Page "Central pontine myelinolysis" Paragraph 5

The currently accepted theory states that the brain cells adjust their osmolarities by changing levels of certain osmolytes like inositol, betaine, and glutamine.

In hyponatremia the levels of these osmolytes fall, causing the cells to absorb free-water.

The reverse is true for hypernatremia, in which cells will shrink to dilute the hypernatremic fluid.

So rapid correction of sodium in hyponatremia would cause the extracellular fluid to be relatively hypertonic.

Free water would then move out of the cells to decrease this relative hypertonicity.

This leads to a central pontine myelinolysis, manifesting as the paralysis.

This would look like the brain is shrinking.