Renal · Electrolyte Disorders
The facts most likely to be tested
Severe symptomatic hyponatremia (seizures, coma) requires immediate treatment with hypertonic 3% saline to prevent cerebral edema and herniation.
Rapid overcorrection of chronic hyponatremia risks osmotic demyelination syndrome (formerly central pontine myelinolysis), characterized by quadriplegia and pseudobulbar palsy.
SIADH is a diagnosis of exclusion characterized by euvolemic hyponatremia, low serum osmolality, and inappropriately concentrated urine (>100 mOsm/kg).
Hypovolemic hyponatremia is differentiated from SIADH by the presence of orthostatic hypotension, tachycardia, and a low urine sodium (<20 mEq/L) in the setting of extrarenal volume loss.
Pseudohyponatremia occurs in patients with hyperlipidemia or hyperproteinemia because these substances displace water in the serum sample, leading to a falsely low sodium measurement.
Thiazide diuretics are a classic cause of hypovolemic hyponatremia in elderly patients due to impaired diluting capacity of the distal tubule.
The correction rate for chronic hyponatremia should not exceed 6–8 mEq/L in any 24-hour period to minimize the risk of osmotic demyelination.
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A 72-year-old woman is brought to the emergency department after a generalized tonic-clonic seizure. Her medical history is significant for hypertension treated with hydrochlorothiazide. Physical examination reveals dry mucous membranes, poor skin turgor, and orthostatic hypotension. Laboratory studies show a serum sodium of 118 mEq/L, serum osmolality of 245 mOsm/kg, and a urine sodium of 12 mEq/L.
What is the most appropriate initial management for this patient?
Hypertonic 3% saline infusion
The patient presents with severe, symptomatic hyponatremia (seizures), which mandates immediate administration of hypertonic saline regardless of the underlying etiology to prevent permanent neurological damage.
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Etiology / Epidemiology
Common in elderly and hospitalized patients; caused by excess free water relative to sodium.
Clinical Manifestations
Symptoms range from lethargy to seizures; cerebral edema is the primary concern.
Diagnosis
Serum sodium < 135 mEq/L is diagnostic; assess serum osmolality to categorize.
Treatment
Hypertonic saline (3%) for severe symptoms; fluid restriction for euvolemic cases.
Prognosis
Rapid correction risks osmotic demyelination syndrome; limit correction to < 8 mEq/L/24h.
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Epidemiology & Etiology
Most common electrolyte disorder in clinical practice, frequently seen in patients with CHF, cirrhosis, or SIADH. Iatrogenic causes include thiazide diuretics and excessive hypotonic IV fluids. Elderly patients are at highest risk due to impaired renal water excretion and polydipsia.
Pertinent Anatomy
The hypothalamus regulates thirst and antidiuretic hormone (ADH) release. The collecting duct of the nephron is the primary site of water reabsorption regulated by ADH, which dictates final urine concentration.
Pathophysiology
Hyponatremia is almost always a disorder of water balance rather than total body sodium. ADH release leads to water retention, diluting serum sodium. Severe cases cause cellular swelling as water shifts into the intracellular space, leading to increased intracranial pressure.
Clinical Manifestations
Early signs include nausea, malaise, and headache. Progression leads to seizures, coma, and respiratory arrest due to cerebral edema. Chronic hyponatremia is often asymptomatic as the brain adapts by extruding intracellular solutes.
Diagnosis
Confirm with serum sodium < 135 mEq/L. Calculate serum osmolality to distinguish between isotonic, hypertonic, and hypotonic states. Check urine sodium and urine osmolality to differentiate between hypovolemic, euvolemic, and hypervolemic etiologies.
Treatment
For severe symptoms, use 3% hypertonic saline with frequent monitoring. For hypovolemic patients, use isotonic saline. For SIADH, initiate fluid restriction or demeclocycline. Avoid rapid correction to prevent osmotic demyelination syndrome.
Prognosis
Prognosis depends on the rate of correction. Osmotic demyelination syndrome (formerly central pontine myelinolysis) presents with quadriplegia and pseudobulbar palsy. Maintain strict correction limits of < 8 mEq/L per 24 hours.
Differential Diagnosis
SIADH: euvolemic with high urine osmolality
Hypovolemic hyponatremia: low urine sodium (< 20 mEq/L)
Pseudohyponatremia: normal serum osmolality (hyperlipidemia/proteinemia)
Hyperglycemia: low sodium due to osmotic shift (1.6 mEq/L drop per 100 mg/dL glucose)
Psychogenic polydipsia: low urine osmolality (< 100 mOsm/kg)