Even small sodium imbalances can be life-threatening. With hypernatremia, sodium over 145 mEq/L dehydrates and shrinks the brain, causing symptoms that can swiftly lead to seizures, coma, and death if untreated. Hyponatremia under 135 mEq/L floods cells with fluid, progressing rapidly from nausea to deadly cerebral and pulmonary edema. Sodium is vital, but too much or too little is dangerous. At the first sign of imbalance, urgent treatment is needed to avoid catastrophe.