The Purple Book

Hyponatraemia

Definition

Sodium<135mmol/L

Cause

• Many medications: commonly: thiazides and thiazide-like diuretics, SSRI's (esp citalopram), anti-psychotics and many, many others. Hyponatraemia does not always start on commencement of that med.
• respiratory pathology: pneumonia, active TB, abscess, neoplasm, or asthma
• CNS pathology: infection, trauma, or neoplasm.
• Various carcinomas: nasopharynx, duodenum, stomach, pancreas, ureter, prostate, or uterus
• hypothyroidism, adrenal insufficiency
• Sodium losses: diarrhoea and sweating
• In setting of hypervolaemia:
  • Urinary Na+ >20
    • Renal failure
  • Urinary Na+ <20
    • Nephrotic syndrome
    • Cirrhosis
    • CHF

Symptoms

• Mild or absent
• often neurological if very low (esp <115)
  • headache
  • confusion
  • lethargy, irritability
  • seizures

Signs

• Delirium, altered consciousness
• Fluid status assessment:
  • Are they dehydrated? (dry mucous membranes, reduced skin turgor, low BP/tachycardia, postural hypotension)
  • Evidence of fluid overload? (Oedema, chest crackles, ascites, elevated JVP)

Diagnostics

• U&E, TFTs if appropriate, serum osmolality, urinary sodium/osmolality

Management

• treat underlying cause (check Glu and correct) - excessive water intake, stop relevant medications
• water restriction
• diuresis
• beware overly rapid correction - aim for 5mmol/day to avoid Central Pontine Myelinolysis