Sodium Imbalances

Sodium (Na+) is the most abundant cation in extracellular fluid, maintaining a normal serum level between 135-145 mEq/L. It serves as the primary determinant of extracellular fluid osmolality and volume. Because water follows sodium by osmosis, the concentration of sodium directly influences the distribution of fluid between intracellular and extracellular compartments. When sodium levels fall below 135 mEq/L, a condition known as hyponatremia, water shifts into cells, causing cellular edema. In the brain, this cellular swelling leads to cerebral edema, which manifests as headache, confusion, nausea, and can progress to seizures and death. Conversely, when sodium levels exceed 145 mEq/L, hypernatremia occurs, causing water to move out of cells and resulting in cellular dehydration. In the brain, this dehydration causes cell shrinkage, leading to symptoms such as restlessness, irritability, lethargy, hyperreflexia, and potentially seizures and coma. Elderly patients are particularly vulnerable to sodium imbalances due to factors such as an impaired thirst mechanism, reduced renal function, and polypharmacy.