Osmotic demyelination syndrome
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Osmotic demyelination syndrome refers to acute demyelination seen in the setting of osmotic changes, typically with the rapid correction of hyponatremia. 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 recognized in other patient groups, but usually in the setting of rapidly corrected electrolyte disturbance 1:
chronically debilitated patients
pregnancy and hyperemesis
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:
changes in levels of consciousness
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 characterized by intramyelinic splitting, vacuolation, and myelin sheath rupture 3. After many days, macrophages can be identified.
CT may demonstrate low attenuation crossing the midline in the lower pons. However, CT assessment of the skull base can be difficult due to beam hardening artifact and, if available, MRI is preferred.
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 a low T1 signal. The T1 and T2 changes may take up to two weeks to develop. This region has a classic trident-shaped appearance. The overall appearance on T2/FLAIR axial MRI has also been likened to the face of a pig, referred to as piglet sign. Occasionally gadolinium enhancement is also demonstrated, just as in the acute phase of multiple sclerosis (MS) plaque. The peripheral fibers (ventrolateral longitudinal fibers, corticospinal tracts), 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 the affected region include:
T1: mildly or moderately hypointense
T2/FLAIR: hyperintense, sparing the periphery and corticospinal tracts
ADC: signal low or signal loss
T1 C+ (Gd): usually there is no enhancement, but some authors reported that it may occur 5,6
Affected regions may demonstrate initial high uptake followed by subsequent low uptake with 18-FDG.
Treatment and prognosis
One recent article states that sodium level correction should be 4 mEq/L to 6 mEq/L within 1 to 2 hours, no more than 10 mEq/L, is an upper limit within the initial 24 hours 7.
Some patients show complete recovery however six-month survival rate is only 5-10%.
General imaging differential considerations include:
demyelination including multiple sclerosis (MS)
brainstem metastasis: usually enhances
vigabatrin toxicity: bilateral dorsal pontine longitudinal restricted diffusion
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