Hyponatraemia

Clinical Cases

• What is sodium?
• Exercise-associated Hyponatremia
• HYPOnatremia Interpretation flow diagram [PDF]
• Seizures, hyponatremia and ADH
• Tracking a killer – Electrolyte imbalance

Cause of HYPOnatraemia

• Definition: Serum sodium <130mEq/l
• Cause: Excess of Water relative to Sodium
• Affects: 1% of hospital population

• Hypovolemic (Na deficit in xs of water deficit) (test urine sodium)
  • RENAL (urinary Na >20mmol)
    • Addison’s
    • Renal failure (Diuretic stage)
    • Salt losing nephropathy (RTA)
    • Diuretic- Thiazides and osmotic diuretics
    • Cerebral salt wasting
  • PRE-RENAL (Urinary Na <20mmol)
    • Third space losses (Burns, pancreatitis, bowel obstruction, cirrhosis)
    • Sweating/vomiting/diarrhoea with continued water intake
      • Alkalosis with upper GI and skin loss
      • Acidosis with lower GI loss
• Normovolemic (Test urine osmolality)
  • Urine osmolality <serum osmolality
    • Tea and toast diet, Beer potomania
    • Psychogenic polydipsia (>15 L/Day if normal kidneys)
    • Iatrogenic water overload in ED
    • Amphetamines
    • Exercise-Associated Hyponatraemia [Reference]
  • Urine osmolality > serum osmolality
    • SIADH (urine Na >20mmol
      • Malignancy (lung, pancreas, prostate, lymphoma, others)
      • CNS, Lung infection and granulomatous disease, Porphyria, positive pressure ventilation
      • Drugs: Psychoactive- MAOI, SSRI, TCA, NSAID, chlorpromazine, Chemotherapeutic (induce SIADH)
• Hypervolemic (increased TBW relative to Na) (oedematous states)
  • Urinary sodium <20mmol/l
    • Increased interstitial salt
    • Low albumin and secondary hyperaldosteronism
    • CCF, cirrhosis, nephrotic syndrome, hepato-renal syndrome
  • Urinary sodium >20mmol/l
    • Renal failure
    • Hypertonic saline, early diuretics
    • Hypothyroidism
• Fictitious (Pseudohyponatraemia)
  • Hyperglycaemia (draws water to ECF)
    • Corrected sodium…Measured Na + [glucose - 5]/4
  • Hyperproteinemia, hyperlipidemia
  • Mannitol
  • Glyceine washout for TURP and hysteroscopy

Click to enlarge

Clinical

Severity of symptoms associated with rapidity of loss and extent of fall

• >125      Asymptomatic
• 115-125 Lethargy, confusion, anorexia, nausea, vomiting
• <115      Muscle cramps and weakness, convulsions, coma

Complication

• Cerebral oedema
  • Secondary to abrupt sodium losses and free water shift from vascular to interstitial space
• ECG changes
  • Cause of non-ischaemic ST elevation on ECG
• Pontine demyelinosis (no clear evidence that associated with rapid correction)
  • Develops 3-5 days after treatment
    • Demyelination of central pons, corticobulbar and corticospinal tracts
    • Altered mental state, pseudobulbar palsies
    • Dysphasia and spastic quadriparesis
  • More likely in chronic hyponatraemia

Correction

Depends on rapidity of onset and clinical symptoms

• Asymptomatic hyponatremia
  • Slow at 0.5mEq/hr (max 12mmol/24 hours)
  • Rapid correction may lead to pontine myelinolysis
    • Water restriction is usually used (especially in SIADH)
    • Salt tablets and diuretics
    • Demeclocycline
  • Induces nephrogenic diabetes insipidus
  • Use with caution, potentially nephrotoxic
• Symptomatic with neurological symptoms
  • Initial rate 1-2mmol/hr for first few hours
  • Monitor CNS symptoms regularly
  • Revert to 0.5mEq/hr after stable

Calculations:

• Calculate Na deficit

Na deficit= (desired Na-current Na) x (0.6 x body weight)

Correction

• In acute severe hyponatraemia, aim for 1-2mEq/hour correction
• In chronic severe hyponatraemia aim for 0.5-1mEq/hour correction.
• Hypertonic saline replacement
  • 3% saline (513mEq/L) by giving (deficit/513) to the patient at the rate of 1mEq/hour over 4 hours
• New AVP receptor antagonists are currently undergoing phase III clinical trials and show promise for the treatment of hyponatraemia.  The increase serum sodium by stimulating free water excretion.