lesions with cerebellar peduncle involvement.

Complete lesion resolution favors MOGAD. The proposed criteria require further validation and do not settle every atypical case.

Acute disseminated encephalomyelitis

ADEM is a rare monophasic demyelinating disorder, mostly of children, with an incidence of about 0.3 to 0.6 per 100,000. It follows viral or bacterial infection, especially upper respiratory infection, or vaccination such as MMR, by 1 to 2 weeks, through molecular mimicry or non-specific activation of myelin-reactive T cells. Histology shows sleeves of demyelination with mild perivascular T-cell and macrophage infiltrates.

The defining clinical feature is encephalopathy with decreased consciousness plus multifocal deficits after prodromal fever, malaise, and headache, which separates it from MS. Cerebrospinal fluid shows lymphocytic pleocytosis with mildly raised protein and absent or rare oligoclonal bands. Brain MRI shows patchy, poorly marginated lesions of varying sizes, sometimes large with mild or absent mass effect, predominantly subcortical at the grey-white interface, with cortical grey involvement in about 30%, symmetric basal ganglia and thalami in children, and infratentorial involvement in about half. Enhancement is usually absent, and when present it affects most lesions simultaneously rather than in the mixed-age pattern of MS. The cord is affected in about 30%, usually thoracic, with central longitudinally extensive lesions.

Most lesions resolve completely and most patients recover fully, with residual disability in about 20%. The course is monophasic: no new lesions after 3 months, although existing lesions may evolve within that window. New lesions beyond 3 months occur in about 10 to 29% and usually mean reclassification as MS, NMOSD, or MOGAD. A second encephalopathy event after 3 months, called multiphasic ADEM and seen in about 4%, usually reclassifies as MOGAD when MOG antibodies persist.

In children after a first demyelinating event, Callen MRI criteria predict progression to MS: at least two of absence of a diffuse bilateral pattern, presence of black holes, and more than 2 periventricular lesions. About 10 to 25% of children with ADEM are eventually diagnosed with MS. A rare fulminant variant with hemorrhages, acute hemorrhagic leukoencephalitis, is rapidly progressive and often fatal.

Why the distinction changes treatment

MS disease-modifying therapies target MS-type inflammation and do not treat AQP4-positive NMOSD, which instead requires its own immunotherapies. Starting an MS therapy in unrecognized NMOSD can allow severe relapses to continue unchecked and some agents can worsen the disease. This is why AQP4 and MOG antibody testing, cerebrospinal fluid analysis, and careful spinal and optic nerve imaging belong in the workup before the diagnosis is considered settled.

Evidence anchors

• Wingerchuk DM, Banwell B, Bennett JL, et al. International consensus diagnostic criteria for neuromyelitis optica spectrum disorders. Neurology. 2015;85(2):177-189. doi:10.1212/WNL.0000000000001729
• Cobo-Calvo A, Ruiz A, Maillart E, et al. Clinical spectrum and prognostic value of CNS MOG autoimmunity in adults: the MOGADOR study. Neurology. 2018;90(21):e1858-e1869. doi:10.1212/WNL.0000000000005560
• Callen DJ, Shroff MM, Branson HM, et al. Role of MRI in the differentiation of ADEM from MS in children. Neurology. 2009;72(11):968-973. doi:10.1212/01.wnl.0000338630.20412.45
• Banwell B, et al. Diagnosis of myelin oligodendrocyte glycoprotein antibody-associated disease: International MOGAD Panel proposed criteria. Lancet Neurol. 2023. https://pubmed.ncbi.nlm.nih.gov/36706773