- In cases of severe dehydration, the most critical electrolyte disturbance is hypernatremia.
- This occurs because dehydration leads to a significant loss of free water relative to sodium, resulting in elevated serum sodium levels.

- The key concern with hypernatremia is its impact on the brain.
- Elevated sodium causes hyperosmolarity of the extracellular fluid, which draws water out of brain cells, leading to cellular dehydration and brain shrinkage.
- This can cause severe neurological symptoms such as confusion, seizures, coma, and even death if not corrected promptly.

- While other electrolyte imbalances like hyperkalemia pose significant risks (e.g., cardiac arrhythmias), in the context of severe dehydration hypernatremia represents the greatest immediate threat due to its effects on the central nervous system and rapid progression.

- Hyponatremia and hypokalemia can also occur but are less common and generally less immediately life-threatening in the setting of dehydration. Proper management involves cautious and controlled rehydration to avoid rapid shifts in sodium concentration, which can cause cerebral edema.

Reference: Harrison’s Principles of Internal Medicine, 20th Edition, Volume 1, Chapter 337: Disorders of Water and Sodium Balance