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BASICS

  • Defined as plasma sodium (Na⁺) ≤135 mEq/L
  • Can be hypovolemic, euvolemic, or hypervolemic
  • Affects endocrine, renal, cardiovascular, CNS systems

EPIDEMIOLOGY

  • Most common electrolyte disorder in hospitals (2.5-7.7%)
  • Elderly at increased risk due to impaired renal response and comorbidities
  • Children at risk of brain herniation from cerebral edema

ETIOLOGY AND PATHOPHYSIOLOGY

  • Serum osmolality categories:
  • Hypertonic (>295 mOsm/kg): solute accumulation (hyperglycemia, mannitol)
  • Isotonic (“pseudohyponatremia”, 275–295 mOsm/kg): lab artifact, hyperlipidemia

  • Hypotonic (<275 mOsm/kg): subdivided by volume status

  • Hypovolemic hypotonic hyponatremia:

  • Low TBW and Na⁺
  • Extrarenal Na loss: urine Na⁺ <30 mmol/L (vomiting, diarrhea, burns)

  • Renal Na loss: urine Na⁺ >30 mmol/L (diuretics, cerebral salt wasting, adrenal insufficiency)

  • Euvolemic hypotonic hyponatremia:

  • Mild increase in TBW, normal Na⁺
  • Urine Osm >100 mOsm/kg: SIADH, hypothyroidism, adrenal insufficiency, meds

  • Urine Osm <100 mOsm/kg: primary polydipsia, beer potomania

  • Hypervolemic hypotonic hyponatremia:

  • Increased TBW and Na⁺

  • Urine Na⁺ <30 mmol/L: CHF, cirrhosis, nephrotic syndrome, renal failure

  • Genetics:

  • Mutations in nephrogenic SIADH syndrome

COMMONLY ASSOCIATED CONDITIONS

  • Hypothyroidism, hypopituitarism
  • Cirrhosis, CHF, nephrotic syndrome
  • SIADH causes: cancers, infections, CNS diseases, HIV
  • Beer potomania, tea-and-toast diet
  • Ecstasy use, marathon runners in heat

DIAGNOSIS

  • Symptoms depend on severity and speed of Na decline:
  • Mild (130-135 mEq/L): often asymptomatic, fatigue
  • Moderate (120-130 mEq/L): nausea, vomiting, lethargy

  • Severe (<120 mEq/L): headache, disorientation, seizures, coma

  • Physical exam: volume status, neurological signs, underlying illness signs

  • Labs: serum osmolality, urine Na⁺, urine osmolality, BUN, creatinine, glucose, TSH, lipids
  • Imaging: chest X-ray, CT head if CNS pathology suspected

TREATMENT

  • Correct underlying cause
  • Hypovolemia: fluid resuscitation
  • Euvolemia/Hypervolemia: fluid restriction (1–1.5 L/day for euvolemic)
  • Severe symptomatic hyponatremia (<125 mEq/L): hypertonic saline 3%
  • Bolus 100–150 mL, goal increase serum Na⁺ by 2–3 mEq/L
  • Continuous infusion aiming ≤10–12 mEq/L/day correction
  • Serum Na⁺ monitored every 2 hours

  • Desmopressin may prevent overcorrection

  • Vasopressin V2 receptor antagonists (tolvaptan, conivaptan) for refractory cases (avoid in liver disease)

  • Demeclocycline for chronic SIADH if fluid restriction insufficient (caution in renal/hepatic disease)
  • Avoid rapid correction to prevent osmotic demyelination syndrome

ADMISSION AND NURSING CONSIDERATIONS

  • Admit symptomatic or acute hyponatremia (<48 hours) cases
  • Admit asymptomatic with Na⁺ <125 mEq/L for monitoring

ONGOING CARE

  • Fluid restriction for euvolemic/hypervolemic states
  • Monitor serum Na⁺ closely during treatment
  • Regular neurological assessments

DIET

  • Euvolemic: restrict water 1–1.5 L/day
  • Hypervolemic: restrict water and sodium

PROGNOSIS

  • Associated with increased morbidity and mortality in hospitalized patients
  • Chronic hyponatremia predicts poor outcomes in pulmonary embolism, liver disease
  • Risk of brain herniation in severe, untreated acute hyponatremia

COMPLICATIONS

  • Central pontine and extrapontine myelinolysis with rapid correction
  • Cerebral edema and herniation if untreated
  • Seizures, coma, respiratory arrest
  • Increased risk of osteoporosis, gait disturbances, falls in chronic cases

REFERENCES

  1. Rondon-Berrios H, Agaba EI, Tzamaloukas AH. Hyponatremia: pathophysiology, classification, manifestations and management. Int Urol Nephrol. 2014;46(11):2153-2165.
  2. Braun MM, Barstow CH, Pyzocha NJ. Diagnosis and management of sodium disorders: hyponatremia and hypernatremia. Am Fam Physician. 2015;91(5):299-307.
  3. Williams DM, Gallagher M, Handley J, et al. The clinical management of hyponatraemia. Postgrad Med J. 2016;92(1089):407-411.
  4. Singh TD, Fugate JE, Rabinstein AA. Central pontine and extrapontine myelinolysis: a systematic review. Eur J Neurol. 2014;21(12):1443-1450.
  5. Rozen-Zvi B, Yahav D, Gheorghiade M, et al. Vasopressin receptor antagonists for the treatment of hyponatremia: systematic review and meta-analysis. Am J Kidney Dis. 2010;56(2):325-337.
  6. Basu A, Ryder REJ. The syndrome of inappropriate antidiuresis is associated with excess long-term mortality: a retrospective cohort analyses. J Clin Pathol. 2014;67(9):802-806.

Clinical Pearls

  • Review all medications for hyponatremia risk.
  • Alcoholic and elderly women on thiazides are at risk of osmotic demyelination with rapid correction.
  • Common SIADH-associated cancers include bronchogenic carcinoma, pancreatic, duodenal, prostate cancers, thymoma, lymphoma, and mesothelioma.
  • Use Adrogue-Madias formula to guide safe correction of sodium levels:
    Sodium Correction Calculator