In SIADH, the serum sodium is 118 mEq/L. A patient is given hypertonic saline to raise serum Na⁺. The maximum recommended rate of Na⁺ correction is approximately 8–10 mEq/L per 24 hours. Exceeding this rate risks which specific neurological complication?

• A Cerebral edema due to osmotic fluid shift into neurons in a hyperosmolar environment
• B Hypernatremic seizures from rapid shift of intracellular water causing neuronal shrinkage
• C Cerebral hemorrhage from rapid vasoconstriction of leptomeningeal arteries
• D Osmotic demyelination syndrome (previously central pontine myelinolysis) due to rapid osmotic stress on oligodendrocytes and myelin ✓

Explanation

In chronic hyponatremia (>48 hours), brain cells adapt to the low osmolality by losing organic osmolytes (taurine, glutamate, myo-inositol). If serum Na⁺ is corrected too rapidly, the extracellular osmolality rises faster than cells can re-accumulate osmolytes, causing acute cell shrinkage. Oligodendrocytes are particularly vulnerable — rapid dehydration disrupts their myelin-maintaining function, causing demyelination, especially in the pons (central pontine myelinolysis) and extrapontine areas. Symptoms include dysarthria, dysphagia, locked-in syndrome, and quadriplegia. Cerebral edema is the risk of UNDER-treating hyponatremia or over-correcting the initial saline dose too rapidly in the other direction.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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