General description

Osmotic demyelination syndrome (ODS) is a demyelinating disease of the central nervous system primarily caused by the rapid correction of chronic hyponatremia. During chronic hyponatremia, brain cells expel osmotic substances to prevent swelling, but a sudden correction in serum sodium levels causes water to drain from the brain cells, leading to atrophy and demyelination. This disease is also known as central pontine myelinolysis (CPM) because the pons is particularly susceptible to damage. However, damage can also occur in other areas, such as the basal ganglia and thalamus, a condition referred to as extrapontine myelinolysis (EPM).

ODS can be classified into two categories based on the site of involvement:

• Central pontine myelinolysis (CPM): Demyelinating lesions primarily affect the central portion of the pons. This can lead to impaired consciousness if the reticular formation is involved, quadriplegia if the pyramidal tract is affected, and dysarthria or dysphagia if the cranial nuclei are impacted. In severe cases, locked-in syndrome may occur.
• Extrapontine myelinolysis (EPM): Involvement of the basal ganglia, thalamus, and subcortical white matter can result in symptoms like parkinsonism, including immobility, muscle rigidity, and tremors.

ODS symptoms often follow a biphasic course: initially, neurological symptoms (such as convulsions and encephalopathy) caused by hyponatremia temporarily improve with rapid sodium correction. However, 2–8 days after correction, neurologic symptoms typically reappear.

Radiographic fatures

Central pontine myelinolysis (CPM)

CPM typically presents as symmetrical high signal intensity in the center of the pons on T2WI or FLAIR images, while the pontine tegmentum and transverse fibers of the pons are usually preserved.

Extrapontine myelinolysis (EPM)

EPM shows bilateral hyperintensities on T2WI and FLAIR in the basal ganglia, external capsule, thalamus, corpus callosum, subcortical white matter, as well as the middle cerebellar peduncles.