HYPONATREMIA
ISOTONIC (pseudohyponatremia) ❶The presence on non-aqueous solute (lipids, proteins) raises the total body of plasma thus lowering the Na concentration per volume of plasma. However, it does not raise the serum tonicity or osmolality.
✧ hyperlipidemiaSeen in hypertriglyceridemia (often associated with pancreatitis, DKA), hypercholesterolemia (associated with cholestatis)
HYPERTONIC ❷The presence on aqueous solute (glucose, mannitol, maltose, sucrose) raises the serum tonicity or osmolality thereby drawing water out of cells and lowering serum Na concentration.
✧ hyperglycemiaSeen in DKA and HHS
✧ mannitol infusionUsed in treatment of traumatic brain injury
✧ ivig infusionOften prepared in mannitol, maltose or sucrose solution
HYPOTONIC
✿ Extra-renal salt loss ❸Low Na intake decreases serum osmolality which inhibits water reabsorption across the renal tubules leading to low urine osmolality (< 100) and low urine Na (< 20)
✧ low salt dietSuch as 'tea and toast' diet
✿ Extra-renal h2o gain ❹High water or hypotonic fluid intake increases blood volume (effective circulating fluid or ECF) which inhibits water reabsorption across the renal tubules leading to low urine osmolality (< 100) and low urine Na (< 20)
✧ beer potomaniaDrinking large amount of beer containing very low solute
✧ primary polydypsiaIncreased water intake due to psychiatric illness or increased thirst (hypothalamic disease)
✿ Renal salt loss/ h2o gain
✧ advanced renal diseaseIn advanced CKD or severe AKI, poor Na resabsorption capability (high urine Na, > 20) lowers serum osmolality thereby inhibiting H2O reabsorption leading to high urine osmolality (200-250)
✧ adrenal insufficiencyLow aldosterone leads to decreased Na absorption (high urine Na) and volume depletion, which stimulates ADH release and increased water absorption. Low cortisol leads to low cardiac output and low ECV, it also stimulates release of CRH, which in turn stimulates release of ADH.
✧ diureticsInclude thiazide and, though less often, loop diuretics. Thiazide inhibits Na reabsorption in distal tubule and increases water permeability and reabsorption at the collecting duct leading to hyponatremia. Loop diuretics inhibits Na reabsorption at the loop of Henle reducing the osmotic gradient in the medullary interstitium resulting in decreased water reabsorption and the net effect might or might not be hyponatremia.
✧ reset osmostatADH release is suppressed at an abnormally low osmolality (hyponatremic level where patients chronically live). Thus at normal osmolality level, ADH activity will not be suppressed leading to hyponatremia. Seen in chronic illnesses, pregnancy.
✧ cerebral salt wastingOften seen in subarrachnoid hemorrhage, meningitis, encephalitis, CNS malignancy, post neurosurgery. Caused by either the removal of central stimulation or inhibition by BNP on renal tubular Na channels leading to decreased Na reabsorption and high urine Na (> 20)
✧ low effective circulating volume (ecv) ❻Low ECV stimulates both water and Na reabsorption via renal tubules causing high urine osmolality and low urine Na (< 20). In addition, ADH release is stimulated leading to a net effect of hyponatremia. Low ECV is seen in hypovolemia (dehydration, GI losses, hemorrhage), heart failure, cirrhosis, or low oncotic pressure (nephrotic syndrome, hypoalbuminemia).
☼ hypovolemiaCaused by ehydration, GI losses (diarrhea, vomiting), hemorrhage, heart failure, cirrhosis, or low oncotic pressure (nephrotic syndrome, hypoalbuminemia).
☼ heart failureCaused by ehydration, GI losses (diarrhea, vomiting), or hemorrhage
☼ cirrhosisSystemic vasodilation leads to leaky vessels thereby decreasing blood volume which then triggers ADH release thereby impairing water excretion and further decrease blood volume
☼ nephrotic syndromeLow serume protein level leads to low oncotic pressure to keep water intravascularly
☼ systemic vasodilationSeen in high inflammatory state such as acute pancreatitis, post op state, sepsis
