Hypernatremia is defined as a sodium concentration greater than 145 mEq/L. All patients with hypernatremia have hyperosmolality, unlike hyponatremic patients who can have a low, normal, or high serum osmolality. Hypernatremia develops when there is a relative loss of water that is inadequately compensated for by water ingestion. Rarely, excess sodium intake contributes to hypernatremia when it is associated with an increase in extracellular volume.
The primary responses to hypernatremia are stimulation of thirst (to increase water intake) and increased secretion of ADH (to minimize water loss in the urine). Cells in the hypothalamus are able to sense minimal changes in serum osmolarity, triggering the thirst mechanism and subsequent intake of water. It is nearly impossible to develop hypernatremia in the context of an intact thirst mechanism with appropriate access to water.
When the patient is dehydrated, orthostatic hypotension and oliguria are typical findings. Because water shifts from the cells to the intravascular space to protect volume status, symptoms may be delayed. Lethargy, irritability, and weakness are early signs. Hyperthermia, delirium, seizures, and coma may be seen with severe hypernatremia (ie, sodium greater than 160 mEq/L). Symptoms in older adults may not be specific.
The first steps in evaluating patients with hypernatremia are assessing the urine volume, osmolality, and the osmole excretion rate. The latter can be calculated by multiplying the urine osmolality with urine volume. The copeptin test is discussed below.
The initial step is to determine whether the patient with hypernatremia is oliguric, ie, urine flow less than 0.5 mL/min, or nonoliguric. Patients who are nonoliguric can be further subdivided by measurement of a urine osmolality.
1. The oliguric patient (urine flow less than 0.5 mL/min)
This is found in several scenarios.
Hypernatremia will develop in patients with reduced water intake secondary to the inability to communicate and/or limited access to water.
Nonrenal sites of water loss include sweat, gastrointestinal (GI) tract, and the respiratory tract. This is most commonly seen in patients with diarrhea or in febrile patients on a ventilator.
C. SHIFT OF WATER INTO CELLS
Rarely, hypernatremia may manifest from a shift of water into cells due to the intracellular gain of an effective osmole. This may be seen with seizures or rhabdomyolysis.