HYPERNATREMIA

hypernatremia

• Hypernatremia is defined as serum sodium concentration above 145 mEq/L.
• Hypernatremia is caused by either decreased water or increased salt concentration in the blood which is driven by the kidneys' handling of sodium and water.

High solute (Na) or hypertonic solution intake increases serum osmolality which stimulates water reabsorption across renal tubules thereby increasing urine osmolality. High serum Na level inhibits Na reabsorption from the urine leading to high urine Na level.
Loss of fluid from the body decreases effective circulating fluid (ECF) which stimulates water and Na reabsorption from renal tubules leading to high urine osmolality and low urine Na level.
With low or absent ADH activity, water is not reabsorpted from the tubular lumen leading to diluted urine (low urine osmolality) and polyuria. Concentrated blood (high serum osmolality) and hypernatremia often cause increased thirst.
Large amount of aqueous solute (glucose, mannitol, urea) in the urine increases urine osmolality which draws more water out from blood into the renal tubules causing high serum osmolality and high serum Na.

HYPERNATREMIA

✿ Extra-renal h2o loss ❷Loss of fluid causes low blood volume (and increased serum osmolality) hence lowers effective circulating fluid (ECF). This stimulates more water and salt reabsorption from renal tubules resulting in increased urine osmolality and decreased urine Na level.

  ✧ skin lossSignificant sweat (and water) loss during exercise or sensible loss during fever, hot weather

  ✧ gi lossIncludes vomiting, GI drainage, and osmotic (not secretory) diarrhea

  ✧ low water intakeLack of access to water, altered mental status

  ✧ hypodipsia Impaired thirst caused by hypothalamic lesions (metastatic cancer, craniopharyngioma, stroke, trauma, SAH, TB, sarcoidosis)


✿ Renal h2o loss
  ✧ diabetes insipidus ❸In central and nephrogenic DIs (causing reduced ADH production and ADH resistance, respectively), water is not reabsorpted from the tubular lumen leading to diluted urine (low urine osmolality) and polyuria. Concentrated blood (high serum osmolality) and hypernatremia often cause increased thirst. More on etiologies of DIs here.

  ✧ osmotic diuresis ❹Large amount of glucose (in diabetes), mannitol, urea (during recovery period after azotemia or post obstruction) in the urine increase urine osmolality which draws more water into the tubules and effectively raising serum Na level and osmolality.


✿ Extra-renal salt gain ❶High serum osmolality stimulates water reabsorption resulting in increased urine osmolality. High serum Na level inhibits Na reabsorption from the urine leading to high urine Na level. The net effect is net salt gain with high serum Na level and serum osmolality.

  ✧ excessive salt ingestionAccidental or surreptitious ingestion

  ✧ hypertonic saline infusionUsed in treatment of traumatic brain injury and hyponatremia

  ✧ sodium bicarb infusionUsed in treatment of metabolic acidosis

  ✧ isotonic saline replacement therapyUsed to replace hypotonic fluid loss from osmotic diuresis (diabetes, recovery from azotemia), nasogastic suction, or loop diuretic therapy


evaluation of hypernatremia


• The causes of hypernatremia can be determined based on urine osmolalilty and urine sodium level.
   Urine osmolalilty: reflects renal water handling (via aquaporins-V2 receptors), decreases when water is excreted and increases when water is reabsorbed, often influenced by the level of ADH activity.
   Urine sodium: reflects renal sodium handling (via Na channels), decreases when Na is reabsorbed and increases when Na is excreted.
• One diagnostic approach for hypernatremia is illustrated below.
   Urine osmolality is low (< 300 mOsm/kg or < serum osmolality) in diabetes insipidus and intermediate (300-600 mOsm/kg and > serum osmolality) in osmotic diuresis.
   If urine osmolality is high (> 600 mOsm/kg), urine sodium concentration (uNa) is used to further differentiate between extra-renal salt gain causes (uNa > 40 mmol/L), and extra-renal net water loss causes (uNa < 25 mmol/L).

hypernatremia algorithm

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