Central Pontine Myelinolysis

Osmotic demyelination (aka central pontine myelinolysis)

Overview

Osmotic demyelination is best known as central pontine myelinolysis, which is a metabolic cause of CNS demyelination.

Imaging

FLAIR imaging is best to view the white matter hyperintensity from demyelination within the center of the pons.

Demyelination can occur in other brain areas (called extrapontine myelinolysis), which is why the disorder is no longer simply called central pontine myelinolysis but it was first recognized as a pontine demyelination disorder and the pontine demyelination still accounts for ~ 90% of the disease, and results in locked-in syndrome.

It occurs from overly rapid correction of hyponatremia (low sodium), which causes such a fast shift in small molecules and water out of CNS cells that demyelination ensues. In short, if the rate of rise of tonicity (from correction of hyponatremia) is too fast, the cell will shrink, and oligodendrocytes are vulnerable to cell death.
It can take up to 6 days for the white matter hyperintensity to show up on MRI.
Anyone with a chronic sodium level less than 120 mEq/L is at risk, this is especially common in alcoholics (it was first described in 1959 in alcoholics and the malnourished).

However, patients with liver disease and diabetes insipidus are known to be susceptible to the disorder with sodium levels even greater than 120 mEq/L.

References