Hypernatraemia

Clinical Cases

Cause of HYPERnatraemia

  • Water loss in excess of salt deficit
    • Hypernatremia is usually due to insufficient water (primarily in patients who either do not experience thirst normally, or cannot act on it)
    • Hypernatraemia occurs less commonly associated with excess salt
  • Each litre of free water loss associated with 3-5mEq rise in sodium
  • Problem associated with rise in serum osmolarity

Decreased water intake (with normal fluid loss)

  • Disordered thirst perception (Normally increase of 2% osmolarity associated with increase in thirst) e.g. hypothalamic lesion
  • Lack of environmental water
  • Inability to communicate water needs e.g. Coma, CVA, Intubated patients and Kids

Hypotonic fluid loss (Water loss in excess of salt loss)

  • Skin:
    • Sweat in hot climate or exercise (Heat stroke, Heat exhaustion)
    • Burns
  • GI disturbances (especially with salt replacement)
    • Vomit, diarrhoea, fistula
  • Renal disease Impaired renal concentrating ability
    • Diabetes Insipidus
      • Central, Nephrogenic or Drugs (alcohol, phenytoin, lithium, colchicine, Amphotericin, gentamicin)
    • Osmotic diuresis
      • CRF, mannitol, Hyperglycaemia, hypokalaemia
    • Renal disease
      • Nephropathy, myeloma, TIN, obstructive uropathy, PKD

Increased salt

  • Acute salt poisoning
    • Ingestion of seawater or salt tablets, IV NaHCO3, hypertonic saline
  • Increased mineralocorticoid
    • Primary hyperaldosteronism (Increased BP, decreased K, and alkalosis)
  • Increased glucocorticoid (Cushings)
  • Ectopic ACTH

Clinical

  • HYPOnatremic dehydration
    • Increased serum osmolality, draws water into vascular space
    • Free water shift form interstitial to vascular space
    • Deceptively normal vital signs
    • Associated with brain cellular dehydration
  • Symptoms worsen if change in Na is rapid
    • Increased risk of intracerebral haemorrhage secondary to rapid brain shrinkage
  • Chronic hypernatremia has milder symptoms with adaptation

Clinical Signs/Symptoms

Signs manifest changes in serum osmolality. Brain shrinkage secondary to free water loss

  • >350            Excessive thirst
  • >375            Weakness and lethargy. Irritability
  • >400            Ataxia, tremor
  • >420            Focal neurological deficit; Hyperreflexia and Spasticity
  • >430            Coma and seizures

Correction

  • Stop ongoing losses
  • Correct water deficit
  • Correct sodium deficit if hypovolaemic
  • Treat the underlying cause

Complications of treatment

  • Intracerebral haemorrhage (especially in neonates: complication of NaHCO3 administration)
  • Coma and seizures
  • Cerebral oedema (if correction too rapid)

Calculation

Calculate Water deficit

Total body water x (serum Na-140)/ (140)

  • TBW is usually 50% in males and 40% in females
  • So male with Na of 160 has free water deficit of (0.5 x 70) x 20/140 = 5 Litres
  • Aim to replace deficit plus maintenance and ongoing losses
  • Replace with oral free water or 5% dextrose over 24-48 hours
  • Rapid correction possible in acute cases
  • Slow correction in chronic cases to avoid cerebral oedema

Correct Na slowly over 24-48 hours with a reduction of Na of 0.5-1.0mmol/hour (Maximum of 10-15 mEq in 24 hours)

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