Osmotic demyelination syndrome

Osmotic demyelination syndrome refers to acute demyelination seen in the setting of osmotic changes, typically with the rapid correction of hyponatraemia. It is the more recent term replacing central pontine myelinolysis, recognizing that extrapontine structures can also be affected, previously known as extrapontine myelinolysis

The initial description of central pontine myelinolysis by Adams et al. in 1959 2 was entirely in a population of chronic alcoholics, and certainly, this scenario is common. Since then it has been increasingly recognised in other patient groups, but usually in the setting of rapidly corrected electrolyte disturbance:

  • chronic alcoholics
  • chronically debilitated patients
  • transplant recipients

Clinically osmotic demyelination syndrome presents in a biphasic pattern. The first phase is usually attributable not to the demyelination but rather to the inciting electrolyte abnormality, with patients being acutely encephalopathic. Following rapid reversal of this abnormality, the patient transiently improves before progressing onto the classic osmotic demyelination syndrome features 2-3 days later. When pontine involvement is prominent, clinical features consist of:

  • spastic quadriparesis
  • pseudobulbar palsy
  • changes in levels of consciousness
  • coma or death

Although the exact mechanism is still uncertain, it is known that oligodendroglial cells are most susceptible to osmotic stresses, leading to their demise. It is not surprising that the distribution of osmotic myelinolysis, therefore, parallels the distribution of these cells.

Histologically, osmotic demyelination syndrome is characterised by intramyelinitic splitting, vacuolisation and myelin sheath rupture 3. After many days, macrophages can be identified.

CT

CT may demonstrate low attenuation crossing the midline in the lower pons, although streak artifact frequently degrades this region and beam hardening phenomenon.

MRI

The earliest change is seen on DWI with restriction in the lower pons. This is seen within 24 hours of the onset of quadriplegia 3. This same region demonstrates eventual high T2 signal and later low T1 signal. The T1 and T2 changes may take up to two weeks to develop. This region has a classic trident shaped appearance. Occasionally gadolinium enhancement is also demonstrated, just as in the acute phase of a multiple sclerosis (MS) plaque. The peripheral fibres (ventrolateral longitudinal fibres), as well as the periventricular and subpial regions, are typically spared.

Similar appearances are seen in other parts of the brain: basal ganglia, midbrain and subcortical white matter. 

Signal characteristics of affected region include:

  • T1: mildly or moderately hypointense
  • T2: hyperintense, sparing the periphery and corticospinal tracts
  • PD: hyperintense
  • FLAIR: hyperintense
  • DWI: hyperintense
  • ADC: signal low or signal loss
  • T1 C+ (Gd): usually there is no enhancement, but some authors reported that it may occur 5-6
PET

Affected regions may demonstrate initial high uptake followed by subsequent low uptake with 18-FDG.

General imaging differential considerations include:

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Article Information

rID: 1077
Section: Pathology
Synonyms or Alternate Spellings:
  • Osmotic demyelination

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