Pathophysiology

Hyponatraemia generally causes low extracellular osmolality causing fluid shift into the intracellular space
This can cause cerebral oedema, precipitating the clinical features
ADH acts primarily on water re-absorption and at a distal end of the nephron, and therefore can cause disorders of serum sodium concentration
- Angiotensin II, Aldosterone and ANP/BNP all cause same directional shifts in sodium and water reabsorption in the kidneys so it is unusual for disorders of these hormones to cause significant derangement in serum sodium concentration

Clinical Features

Mostly neurological
- At sodium 125-130 when acute or sodium 120-125 when chronic:
  - Fatigue
  - Headache
  - Nausea and vomiting
- At sodium 120-125 when acute or sodium 110-120 when chronic:
  - Confusion
  - Seizures
  - Coma
Presence and severity of symptoms are related to severity of hyponatraemia and speed of derangement (as indicated above)

Determine serum osmolality
Hypotonic hyponatraemia
- Determine volume status
  - Hypovolaemic
    - Intra-renal causes: ACE-i, Diuretics, Addison disease, Cerebral sodium wasting syndrome, Acute Tubular Necrosis → would expect urine sodium to be high >20
    - Extra-renal causes: vomiting, diarrhoea, dehydration, burns, bleeding → would expect urine sodium to be low <10
  - Euvolaemic (usually because of limited free water excretion)
    - SIADH
    - Hypothyroidism
    - Psychogenic polydipsia
    - Iatrogenic SIADH (SSRIs, carbamazepine)
    - Beer potomania
  - Hypervolaemic (fluid overload state)
    - Congestive heart failure
    - Cirrhosis
    - Nephrotic syndrome
    - End stage renal disease
Isotonic hyponatraemia
- Hyperlipidaemia, multiple myeloma ⇒ pseudohyponatraemia because of reduction in aqueous plasma
- TURP syndrome
  - Dilutional hypontraemia as water is absorbed through water sinuses at surgical site
Hypertonic hyponatraemia
- Hyperglycaemia
- Mannitol
- Radiocontrast use
- Highly osmotic agents cause increase in water osmosis into intravascular space causing dilutional hyponatraemia