Osmotic demyelination syndrome refers to acute demyelination seen in the setting of osmotic changes, typically with the rapid correction of hyponatremia.
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Terminology
Osmotic demyelination syndrome is the preferred term, replacing central pontine myelinolysis, recognizing that extrapontine structures can also be affected, previously known as extrapontine myelinolysis.
Epidemiology
Osmotic demyelination syndrome usually occurs in the setting of rapidly corrected electrolyte disturbance 1,8:
patients with chronic alcoholism
chronically debilitated patients
transplant recipients (especially liver transplantation)
pregnancy and hyperemesis
Clinical presentation
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 may include:
spastic quadriparesis
altered conscious state
coma
death
Pathology
Although the exact mechanism is 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.
Microscopic appearance
Histologically, osmotic demyelination syndrome is characterized by intramyelinic splitting, vacuolation, and myelin sheath rupture 3. After many days, macrophages can be identified.
Radiographic features
CT
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.
MRI
The earliest change is seen on DWI with diffusion 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, including the 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
PD: 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
Nuclear medicine
PET
Affected regions may demonstrate initial high uptake followed by subsequent low uptake with 18-FDG.
Treatment and prognosis
Once osmotic demyelination syndrome has occurred, management is supportive in nature. Thus, prevention of the syndrome developing is vital, in particular, ensuring that if there is hyponatremia, serum sodium correction should be gradual. One article states that serum sodium level correction should be 4 mEq/L to 6 mEq/L within 1 to 2 hours, with no more than 10 mEq/L correction within the initial 24 hours 7.
Some patients show complete recovery however six-month survival rate is only 5-10%.
History and etymology
The initial description of this pathology as central pontine myelinolysis was made by Adams et al. in 1959 in a a population of patients with chronic alcoholism 2.
Differential diagnosis
General imaging differential considerations include:
demyelination from other causes, such as multiple sclerosis (MS)
pontine ischemic stroke from basilar perforators can be central, although usually, brainstem infarcts stop at the midline
pontine neoplasms including astrocytomas: brainstem is asymmetrically expanded with mass effect upon adjacent cisterns. local vessels may also be displaced
brainstem metastasis: usually enhances
vigabatrin toxicity: bilateral dorsal pontine longitudinal restricted diffusion
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