Hyponatremia — Approach and Treatment

• Inappropriate ADH secretion (SIADH) — The fundamental disturbance in most hyponatremia cases involves non-osmotic ADH release, where ADH continues to be secreted despite low serum osmolality and plasma volume expansion. This occurs through various mechanisms: malignancy (small cell lung cancer produces ectopic ADH), CNS disease (meningitis, encephalitis, head trauma, subarachnoid hemorrhage activate osmoreceptor-independent neural pathways), pulmonary disease (pneumonia, positive-pressure ventilation), and medications (SSRIs enhance ADH sensitivity at the V2 receptor, carbamazepine stimulates ADH release). The persistently elevated ADH concentration maintains aquaporin-2 water channels open in the collecting duct, promoting free water reabsorption and dilution of serum sodium. Hypovolemic SIADH can also occur with severe volume depletion when baroreceptor-mediated ADH release overrides osmotic inhibition. The result is retention of hypotonic fluid, progressive hyponatremia, and inappropriate urinary osmolality (>200 mOsm/kg despite low serum osmolality).

• Renal free water excretion deficiency — The kidney's capacity to excrete free water depends on three elements: adequate glomerular filtration of water and electrolytes, active reabsorption of sodium in the thick ascending limb (which dilutes tubular fluid and makes the collecting duct impermeable to water when ADH is low), and an osmotic gradient for water to follow sodium. Hypokalemia impairs this mechanism by reducing the positive potential in the lumen needed for paracellular sodium reabsorption in the thick ascending limb, and by directly stimulating ADH release through volume depletion of the vasa recta. Hypomagnesemia similarly impairs sodium transport. Advanced renal disease reduces the number of functional nephrons and thus the capacity for free water excretion. The beer potomania phenomenon—where consumption of large volumes of dilute, sodium-poor fluid overwhelms the kidney's maximal free water excretion capacity (approximately 500-800 mL/hour in healthy kidneys)—illustrates this principle. Diuretic use, particularly thiazides, causes sodium loss and hypovolemia; thiazides also impair the thick ascending limb dilution mechanism necessary for free water excretion.

• Volume depletion-mediated ADH activation — Baroreceptors in the carotid sinus and aortic arch detect decreases in effective circulating volume and trigger ADH release independent of osmotic stimuli. This occurs in conditions causing gastrointestinal sodium loss (vomiting, diarrhea, nasogastric suctioning), renal sodium losses (diuretics, adrenal insufficiency), or third-spacing (peritonitis, pancreatitis, sepsis). Even modest volume depletion (5-10%) can suppress the osmotic threshold for ADH suppression from 280 mOsm/kg to as low as 260-265 mOsm/kg, meaning ADH remains detectable despite hyposmolality. This baroreceptor-mediated ADH release is a physiologic defense of circulating volume at the expense of osmotic homeostasis. The kidney, receiving signals to conserve sodium and water, becomes unable to excrete dilute urine despite serum hyposmolality. This mechanism explains why hypovolemic hyponatremia often coexists with oliguria and fractional excretion of sodium (FENa) <1%, resembling prerenal azotemia.

• Extrarenal free water loss with inadequate replacement — Insensible losses (sweat, respiration, skin) normally represent approximately 800-1000 mL/day of free water loss. In conditions increasing these losses (fever, hyperventilation, burns), or gastrointestinal losses (diarrhea), when patients cannot access free water or lack thirst sensation (elderly, altered mental status, severe illness), hyponatremia develops. Diarrheal losses are particularly rich in sodium and potassium but relatively poor in water; severe diarrhea can cause both sodium depletion and free water deficit.

• Psychogenic polydipsia and diabetes insipidus variants — In psychogenic polydipsia, psychiatric patients chronically consume excessive hypotonic fluids beyond renal free water excretion capacity, overwhelming the kidney's defensive mechanisms. In nephrogenic diabetes insipidus (NDI), the collecting duct epithelium is insensitive to ADH due to V2 receptor mutations (genetic forms), medications (lithium, demeclocycline, amphotericin B, NSAIDs), hyperkalemia, or hypercalcemia, resulting in inability to concentrate urine despite high ADH levels and risk of hypernatremia if water intake is inadequate. Central diabetes insipidus causes hyponatremia primarily when patients cannot access water, but if they can drink freely, they typically maintain normal sodium concentration through thirst.

• Euvolemic hyponatremia with high urinary osmolality (>200 mOsm/kg) — This constellation indicates inappropriate ADH secretion despite normal or low plasma osmolality. SIADH is the classic diagnosis and encompasses: malignancy (small cell lung cancer in 10-15% of cases, also adenocarcinoma of lung, pancreas, bladder, GI tract), CNS disease (meningitis/encephalitis from any pathogen, head trauma, subarachnoid hemorrhage, acute hypoxemia, seizures, multiple sclerosis, Guillain-Barré syndrome, pituitary surgery), pulmonary disease (pneumonia, tuberculosis, positive-pressure ventilation, pneumothorax), medications (SSRIs and SNRIs are most common, carbamazepine, oxcarbazepine, vincristine, cisplatin, desmopressin, NSAIDs, chlorpropamide, haloperidol, tricyclic antidepressants, and 3,4-methylenedioxymethamphetamine [MDMA/"Ecstasy"]), and idiopathic forms. Hypothyroidism and adrenal insufficiency reduce ADH clearance and impair free water excretion through reduced cardiac output and renal perfusion, respectively.

• Euvolemic hyponatremia with low urinary osmolality (<200 mOsm/kg) — This pattern indicates appropriate suppression of ADH but excessive free water intake overwhelming renal excretion capacity. Psychogenic polydipsia (compulsive water drinking in psychiatric disease, particularly schizophrenia) is the classic example. Primary polydipsia from impaired thirst osmoregulation (hypothalamic disease, sarcoidosis) also presents this way. These conditions require normal renal function and intact diluting capacity (normal thick ascending limb function).

• Hypovolemic hyponatremia — Loss of sodium-containing fluid (total body water loss exceeds total body sodium loss) with secondary ADH stimulation. GI losses include vomiting, diarrhea (cholera, infectious gastroenteritis), and nasogastric suctioning—these typically cause hypotonic fluid loss because GI secretions contain sodium at 20-100 mEq/L (much less than plasma). Renal losses occur with diuretics (especially thiazides, which cause hyponatremia in 5-15% of users, particularly in elderly women on long-term therapy), adrenal insufficiency (aldosterone deficiency reduces renal sodium reabsorption), renal tubular acidosis (type 2 RTA impairs proximal sodium reabsorption; type 4 RTA causes aldosterone deficiency), and osmotic diuresis (hyperglycemia, mannitol, urea). In volume depletion, urinary sodium is typically <20 mEq/L (the kidney appropriately tries to conserve sodium) and urinary osmolality is high (>400 mOsm/kg) as ADH is stimulated. A useful distinguishing feature is FENa: in hypovolemia from GI losses, FENa is <0.1% (kidney avidly reabsorbs sodium), whereas with diuretics or adrenal insufficiency, FENa may be higher initially before sodium depletion becomes severe.

• Hypervolemic hyponatremia — Expansion of total body water exceeds expansion of total body sodium, causing dilution despite sodium excess. Congestive heart failure (decreased effective circulating volume triggers baroreceptor-mediated ADH release despite volume expansion and hyponatremia; neurohormonal activation of the renin-angiotensin-aldosterone system promotes sodium reabsorption, perpetuating dilution), hepatic cirrhosis with ascites (similar mechanism with splanchnic vasodilation and third-spacing), and nephrotic syndrome (decreased plasma oncotic pressure, third-spacing, and renin-angiotensin-aldosterone system activation) are the typical causes. Acute kidney injury and chronic kidney disease (reduced glomerular filtration impairs water excretion; hypervolemia and secondary hyperaldosteronism). The urine sodium is typically >20 mEq/L (kidney is attempting to excrete sodium but cannot overcome the stimulus to retain fluid from neurohormonal activation) and urinary osmolality is variable but often >200 mOsm/kg.

• Medication-related hyponatremia — Beyond SIADH-inducing agents, medications cause hyponatremia through multiple mechanisms: SSRIs/SNRIs (enhance SIADH), desmopressin (exogenous ADH), NSAIDs (enhance ADH effect and reduce renal perfusion), thiazide diuretics (cause sodium loss and impair free water excretion), carbamazepine (stimulates ADH), vincristine (SIADH), cisplatin (SIADH), chlorpropamide (enhances ADH effect), and haloperidol. Recreational drugs including MDMA cause SIADH through increased ADH release and enhanced renal aquaporin-2 expression.

• Neurological symptoms from cerebral edema — Mild hyponatremia (130-135 mEq/L acutely) causes nausea and malaise as the earliest manifestations. As sodium falls further, headache develops from increased intracranial pressure. With moderate acute hyponatremia (125-130 mEq/L), confusion and altered mental status become prominent; patients appear lethargic and may have difficulty concentrating. The confusion is non-focal and global, reflecting diffuse cerebral edema rather than a focal lesion. Severe acute hyponatremia (<125 mEq/L) causes seizures, coma, and respiratory depression. Seizures occur when increased intracranial pressure causes herniation or when hypotonic edema of the brain reaches critical levels; these seizures are typically generalized tonic-clonic. Abnormal pupillary responses and posturing suggest brainstem herniation from severe, uncontrolled cerebral edema.

• Respiratory symptoms and signs — Dyspnea and cough may result from pulmonary edema if the underlying condition is heart failure or renal failure (hypervolemic hyponatremia). In acute hyponatremia with severe neurological impairment, respiratory depression from cerebral edema affecting the medullary respiratory centers may occur.

• Physical examination findings — Orthostatism, dry mucous membranes, reduced skin turgor, and decreased urine output suggest volume depletion (hypovolemic hyponatremia). Peripheral edema, elevated jugular venous pressure, hepatomegaly, and ascites indicate volume overload (hypervolemic hyponatremia). Euvolemic patients appear clinically well-hydrated. Neurological examination may reveal confusion, lethargy, and hyperreflexia. In severe acute hyponatremia, pupils may become unequal (anisocoria) or fixed, reflecting herniation.

• Important clinical variants — Asymptomatic chronic hyponatremia is common when the process develops slowly (over weeks to months), allowing full cerebral adaptation; such patients may have sodium levels of 115-120 mEq/L yet remain completely asymptomatic. Thiazide-induced hyponatremia typically presents acutely (within 1-2 weeks of starting the drug) in elderly women with symptoms disproportionate to the degree of hyponatremia; this exaggerated symptom burden relates to both the acute nature and the superimposition on chronically impaired water excretion. SIADH-related hyponatremia typically develops over days to weeks (semi-acutely) and may present with vague symptoms (malaise, anorexia) before neurological signs appear. Psychogenic polydipsia usually causes gradual hyponatremia and is often discovered incidentally on laboratory testing; symptomatic presentations reflect extreme sodium levels or rapid development.

• Initial laboratory assessment and diagnostic criteria — Serum sodium <135 mEq/L defines hyponatremia. Serum osmolality should be measured or calculated (2×[Na+] + [glucose]/18 + [BUN]/2.8); truly hypo-osmolar hyponatremia (osmolality <280 mOsm/kg) indicates water excess relative to sodium, whereas hypertonic hyponatremia (osmolality >295 mOsm/kg) occurs with hyperglycemia or mannitol use and is a laboratory artifact (pseudo-hyponatremia). Urine osmolality is the key discriminating test: high urine osmolality (>200 mOsm/kg) indicates ADH effect and includes SIADH, hypovolemia, and hypervolemia; low urine osmolality (<200 mOsm/kg) indicates appropriate ADH suppression and occurs with primary polydipsia or psychogenic polydipsia. Urine sodium concentration helps differentiate the cause of high urine osmolality: in hypovolemic hyponatremia from GI losses, urine sodium is <20 mEq/L

Immediate stabilisation — severe symptoms (seizure, coma, obtundation, respiratory arrest)

• Hypertonic (3%) saline: the only intervention that reverses cerebral edema quickly. The US expert panel endorses a 100 mL IV bolus over ~10 minutes, repeatable up to three times until symptoms abate; a rise of 4–6 mEq/L is usually enough to stop seizures. Bolus dosing is preferred over continuous infusion because it is self-limited and less likely to overshoot.
• Airway protection and ICU-level monitoring, with serum sodium rechecked every 1–2 hours during active correction.
• Correction limits: total rise should not exceed roughly 8 mEq/L in 24 hours in patients at high osmotic demyelination risk (sodium <105 mEq/L, hypokalemia, alcohol use disorder, malnutrition, advanced liver disease); slightly higher ceilings (10–12 mEq/L) are tolerated only in low-risk acute cases. Remember that potassium given to correct concurrent hypokalemia is osmotically active and itself raises serum sodium — it counts toward the 24-hour rise and must be built into the correction ceiling, or overcorrection will occur despite "appropriate" saline dosing.

Cause-directed first-line therapy (asymptomatic or mildly symptomatic)

• Hypovolemic: isotonic crystalloid (0.9% saline). Volume repletion shuts off baroreceptor-driven ADH, which can trigger a brisk water diuresis and abrupt overcorrection — anticipate it.
• Euvolemic/SIADH: fluid restriction is first line; add oral sodium chloride tablets and/or a loop diuretic (furosemide) when urine osmolality is high, since the loop agent blunts urinary concentrating ability. Oral urea is an effective osmotic option.
• Hypervolemic (HFrEF, cirrhosis): sodium and water restriction plus loop diuretics, layered onto guideline-directed therapy — for HFrEF the ACC/AHA/HFSA quadruple regimen of ARNI (or ACEI/ARB), beta blocker, MRA, and SGLT2 inhibitor.

Escalation and definitive measures

• Vasopressin V2-receptor antagonists: tolvaptan (oral) or conivaptan (IV) produce aquaresis in SIADH or heart failure refractory to restriction; FDA labeling requires inpatient initiation and caps duration — tolvaptan for no more than 30 days because of hepatotoxicity, and conivaptan for no more than 4 days.
• Demeclocycline is a rarely used alternative that induces nephrogenic DI.
• Definitive management is treating the driver: stop the offending drug (SSRI, thiazide, carbamazepine, desmopressin), treat pneumonia or CNS disease, replace glucocorticoid in adrenal insufficiency, resect or treat small cell lung cancer.

Contraindicated / avoid

• Isotonic saline in SIADH — when urine osmolality exceeds that of the infusate, the salt is excreted and free water retained, worsening hyponatremia (desalination).
• Vaptans in hypovolemic hyponatremia and, per AASLD, tolvaptan in cirrhosis because of hepatotoxicity.
• Rapid correction of chronic hyponatremia; also avoid fluid restriction as the answer in a seizing patient.

Complications of untreated hyponatremia

• Cerebral edema with herniation — emergency. Hypotonic plasma drives water into astrocytes; in acute hyponatremia the brain has not yet extruded organic osmolytes, so swelling occurs inside a fixed skull. Signalled by headache and vomiting progressing to generalized tonic-clonic seizures, obtundation, anisocoria, posturing, and Cushing reflex. Highest risk in menstruating women, children, marathon runners, and post-operative patients given hypotonic fluids.
• Neurogenic (non-cardiogenic) pulmonary edema and respiratory arrest — brainstem compression and central hypoventilation; hypoxia worsens cerebral swelling in a vicious cycle.
• Falls, gait instability, attention deficits, and osteoporosis with fracture — even "asymptomatic" chronic hyponatremia causes subtle cerebellar/attentional dysfunction, and chronic hyponatremia mobilizes bone sodium stores.

Complications of treatment

• **Osmotic demyelination syndrome (ODS, central pontine myelinolysis)** — emergency and largely irreversible. In chronic hyponatremia the brain has already shed organic osmolytes; too-rapid restoration of tonicity draws water out of oligodendrocytes faster than osmolytes can be regenerated, causing non-inflammatory demyelination of the pons and extrapontine sites. Classically delayed 2–6 days after correction: initial improvement, then dysarthria, dysphagia, spastic quadriparesis, pseudobulbar palsy, and the locked-in syndrome; extrapontine lesions produce parkinsonism and dystonia. MRI is typically normal for the first several days, so a normal early scan does not exclude it. Risk is amplified by hypokalemia, alcohol use disorder, malnutrition, liver disease or transplantation, and sodium below roughly 105 mEq/L.
• Overcorrection after removal of the ADH stimulus — repleting volume, stopping a thiazide, or giving glucocorticoid in adrenal insufficiency abruptly switches off ADH; a sudden dilute water diuresis sends sodium up fast. Signalled by a large-volume, low-osmolality urine output. Rescue with desmopressin plus hypotonic fluid (D5W) to relower sodium, per the US expert panel.
• Volume overload and pulmonary edema from hypertonic or isotonic saline in heart failure or cirrhosis.
• Vaptan-related complications: overly rapid aquaresis, thirst, and tolvaptan hepatotoxicity; demeclocycline causes nephrogenic diabetes insipidus, photosensitivity, and nephrotoxicity.

• Seizing hyponatremic patient — single best next step is 3% hypertonic saline, not fluid restriction, not normal saline, not a vaptan. A 4–6 mEq/L rise stops the seizure; the entire deficit is never corrected acutely.
• The correction speed rule: no more than about 8 mEq/L in 24 hours in anyone at risk for osmotic demyelination, and remember that potassium repleted for concurrent hypokalemia also raises serum sodium and counts toward that ceiling. Too fast → central pontine myelinolysis / locked-in syndrome, appearing days later with a normal early MRI. Too slow in acute symptomatic hyponatremia → herniation. This trade-off is the single association examiners test most.
• SIADH laboratory signature: euvolemia, urine osmolality inappropriately high, urine sodium typically >30–40 mEq/L on a normal salt intake, plus low serum uric acid and low BUN from dilution and enhanced urate clearance. Always exclude hypothyroidism and adrenal insufficiency before calling it SIADH.
• Giving normal saline to a patient with SIADH can lower the sodium further (desalination) when urine osmolality exceeds infusate osmolality. Classic distractor answer choice.
• Correct for hyperglycemia before acting: measured sodium falls roughly 1.6–2.4 mEq/L for every 100 mg/dL rise in glucose. Translocational (hypertonic) hyponatremia from hyperglycemia does not require hypertonic saline — the sodium normalizes as glucose is lowered with insulin and volume is repleted with isotonic fluid per ADA-endorsed DKA/HHS protocols; these patients are profoundly volume depleted, so insulin without fluid resuscitation is harmful.
• Pseudohyponatremia from severe hypertriglyceridemia or paraproteinemia (multiple myeloma) gives a low sodium with a normal measured serum osmolality — a lab artifact of flame photometry/indirect ion-selective electrodes. No treatment.
• Cerebral salt wasting vs SIADH after subarachnoid hemorrhage or neurosurgery: both have high urine sodium, but CSW patients are hypovolemic and are treated with salt and volume, whereas SIADH is treated with restriction. Volume status is the discriminator.
• Thiazides, SSRIs, carbamazepine, desmopressin, and MDMA are the drug stems; small cell lung cancer is the malignancy stem. If overcorrection occurs, desmopressin plus D5W is the rescue.