Hyponatremia: Difference between revisions

Latest revision as of 20:05, 9 March 2024

Etiologies

Medications: thiazide and thiazide-type diuretics, mannitol, IVIG, desmopressin (dDAVP), ecstasy (methylenedioxymethamphetamine), and some antidepressants, antiepileptics, and antipsychotics

Differential Diagnosis

Pseudohyponatremia from lab error
Translational hyponatremia from mannitol or hyperglycemia
Hypovolemic
- U_Na >20: Renal losses, including mineralocorticoid deficiency
- U_Na <10: Non-renal losses
Euvolemic
- U_osm >100: SIADH, hypothyroidism, glucocorticoid deficiency
- U_osm <100: Primary polydipsia, low solute intake
- U_osm variable: Reset osmostat
Hypervolemic
- U_Na <10: CHD, cirrhosis, nephrosis
- U_Na >20: Renal failure

Investigations

Serum and urine osmolality and electrolytes (prior to treatment)
TSH and AM cortisol

Diagnosis by Lab Criteria

Serum Na (S_Na) <134?
Measure serum osmolality (S_osm) and urea level (S_urea)
- S_osm ≥ 280: normotonic/hypertonic
  - Differential includes pseudohyponatremia (from hyperlipidemia, hyperparaproteinemia, etc.), or presence of osmotically active substances (glucose, mannitol)
- S_osm <280 mOsm/kg: true hyponatremia
  - Measure urine osmolality (U_osm)
  - U_osm <100 mOsm/kg: normal water excretion
    - Primary polydipsia or reset osmostat syndrome
  - U_osm ≥ 100 mOsm/kg: impaired water excretion
    - Exclude hypothyroidism and adrenal insufficiency with morning cortisol and TSH
    - Measure urine sodium (U_Na)
    - U_Na < 20 mmol/L: hypovolemia, including EABV depletion (heart failure, cirrhosis, nephrotic syndrome)
    - U_Na > 40 mmol/L: SIADH, reset osmostat, renal salt wasting
      - Salt supplementation and water restriction
      - Normal S_urate and reduced FE_urate: SIADH
      - Hypouricemia and unchanged FE_urate: renal salt wasting
    - U_Na between 20 and 40 mmol/L
      - Bolus 2L/day normal saline for 2 days and trend sodium
      - If S_Na increases by ≥ 5 mmol/L: hypovolemia
      - If S_Na increases by <5 mmol/L: SIADH or reset osmostat
        FE_urea >55%, S_urate <0.24, and FE_urate >10%: SIADH
        
        Oral or IV water-loading test: reset osmostat

Management

Depends on cause
Goal rate of correction in chronic hyponatremia should be 4 to 6 mEq/L in 24 hours (max of 8 mEq) in order to reduce the risk of osmotic demyelination syndrome

@@ Line 25: / Line 25: @@
 === Diagnosis by Lab Criteria ===
-* Serum Na <134?
+* Serum Na (S<sub>Na</sub>) <134?
-* Measure serum osmolality and urea level
+* Measure serum osmolality (S<sub>osm</sub>) and urea level (S<sub>urea</sub>)
-** Sosm ≥ 280: normotonic/hypertonic
+** S<sub>osm</sub> ≥ 280: normotonic/hypertonic
-*** Ddx includes [[pseudohyponatremia]] (from [[hyperlipidemia]], [[hyperparaproteinemia]], etc.), or presence of osmotically active substances ([[glucose]], [[mannitol]])
+*** Differential includes [[pseudohyponatremia]] (from [[hyperlipidemia]], [[hyperparaproteinemia]], etc.), or presence of osmotically active substances ([[glucose]], [[mannitol]])
-** Sosm <280 mOsm/kg: true hyponatremia
+** S<sub>osm</sub> <280 mOsm/kg: true hyponatremia
-*** Measure urine osmolality
+*** Measure urine osmolality (U<sub>osm</sub>)
-*** Uosm <100 mOsm/kg: normal water excretion
+*** U<sub>osm</sub> <100 mOsm/kg: normal water excretion
-**** [[Primary polydipsea]] or [[reset osmostat syndrome]]
+**** [[Primary polydipsia]] or [[reset osmostat syndrome]]
-*** Uosm ≥ 100 mOsm/kg: impaired water excretion
+*** U<sub>osm</sub> ≥ 100 mOsm/kg: impaired water excretion
 **** Exclude [[hypothyroidism]] and [[adrenal insufficiency]] with morning cortisol and TSH
-**** Measure urine sodium
+**** Measure urine sodium (U<sub>Na</sub>)
-**** UNa < 20 mmol/L: [[hypovolemia]], including EABV depletion ([[heart failure]], [[cirrhosis]], [[nephrotic syndrome]])
+**** U<sub>Na</sub> < 20 mmol/L: [[hypovolemia]], including EABV depletion ([[heart failure]], [[cirrhosis]], [[nephrotic syndrome]])
-**** UNa > 40 mmol/L: [[SIADH]], [[Reset ostmostat syndrome|reset ostmostat]], renal salt wasting
+**** U<sub>Na</sub> > 40 mmol/L: [[SIADH]], [[Reset osmostat syndrome|reset osmostat]], renal salt wasting
 ***** Salt supplementation and water restriction
-***** Normal serum urate and reduced FEurate: SIADH
+***** Normal S<sub>urate</sub> and reduced FE<sub>urate</sub>: SIADH
-***** Hypouricemia and unchanged FEurate: renal salt wasting
+***** Hypouricemia and unchanged FE<sub>urate</sub>: renal salt wasting
-**** UNa between 20 and 40 mmol/L
+**** U<sub>Na</sub> between 20 and 40 mmol/L
 ***** Bolus 2L/day normal saline for 2 days and trend sodium
-***** If SNa increases by ≥ 5 mmol/L: hypovolemia
+***** If S<sub>Na</sub> increases by ≥ 5 mmol/L: hypovolemia
-***** If SNa increases by <5 mmol/L: [[SIADH]] or [[Reset ostmostat syndrome|reset ostmostat]]
+***** If S<sub>Na</sub> increases by <5 mmol/L: [[SIADH]] or [[Reset osmostat syndrome|reset osmostat]]
-****** FEurea >55%, serum urate <0.24, and FEurate >10%: SIADH
+****** FE<sub>urea</sub> >55%, S<sub>urate</sub> <0.24, and FE<sub>urate</sub> >10%: SIADH
-****** Oral or IV water-loading test: [[Reset ostmostat syndrome|reset ostmostat]]
+****** Oral or IV water-loading test: [[Reset osmostat syndrome|reset osmostat]]
+== Management ==
+* Depends on cause
+* Goal rate of correction in chronic hyponatremia should be 4 to 6 mEq/L in 24 hours (max of 8 mEq) in order to reduce the risk of [[osmotic demyelination syndrome]]
 ==Further Reading==