Case of the Week
Section Editors: Matylda Machnowska1 and Anvita Pauranik2
1University of Toronto, Toronto, Ontario, Canada
2BC Children's Hospital, University of British Columbia, Vancouver, British Columbia, Canada
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June 3, 2021
Pontine and Extrapontine Osmotic Demyelination Syndrome
- Background:
- Astrocytes usually adapt to chronic hyponatremia (>48 hours). However, overly rapid correction leads to astrocyte apoptosis.
- There is selective predilection for areas with rich gray-white matter apposition due to the release of myelinotoxic factors by gray matter in response to osmotic insults.
- Relative proportion: central pontine (1/2), extrapontine myelinolysis (2/5), or both (3/5)
- Clinical Presentation:
- This pathology shows a phasic clinical course.
- Initially, the patient is encephalopathic or presents with seizures from hyponatremia.
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Then there is rapid recovery as normonatremia is restored.
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Finally, there is deterioration several days later which presents as dysarthria, dysphagia, and flaccidity. (Osmotic demyelination reflects this phase.)
- There can be quadriparesis which later may become spastic. Classic “locked-in syndrome” is also seen (GCS-E4VtM2 in our case).
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If lesions are extrapontine, the clinical picture may be very confusing, ranging from apparently psychiatric and behavioral changes to movement disorders.
- Key Diagnostic Features:
- CT is less sensitive than MRI with areas of myelinolysis appearing as hypoattenuating regions.
- On MRI: Symmetric trident-shaped area in the central pons is a characteristic finding on T2-weighted imaging and FLAIR with restricted diffusion. The ventrolateral pons and the pontine portion of the corticospinal tracts typically are spared. Lesions typically do not enhance after the administration of contrast.
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Diagnosis of osmotic demyelination syndrome cannot be ruled out in the setting of normal MRI (can appear normal in conventional MRI sequences up to 10 days after the insult, but DWI can detect changes within 24 hours).
- Differential Diagnoses:
- Common lesions involving bilateral basal ganglia can be considered in the differential diagnosis of extrapontine myelinolysis.
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Toxic poisoning and drug abuse:
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Carbon monoxide poisoning: typically involves the globi pallidi with delayed involvement of white matter
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Heroin: basal ganglia and deep white matter
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Methanol: putamen and deep white matter
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Cyanide: putamen (often hemorrhagic)
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Uremic encephalopathy: basal ganglia, thalamus, midbrain, and mesial temporal lobes; lentiform fork sign described for this entity
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Other toxic encephalopathies like hepatic hyperammonemic encephalopathy
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- Metabolic: Wernicke encephalopathy: medial thalami, periaqueductal gray matter, mammillary bodies
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Infections: viral, especially flaviviral encephalitides (West Nile virus, Japanese encephalitis); may show areas of hemorrhage
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Acute disseminated encephalomyelitis: patchy areas of involvement with involvement of white matter, thalami, and cord
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The major differential diagnosis of central pontine myelinolysis is pontine ischemia or infarct. Basilar perforating artery infarcts involve the surface of the pons and are usually asymmetric.
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Demyelinating diseases can involve the pons, but they are rarely symmetric. Also, typically they involve the callososeptal interface.
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Hypertensive encephalopathy (PRES) can involve the pons but does not spare the peripheral white matter tracts.
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Treatment:
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​Guarded electrolyte correction is the mainstay. Over the years, many guidelines have been laid down for same (eg, sodium correction should not exceed 8 mmol/L on any day of correction).
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The prognosis for osmotic demyelination syndrome varies and there is no apparent connection to clinical features or imaging findings.
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In summary, the outcome may be death, disability, or recovery to a virtually normal level of function.
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