Water moves in the direction of Na in the body, Na is principal electrolyte controlling osmolarity in body.
In cases of Hypernatremia that has lasted over >48-72 hours, the brain cells generate Idiogenic osmoles to keep water in cell and balance osmosis. Hypernatremia tend to pull out water from cell and shrink the cells causing intracellular dehydration. Idiogenic osmoles balances the water shift.
When Sodium is rapidly lowered in ECF, the water moves into the cell as now cell has more solute compared to ECF, so water shift into the cell. This leads to cerebral swelling and Edema and worsening of Clinical conditions.
Therefore correction rate should not exceed, 0.5 mEq/L/hr or 12mEq/L/day
Similarly in Hyponatremia, quick increase in sodium pulls out water from the cell and cause cell shrinkage rapidly, which lead to a condition called Central Potine Myelinosis.