# Hypernatremia

## Introduction

Hypernatremia, defined as a serum sodium concentration exceeding 145 mEq/L, invariably indicates a hyperosmolar state. Unlike hyponatremia, there is no "pseudo" or "translocational" hypernatremia; an elevated serum sodium always reflects a true deficit of water relative to sodium. In virtually all cases, hypernatremia results from impaired thirst perception or the inability to access water, because an intact thirst mechanism coupled with water availability would drive sufficient intake to prevent hypernatremia regardless of the magnitude of water losses. The prevalence is 1 to 3 percent among hospitalized patients and up to 9 percent in the intensive care unit. Hospital-acquired hypernatremia is more common than community-acquired hypernatremia, reflecting the iatrogenic potential of inadequate free water prescribing. Severe hypernatremia carries substantial mortality of 40 to 70 percent, though this high mortality is largely a reflection of the severity of the underlying conditions that predispose to hypernatremia rather than a direct effect of the sodium elevation itself.

## Pathophysiology

### Normal Water Balance

The normal adult ingests approximately 2 to 3 liters of water daily through a combination of thirst-driven drinking and water content in food. Daily water losses occur through insensible routes (skin and respiratory, approximately 800 to 1000 mL), renal excretion (approximately 1 to 1.5 liters, modulated by ADH), and gastrointestinal losses (approximately 100 to 200 mL under normal conditions). Two defense mechanisms protect against the development of hypernatremia: antidiuretic hormone (ADH), which concentrates the urine and minimizes renal water losses, and thirst, which drives water intake in response to rising plasma osmolality. If either defense mechanism is intact and the patient has access to water, hypernatremia will not develop. This fundamental principle has critical clinical implications for identifying vulnerable patients.

### Who Gets Hypernatremic?

Patients who develop hypernatremia almost invariably have one or more impediments to water intake. Impaired thirst perception affects elderly patients with altered sensorium, patients with hypothalamic lesions disrupting the osmoreceptor-thirst axis (adipsic diabetes insipidus), and intubated or sedated patients in the ICU who cannot communicate thirst. Inability to access water independently affects infants, immobilized nursing home residents, and hospitalized patients whose free water intake depends entirely on intravenous fluid prescriptions. In a third group, water losses are so overwhelming that even maximal water intake cannot compensate: this includes patients with diabetes insipidus producing massive volumes of dilute urine, those with osmotic diuresis from uncontrolled hyperglycemia or high-protein enteral feeds, and those with severe diarrheal illness or extensive burns.

## Etiology

### Unreplaced Water Losses (Most Common)

Renal water losses producing dilute urine with an osmolality below 300 mOsm/kg indicate failure of the urinary concentrating mechanism, as seen in central or nephrogenic diabetes insipidus. Osmotic diuresis, characterized by a urine osmolality above 300 mOsm/kg but with inappropriately large urine volumes, occurs with hyperglycemia, mannitol administration, urea loading from high-protein tube feeds, and the post-obstructive diuresis that follows relief of bilateral ureteral obstruction.

Extrarenal water losses are accompanied by appropriately concentrated urine with an osmolality exceeding 600 mOsm/kg, reflecting intact renal concentrating ability. Insensible losses increase substantially with fever (each degree Celsius above 38 adds approximately 100 to 150 mL per day of insensible loss), extensive burns, and hyperventilation. Gastrointestinal losses, particularly osmotic diarrhea from lactulose or infectious enteritis, are another important category. Vomiting, though a potential contributor, is a less common cause of hypernatremia because emesis is relatively hypotonic and the volume is typically less than diarrheal losses.

### Sodium Gain (Less Common)

Hypernatremia from sodium gain occurs in specific iatrogenic contexts: hypertonic saline infusion during resuscitation, sodium bicarbonate administration during cardiac arrest (each 50 mL ampule delivers 50 mEq of sodium), salt water ingestion during near-drowning, and improperly prepared infant formula. Hyperaldosteronism produces mild hypernatremia at most, as the mineralocorticoid escape phenomenon limits sodium retention.

### Diabetes Insipidus

#### Central DI

Central diabetes insipidus results from inadequate production or secretion of ADH by the hypothalamus and posterior pituitary. Causes include neurosurgery (particularly transsphenoidal pituitary surgery), traumatic brain injury, pituitary tumors (craniopharyngioma being the classic example, along with metastatic disease), infiltrative diseases such as sarcoidosis and Langerhans cell histiocytosis, lymphocytic hypophysitis, brain death, and idiopathic cases accounting for approximately 30 percent.

Post-surgical diabetes insipidus deserves special discussion because of its characteristic triphasic response. During days 1 through 5, axonal shock in the hypothalamic-neurohypophyseal tract produces an initial phase of DI with polyuria and rising sodium. From approximately days 5 through 10, uncontrolled ADH release from degenerating neurons causes a phase of SIADH with water retention and potential hyponatremia. If more than 80 percent of magnocellular neurons are destroyed, a third phase of permanent DI ensues beyond day 10. Recognition of this pattern is essential to avoid the potentially dangerous alternation between hypernatremia and hyponatremia in the post-surgical setting.

The diagnosis of central DI rests on the triad of polyuria exceeding 3 liters per day, dilute urine with an osmolality below 300 mOsm/kg, and elevated plasma osmolality above 295 mOsm/kg with elevated serum sodium. Treatment is with desmopressin (DDAVP), a synthetic vasopressin analog selective for the V2 receptor, administered as 10 to 20 mcg intranasally twice daily or 0.1 to 0.4 mg orally two to three times daily, titrated to prevent nocturia while avoiding over-replacement and iatrogenic hyponatremia.

#### Nephrogenic DI

Nephrogenic diabetes insipidus is characterized by renal resistance to ADH despite adequate or elevated circulating ADH levels. The kidneys cannot concentrate urine despite maximum ADH stimulation. The most common acquired cause is lithium therapy, which affects 40 percent of chronic users by downregulating aquaporin-2 expression in the collecting duct through a mechanism involving glycogen synthase kinase-3 beta (GSK-3 beta). Other drug causes include demeclocycline, amphotericin B, foscarnet, and cidofovir.

Electrolyte disorders that cause nephrogenic DI include hypercalcemia, which impairs both aquaporin-2 trafficking and the sodium-potassium-2-chloride cotransporter in the thick ascending limb, and hypokalemia, which downregulates aquaporin-2. Renal structural diseases that disrupt the medullary architecture, including chronic obstruction, sickle cell disease and trait (which causes medullary damage from sickling in the hypertonic medullary environment), polycystic kidney disease, and amyloidosis, can produce nephrogenic DI. Hereditary forms present in infancy with failure to thrive and hypernatremic dehydration: X-linked forms result from AVPR2 mutations affecting the V2 receptor, while autosomal recessive and dominant forms result from AQP2 mutations affecting the aquaporin-2 water channel.

| Feature | Central Diabetes Insipidus | Nephrogenic Diabetes Insipidus |
|---------|---------------------------|-------------------------------|
| Defect | Inadequate ADH production/secretion | Renal resistance to ADH |
| ADH levels | Low/absent | Normal or elevated |
| Common causes | Post-neurosurgery (transsphenoidal), TBI, pituitary tumors (craniopharyngioma), infiltrative disease (sarcoidosis, LCH), idiopathic (30%) | Lithium (40% of chronic users), hypercalcemia, hypokalemia, hereditary (AVPR2, AQP2 mutations) |
| Urine osmolality | <300 mOsm/kg (dilute) | <300 mOsm/kg (dilute) |
| Response to desmopressin | **>50% increase** in urine osmolality | **No response** |
| Treatment | Desmopressin (DDAVP): intranasal 10–20 mcg BID, oral 0.1–0.4 mg BID-TID | Remove offending agent; thiazide + amiloride (lithium); low-Na/low-protein diet; indomethacin |
| Key management pearl | Allow breakthrough polyuria before next DDAVP dose to prevent hyponatremia | Amiloride blocks lithium entry via ENaC (drug of choice for lithium-induced NDI) |
| Post-surgical pattern | Triphasic response (DI → SIADH → permanent DI) | N/A |

The diagnosis of nephrogenic DI is confirmed by demonstrating polyuria with dilute urine in the setting of elevated ADH levels and, critically, the absence of a response to exogenous desmopressin, which distinguishes it from central DI. Treatment focuses on removing the offending agent when possible, reducing renal solute load through a low-sodium and low-protein diet, and pharmacologic interventions. Thiazide diuretics paradoxically reduce polyuria by 30 to 50 percent through inducing mild volume contraction that enhances proximal tubular sodium and water reabsorption, thereby reducing distal delivery. Amiloride at 5 to 10 mg twice daily is specifically useful for lithium-induced nephrogenic DI because it blocks lithium entry into collecting duct principal cells via the epithelial sodium channel (ENaC), the same channel through which lithium gains access to the intracellular compartment. Nonsteroidal anti-inflammatory drugs, particularly indomethacin at 50 mg three times daily, reduce prostaglandin-mediated antagonism of ADH action, though their use must be weighed against the risk of further renal impairment. Adequate free water intake, with the patient having unrestricted access to water, is a fundamental component of management.

<image>Diagnostic algorithm for hypernatremia. Start with serum Na >145 mEq/L and confirm by checking serum osmolality (always >295 in true hypernatremia). Step 1: Assess urine osmolality and urine volume. If Uosm >600 mOsm/kg (concentrated urine, low volume): appropriate renal response → extrarenal water loss (insensible losses, GI losses) or sodium gain (hypertonic saline, NaHCO3). If Uosm <300 mOsm/kg (dilute urine, high volume): renal water loss → diabetes insipidus. If Uosm 300-600 mOsm/kg with high volume: partial DI or osmotic diuresis. Step 2: For DI, perform desmopressin test: if Uosm increases >50% → central DI; if no response → nephrogenic DI. Include causes in boxes for each DI type: central (post-surgical, trauma, tumor, idiopathic) and nephrogenic (lithium, hypercalcemia, hypokalemia, hereditary). Step 3: For osmotic diuresis, check urine glucose, urea, mannitol.</image>

## Water Deprivation Test (Miller-Moses Test)

The water deprivation test remains the gold standard for diagnosing diabetes insipidus when the diagnosis is not clinically obvious. The protocol involves withholding all water intake while closely monitoring the patient's body weight, urine osmolality, serum sodium, and serum osmolality at one- to two-hour intervals. The test is terminated when any of three endpoints is reached: body weight loss exceeding 3 percent (indicating clinically significant dehydration), serum sodium rising above 145 mEq/L, or urine osmolality reaching a plateau despite continued rises in serum osmolality. At this endpoint, desmopressin 2 mcg is administered subcutaneously or intravenously, and urine osmolality is measured at 1 and 2 hours post-injection.

### Interpretation

A normal response is urine concentration above 600 mOsm/kg during the deprivation phase itself, indicating intact ADH release and renal responsiveness. Complete central diabetes insipidus produces dilute urine below 300 mOsm/kg throughout the deprivation phase with a greater than 50 percent increase in urine osmolality after desmopressin administration, confirming that the kidneys can respond to ADH but endogenous ADH is absent. Partial central DI produces intermediate concentration of urine, in the range of 300 to 600 mOsm/kg, during deprivation with a modest further increase after desmopressin. Nephrogenic DI produces persistently dilute urine with no response to exogenous desmopressin, confirming renal resistance. Primary polydipsia produces concentrated urine above 500 mOsm/kg during the deprivation phase (because ADH release and renal function are normal, the kidneys concentrate urine appropriately once water intake is restricted), with normal serum osmolality maintained throughout.

### Copeptin (Emerging Biomarker)

Copeptin is the C-terminal fragment of pre-provasopressin that is co-secreted with ADH in equimolar amounts from the posterior pituitary. Unlike ADH itself, which is unstable and technically difficult to measure, copeptin is stable in plasma and can be reliably assayed. The stimulated copeptin test, performed after hypertonic saline infusion or arginine stimulation, uses a threshold of 4.9 pmol/L to distinguish central DI (levels below the threshold) from nephrogenic DI (levels above). The WASP study and the Swiss DI cohort study demonstrated that the hypertonic saline stimulation test with copeptin measurement achieved 95 percent diagnostic accuracy, potentially surpassing the water deprivation test and offering a faster, more standardized diagnostic approach. However, copeptin assays are not yet widely available in routine clinical practice.

## Management

### Free Water Deficit Calculation

The free water deficit is calculated using the formula: free water deficit = total body water multiplied by [(current sodium / 140) minus 1]. Total body water is estimated as 0.6 times body weight in kilograms for younger men, 0.5 for younger women, and 0.5 for elderly men and 0.45 for elderly women. For example, a 70 kg man with a serum sodium of 160 mEq/L has a free water deficit of 42 multiplied by [(160/140) minus 1], which equals 42 multiplied by 0.143, or approximately 6 liters. This calculation estimates only the static deficit at the time of measurement; ongoing losses from insensible, renal, and gastrointestinal routes must be added to the total replacement plan, a common source of error that leads to underestimation of fluid requirements.

### Correction Rate

The rate of sodium correction depends on the chronicity of the hypernatremia. Acute hypernatremia developing over less than 48 hours can be corrected rapidly at 1 to 2 mEq/L per hour until sodium normalizes, because the brain has not yet undergone adaptive changes. Chronic hypernatremia of more than 48 hours' duration, or hypernatremia of unknown duration, must be corrected slowly, with a maximum rate of 10 mEq/L per 24 hours. Rapid correction of chronic hypernatremia drives water into brain cells that have adapted to the hyperosmolar state by accumulating intracellular organic osmolytes; the resulting cellular swelling produces cerebral edema, analogous in principle to the osmotic demyelination syndrome that complicates overly rapid correction of chronic hyponatremia. Serum sodium should be monitored every 4 to 6 hours during active correction, with adjustment of the infusion rate based on the observed response.

### Fluid Selection

| Fluid | Sodium Content | Free Water per Liter | Primary Indication | Key Consideration |
|-------|---------------|---------------------|-------------------|-------------------|
| D5W (5% dextrose in water) | 0 mEq/L | **1000 mL** (1 L free water/L) | Pure water deficit (euvolemic hypernatremia, DI) | Glucose metabolized rapidly; avoid in hyperglycemia |
| 0.45% NaCl (half-normal saline) | 77 mEq/L | **500 mL** | Combined volume + water deficit | Provides some volume expansion + free water |
| 0.9% NaCl (normal saline) | 154 mEq/L | **0 mL** | Hypovolemic hypernatremia (restore hemodynamics first) | No free water; use only for volume resuscitation phase |
| Oral/enteral free water | 0 mEq/L | 1000 mL | Any hypernatremia with functional GI tract | **Preferred route** when feasible; most physiologic |

Dextrose 5 percent in water (D5W) provides 1 liter of free water per liter infused, as the dextrose is rapidly metabolized. Half-normal saline (0.45 percent sodium chloride) provides approximately 500 mL of free water per liter. Normal saline (0.9 percent sodium chloride) is isotonic and provides no free water; it should be used only when concurrent hypovolemia necessitates intravascular volume resuscitation. Oral or enteral free water, administered as tap water by mouth or through a nasogastric tube, is the preferred route whenever the gastrointestinal tract is functional. In hypernatremia with concurrent hypovolemia, the management is sequential: first restore intravascular volume with isotonic saline to maintain hemodynamic stability, then transition to hypotonic fluids for free water replacement once the patient is euvolemic.

### Treatment of Diabetes Insipidus

Central DI is treated with desmopressin (DDAVP) in one of several formulations: intranasal at 10 to 20 mcg twice daily, oral at 0.1 to 0.4 mg two to three times daily, or subcutaneous/intravenous at 1 to 2 mcg twice daily. A key management principle is to allow breakthrough polyuria ("escape polyuria") before each subsequent dose to prevent the accumulation of free water and iatrogenic hyponatremia. Sodium monitoring must be particularly vigilant in post-surgical patients because of the triphasic response.

Nephrogenic DI management centers on removing the offending agent when possible and employing multiple complementary strategies. Hydrochlorothiazide at 25 mg twice daily reduces polyuria by 30 to 50 percent through its volume-contracting effect, which enhances proximal tubular reabsorption and reduces distal delivery. Amiloride at 5 to 10 mg twice daily is specifically indicated for lithium-induced nephrogenic DI, as it blocks lithium entry through ENaC. A low-sodium, low-protein diet reduces the renal solute load and thereby reduces the obligatory water excretion needed to eliminate that solute. Indomethacin at 50 mg three times daily reduces prostaglandin-mediated antagonism of ADH but must be used cautiously given its nephrotoxic potential. Above all, the patient must have unrestricted access to water and the ability to drink ad libitum.

<image>Treatment decision tree for hypernatremia management. Start with volume status assessment. If hypovolemic + hypernatremic: Phase 1 - restore intravascular volume with 0.9% NaCl until hemodynamically stable; Phase 2 - switch to 0.45% NaCl or D5W for free water replacement. If euvolemic + hypernatremic (DI): central DI → desmopressin + free water; nephrogenic DI → remove cause, thiazide + amiloride (lithium), low-solute diet, free water. If hypervolemic + hypernatremic (sodium gain): loop diuretic + D5W; consider dialysis if severe renal failure. For all: show free water deficit formula, correction rate (≤10 mEq/L/24h for chronic, faster for acute), preferred fluids (D5W: 1L = 1L free water; 0.45% NaCl: 1L = 0.5L free water), and monitoring schedule (sodium every 4-6 hours).</image>

## Hospital-Acquired Hypernatremia

Hospital-acquired hypernatremia is more common than community-acquired hypernatremia, accounting for up to 60 percent of cases in hospitalized patients. The causes are largely preventable and reflect gaps in fluid management: inadequate free water prescribing in NPO patients, excessive isotonic saline without supplemental free water, failure to account for increased insensible losses in febrile patients or those with open wounds, osmotic diuresis from uncontrolled hyperglycemia, mannitol, or hyperosmolar tube feeds, and medication-induced water losses from lactulose or hypertonic nebulized saline. Prevention requires a proactive approach: prescribing maintenance free water for all NPO patients at a minimum of 30 mL/kg/day or 1 to 1.5 mL/kg/hr, monitoring serum sodium daily in ICU patients, and anticipating the increased free water requirements of febrile, mechanically ventilated, and surgically exposed patients.

## Key Clinical Pearls

- Hypernatremia in an alert patient with access to water is essentially impossible (intact thirst prevents it); always consider why the patient cannot drink: altered mental status, intubation, extremes of age, or hypothalamic lesion
- The free water deficit formula estimates the static deficit only; ongoing losses (insensible, renal, GI) must be added to the replacement plan -- a common error is underestimating total fluid requirements
- Chronic hypernatremia should be corrected at <=10 mEq/L per 24 hours to prevent cerebral edema; this is analogous to the slow correction principle in chronic hyponatremia (to prevent ODS)
- In lithium-induced nephrogenic DI, amiloride is the drug of choice because it blocks lithium entry into collecting duct principal cells via ENaC; thiazide diuretics reduce polyuria through a different mechanism (volume contraction) and can be used concomitantly
- Hospital-acquired hypernatremia is preventable: prescribe adequate free water for all NPO patients, especially those receiving isotonic saline, tube feeds, or osmotic agents

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