Hypernatremia

• Water depletion relative to sodium: The primary mechanism underlying nearly all cases of hypernatremia is absolute or relative loss of free water. Because sodium is largely confined to the extracellular compartment (ECF) and water distributes freely across all body compartments, loss of hypotonic fluid (water losses exceeding sodium losses) or inadequate water intake increases the concentration of dissolved solutes. Mathematically, serum sodium [Na+] = (total body Na+ + total body K+)/(total body water). When the denominator decreases without proportional decrease in numerator, [Na+] rises. This occurs through renal losses (diabetes insipidus, osmotic diuresis), extrarenal losses (insensible losses from skin and respiratory tract, gastrointestinal losses), or inadequate intake (impaired thirst, inability to access water).

• Osmoreceptor activation and ADH suppression paradox: The hypothalamic osmoreceptors, located in the supraoptic and paraventricular nuclei, detect increased plasma osmolality (even increases of 1-2% trigger response) and trigger two compensatory mechanisms: enhanced antidiuretic hormone (ADH/vasopressin) secretion and activation of thirst. ADH acts on V2 receptors on collecting duct principal cells to increase aquaporin-2 (AQP2) water channel insertion into the apical membrane, promoting water reabsorption and diluting urine. However, in central diabetes insipidus, absent ADH secretion prevents this compensation. In nephrogenic diabetes insipidus, kidneys are resistant to ADH signaling, so despite maximal ADH levels, aquaporin-2 channels fail to insert appropriately. Paradoxically, impaired thirst (seen in elderly patients, CNS pathology, or critical illness) prevents the normally adequate compensatory response of increased free water intake.

• Cellular dehydration and osmotic shifts: The increased extracellular osmolality creates an osmotic gradient drawing water out of intracellular compartments (ICF), particularly affecting neurons. Cell volume decreases as water exits to maintain osmotic equilibrium. In the brain, this cellular shrinkage can stretch and tear cerebral blood vessels, causing intraparenchymal and subarachnoid hemorrhage—a life-threatening acute complication. Over hours to days, neurons activate adaptive mechanisms by generating organic osmolytes (sorbitol, taurine, betaine, myo-inositol) through gene expression changes, which increase intracellular osmolality and restore cell volume (cerebral adaptation). This adaptation explains why chronically hypernatremic patients tolerate higher sodium levels with less severe symptoms, but also why overly rapid correction causes cerebral edema.

• Renal compensation mechanisms: The kidney's primary defense against hypernatremia is the ability to produce maximally dilute urine (osmolality <100 mOsm/kg). This requires both suppression of ADH and adequate glomerular filtration to deliver solute to the collecting duct (solute diuresis). When ADH is suppressed and collecting ducts are impermeable to water, urine remains dilute. However, in states of volume depletion or hypotension (common in hypernatremia from extrarenal losses), the kidney preferentially reabsorbs sodium and water in proximal tubule and loop of Henle to restore intravascular volume, preventing adequate distal delivery of solute. This creates a secondary nephrogenic mechanism exacerbating hypernatremia. Additionally, sodium reabsorption in the collecting duct via epithelial sodium channels (ENaC) increases sodium concentration in the already concentrated medullary interstitium, perpetuating the osmotic gradient that opposes free water excretion.

• Central diabetes insipidus (renal water loss): Characterized by insufficient ADH secretion from the posterior pituitary. Common causes include pituitary surgery or traumatic brain injury (often transient), pituitary tumors (craniopharyngiomas, metastases), infiltrative diseases (sarcoidosis, histiocytosis X, tuberculosis), infections (meningitis, encephalitis), vascular events (Sheehan syndrome, cerebral aneurysm), and idiopathic forms. Central DI accounts for 10-15% of hypernatremia cases. The clinical hallmark is dramatic polyuria (urine output >15-20 L/day) with appropriately dilute urine (osmolality <100 mOsm/kg), which improves with ADH administration (desmopressin).

• Nephrogenic diabetes insipidus (renal water loss with ADH resistance): Results from kidney unresponsiveness to ADH despite high or normal ADH levels. Congenital forms arise from mutations in the AVPR2 gene (V2 receptor, X-linked) or AQP2 gene (aquaporin-2). Acquired nephrogenic DI occurs with chronic lithium use (blocks aquaporin-2 insertion and causes direct toxicity to collecting duct cells), amphotericin B (causes collecting duct damage), hypercalcemia (suppresses ADH effect through uncertain mechanisms), hypokalemia (impairs ADH-mediated water reabsorption), chronic kidney disease, and recovery phase of acute kidney injury (post-obstructive diuresis). Nephrogenic DI is characterized by high ADH levels with failure to concentrate urine appropriately—a key distinguishing feature from central DI.

• Osmotic diuresis (renal water loss): Occurs when non-reabsorbable solutes in the tubular lumen osmotically draw water into the filtrate, preventing water reabsorption despite adequate ADH. Common causes include hyperglycemia in diabetic ketoacidosis (glucose exceeds renal threshold and acts as osmotic diuretic), mannitol administration (used therapeutically for cerebral edema), contrast agents, and excessive urea from high-protein feeding. In diabetic hyperglycemic hyperosmolar state, glucose-induced osmotic diuresis combines with impaired thirst and access to hyperosmolar fluids to cause profound hypernatremia.

• Insensible losses (extrarenal water loss): Fever increases insensible losses through skin and respiratory tract by approximately 10-15% per degree Celsius above 37°C. Mechanical ventilation increases respiratory water loss, particularly in hot, dry environments. Burns cause massive insensible losses through damaged skin. Inadequate humidification in ventilator circuits or high-flow oxygen therapy increases respiratory losses. These losses are typically hypotonic (low sodium content) and thus selectively deplete free water.

• Gastrointestinal losses: Diarrhea and small bowel/ileostomy output cause water losses; because these fluids contain less sodium than plasma (typically 40-80 mEq/L), the net result is relative free water depletion. Patients with small bowel obstruction or high-output fistulas may lose 2-3 L/day of fluid. Vomiting causes sodium and water loss but also volume depletion stimulating ADH secretion, partially offsetting hypernatremia.

• Inadequate water intake: Occurs in patients unable to communicate thirst (sedation, intubation, dementia, stroke) or unable to access water (physical restraint, neglect). The elderly have impaired thirst mechanisms and reduced ability to concentrate urine, making them particularly vulnerable even with modest water deficits. Patients on restrictive fluid diets may develop hypernatremia, particularly if concurrently experiencing insensible losses or on diuretics. Infants cannot communicate thirst and depend entirely on caregivers for adequate free water intake, making formula preparation errors (excessive powder without adequate water) a significant cause.

• Hypertonic sodium intake (less common, but important): Rare but dramatic causes include accidental or iatrogenic hypertonic saline administration (3% or 23.4% NaCl), primary hyperaldosteronism (though typically mild hypernatremia), ingestion of sea water in drowning near-recovery, and rarely, salt poisoning from salt tablets or industrial accidents.

• Risk factors amplifying risk: Age extremes (infants, elderly), altered consciousness or inability to communicate, critical illness with mechanical ventilation, diuretic use, and CNS pathology.

• Thirst: Often the earliest symptom in conscious, alert patients with access to water. Stimulated by osmoreceptors when serum osmolality exceeds ~295 mOsm/kg (corresponding to [Na+] >145 mEq/L). Thirst is so sensitive that it is considered the most reliable defense against hypernatremia—its absence in a patient developing hypernatremia is a red flag for impaired thirst mechanisms (elderly, CNS pathology, sedation, dementia).

• Neurologic symptoms (acute hypernatremia): Cerebral cellular dehydration causes a spectrum of neurologic manifestations. Early symptoms include restlessness, irritability, lethargy, and confusion. As hypernatremia progresses, patients develop altered mental status, hallucinations, and delirium. In severe acute hypernatremia ([Na+] >160 mEq/L with rapid development), seizures may occur due to cerebral edema from ruptured vessels and hemorrhage. The mechanism is that water shifts out of neurons when osmolality increases, causing cell shrinkage and stretching of cerebral vessels, leading to rupture and hemorrhage into the brain parenchyma and subarachnoid space. Coma and death can result from severe cerebral hemorrhage or from the metabolic consequences of extreme hypernatremia.

• Neurologic symptoms (chronic hypernatremia): Patients with slowly developing hypernatremia to [Na+] 150-160 mEq/L may have minimal neurologic symptoms due to adaptive mechanisms. However, they may experience mild lethargy, weakness, or non-specific malaise. Older patients may present with insidious decline in mental status mistaken for dementia. Chronic hypernatremia increases risk of falls from weakness and altered sensorium.

• Thirst-driven polyuria cycle: In central DI, patients develop profound polydipsia and polyuria (often >15-20 L/day). This biphasic presentation—massive urine output coupled with compensatory massive fluid intake—is pathognomonic. If water intake is interrupted (hospitalization with IV dextrose instead of free water, inability to access water), rapid hypernatremia develops. Some patients develop psychogenic polydipsia as a psychiatric symptom, leading to habitual excessive water intake.

• Symptoms from underlying etiology: Fever suggests infection (meningitis causing central DI, sepsis causing insensible losses). Polyuria in context of recent neurosurgery suggests post-operative DI. High urine output in a patient on lithium suggests drug-induced nephrogenic DI. Hyperglycemia with fruity breath suggests DKA with osmotic diuresis. Diarrhea suggests gastroenteritis as cause of extrarenal loss.

• Physical exam findings: Dry mucous membranes and tongue (though may be absent if patient has received IV fluids). Skin turgor changes (skin tenting, though less reliable in elderly due to loss of skin elasticity). Tachycardia from volume depletion (if extrarenal losses predominate). Orthostatic hypotension if significant intravascular volume depletion. Muscle weakness and hyporeflexia from intracellular dehydration. Altered mental status ranging from confusion to coma depending on severity and acuity. Seizures or focal neurologic deficits (from hemorrhage) in severe acute cases. Importantly, absence of signs of volume depletion (preserved blood pressure, normal skin turgor, no tachycardia) suggests pure water loss without sodium loss (DI) rather than mixed losses.

• Infants and young children: Present with irritability, high-pitched cry, lethargy, and poor feeding. May have convulsions even at lower sodium levels. Fontanel may be sunken (in contrast to bulging seen with cerebral edema). Severe cases present with agonal respirations and cardiovascular collapse.

• Serum sodium concentration ([Na+] >145 mEq/L): Diagnostic threshold. Values 145-150 mEq/L represent mild hypernatremia, 150-160 mEq/L moderate, and >160 mEq/L severe (associated with mortality ~70%). Measured by standard electrolyte panel; pseudohypernatremia can occur with extreme hyperlipidemia or hyperproteinemia (flame photometry artifact), though ion-selective electrode methods avoid this error. Calculated serum osmolality = 2[Na+] + [glucose]/18 + [BUN]/2.8; in hypernatremia, measured osmolality typically exceeds 295 mOsm/kg.

• Urine osmolality and volume: Critical next step after confirming hypernatremia. Normal kidney response to hypernatremia is production of maximally dilute urine with osmolality <100 mOsm/kg and high volume (reflecting attempted compensation). A spot urine osmolality >300 mOsm/kg in the setting of hypernatremia is abnormal and suggests either nephrogenic DI (with ADH resistance) or extrarenal losses (volume depletion stimulating ADH-mediated reabsorption). Urine osmolality 100-300 mOsm/kg is intermediate and suggests partial ADH deficiency (partial central DI) or early stages of disease. Measurement of 24-hour urine volume >3 L with low osmolality confirms polyuric DI. In contrast, oliguric hypernatremia (low urine output, elevated osmolality) indicates extrarenal losses with secondary ADH stimulation.

• ADH (vasopressin) level: Measured concurrently with serum and urine osmolality. Low or undetectable ADH with elevated serum osmolality (osmolality >295 mOsm/kg) confirms central DI. High ADH (typically >5 pg/mL, normal being 0-5) with inability to concentrate urine confirms nephrogenic DI. This distinction is crucial for treatment selection.

• Desmopressin stimulation test: If ADH levels are unavailable or initial presentation is unclear. Administer desmopressin (synthetic ADH) 10 mcg intranasal or 1 mcg subcutaneously and reassess urine osmolality 60 minutes later. Increase in urine osmolality to >600 mOsm/kg indicates central DI (kidneys respond to exogenous ADH). Failure to concentrate urine despite desmopressin indicates nephrogenic DI (kidneys unresponsive). This test is considered diagnostic gold standard for distinguishing central from nephrogenic DI.

• Urine sodium concentration: Helps distinguish extrarenal losses (low urine [Na+], typically <20 mEq/L, as kidney tries to conserve sodium) from renal losses via diuretics or osmotic agents (high urine [Na

Step 1 — Stabilize hemodynamics first

• Isotonic crystalloid (0.9% NaCl or lactated Ringer's): if hypotensive, tachycardic, or in shock, restore intravascular volume before free-water repletion. Normal saline is hypotonic relative to a patient with Na⁺ 170 and will still lower serum sodium.

Step 2 — Calculate and replace the free water deficit

• Free water deficit = total body water × ([Na⁺]/140 − 1), with TBW ≈ 0.6 × kg (men), 0.5 × kg (women), less in the elderly. The Adrogué–Madias formula estimates the ΔNa⁺ per liter of a chosen infusate.
• Enteral free water (PO or via NG tube) is preferred when the gut works — safest and avoids dextrose load.
• Hypotonic IV fluid: D5W, or 0.45% NaCl when concurrent volume depletion exists. Add ongoing urinary and insensible losses to the calculated deficit.
• Rate: correct chronic (>48 h) hypernatremia slowly — roughly 0.5 mEq/L/hour, not more than about 10 mEq/L in 24 hours — because cerebral organic osmolytes take days to dissipate. Acute hypernatremia (<48 h, e.g., salt ingestion, post-op DI) may be corrected faster. Recheck sodium every 2–4 hours.

Step 3 — Treat the cause

• Central DI: vasopressin analog — desmopressin (DDAVP), intranasal, oral, or parenteral.
• Nephrogenic DI: stop the offender (lithium, amphotericin B); correct hypercalcemia/hypokalemia. Thiazide diuretic (hydrochlorothiazide) with low-solute diet induces mild volume contraction and proximal reabsorption; amiloride blocks ENaC-mediated lithium uptake and is preferred in lithium-induced disease; NSAID (indomethacin) is adjunctive.
• Osmotic diuresis: insulin therapy per ADA for hyperglycemia; stop mannitol.
• Sodium overload: loop diuretic plus free water; hemodialysis if oliguric renal failure.

Complications of hypernatremia itself

• Intracranial hemorrhage (EMERGENCY): acute brain shrinkage stretches and tears bridging veins and small parenchymal vessels, producing subdural, subarachnoid, or intraparenchymal hemorrhage. Signaled by seizure, focal deficit, or abrupt coma out of proportion to the sodium level; obtain non-contrast head CT.
• Cerebral venous / dural sinus thrombosis: hyperosmolar hemoconcentration and sluggish flow; classically described in dehydrated infants. Signaled by seizures, bulging fontanelle, or hemorrhagic venous infarct on imaging.
• Rhabdomyolysis: myocyte shrinkage and membrane dysfunction in severe hyperosmolality. Signaled by weakness with markedly elevated creatine kinase and pigmented granular casts; may precipitate AKI and hyperkalemia.
• Hypovolemic shock and prerenal AKI: when hypernatremia stems from GI, insensible, or osmotic losses. Rising BUN/creatinine ratio with low urine sodium.
• Insulin resistance and impaired hepatic gluconeogenesis in critical illness contribute to the high attributable mortality noted earlier.

Complications of treatment

• Cerebral edema from overrapid correction (EMERGENCY): brain cells that generated organic osmolytes cannot shed them quickly; a falling extracellular osmolality drives water into neurons. Signaled by headache, vomiting, new seizure, or declining mental status while sodium is improving — stop hypotonic fluid, consider hypertonic saline, and image.
• Hyperglycemia and worsened osmotic diuresis from high-rate D5W — monitor glucose.
• Volume overload / pulmonary edema in heart failure or CKD receiving large repletion volumes.
• Desmopressin-induced hyponatremia: unopposed V2 water retention if free water intake continues; the classic overshoot after treating central DI.
• Thiazide therapy in nephrogenic DI: hypokalemia, hyponatremia, and — importantly — reduced lithium clearance with lithium toxicity, which is why amiloride is favored. NSAIDs risk AKI and GI bleeding.

• Hypernatremia is a thirst/access problem until proven otherwise: an awake patient with an intact thirst mechanism and a water pitcher will not stay hypernatremic even with florid DI. A hypernatremic adult means impaired thirst (hypothalamic lesion, dementia, intubation/sedation) or no access (restraints, infancy, neglect).
• Single best next step after confirming Na⁺ >145 is urine osmolality, not head imaging and not empiric desmopressin. Dilute urine (<300 mOsm/kg) with polyuria = DI or osmotic-free water loss; concentrated urine (>600–800) points to extrarenal or insensible loss with intact ADH.
• Desmopressin challenge separates the two DIs: urine osmolality rises substantially → central DI; no response → nephrogenic. Copeptin (a surrogate for AVP) is the modern replacement for direct ADH assays.
• Lithium is the examiners' favorite cause of nephrogenic DI — it blocks aquaporin-2 insertion via ENaC-mediated entry into principal cells. The tested answer is amiloride, not a thiazide, because thiazides raise lithium levels.
• Stabilize before you dilute: in a hypotensive patient, isotonic saline comes first; free water repletion follows. Boards punish giving D5W to a patient in shock.
• Correct chronic hypernatremia slowly (about 10 mEq/L per 24 h). The complication of overrapid correction is cerebral edema and seizures — mirror image of hyponatremia.
• Common distractor: osmotic demyelination / central pontine myelinolysis belongs to overrapid correction of hyponatremia. Do not select it for hypernatremia.
• Infants: hypernatremic dehydration shows a sunken fontanelle and doughy skin; a bulging fontanelle during therapy signals iatrogenic cerebral edema. Improperly concentrated formula is the classic stem.
• In hyperglycemia, use the corrected sodium — measured sodium is diluted by glucose-driven water shift, so a "normal" Na⁺ in HHS may conceal a large free water deficit.