Hyponatremia

Acute hyponatraemia (onset <48 hours) is rare and frequently symptomatic, causing confusion, coma and even death. Chronic hyponatraemia is much more common and usually asymptomatic. This is because brain cells are capable of adapting to moderate changes in plasma tonicity (Soiza et al, 2008). This adaptive process takes 24-48 hours and hence the threshold of 48 hours for classification (Spasovski et al, 2014).

The severity of hyponatraemia is classically divided into

• mild (130-134 mmol/L),
• moderate (125-129 mmol/L) and
• severe (<125 mmol/L)

but symptoms and outcomes relate more to the speed of the drop in sodium levels than the actual sodium concentration (Gill et al, 2006; Chua et al, 2007).

Chronic:

Usually asymptomatic and incidental finding during routine test

Geriatric: may have gait disturbances -> increased risk of falls and immobility, bone demineralisation (Verbalis et al, 2010), hip fractures (Gankam Kengne et al, 2008) and cognitive impairment. --> increased mortality

Symptoms:

• Moderately severe
  • Nausea without vomiting
  • Confusion
  • Headache
• Severe
  • Vomiting
  • Cardiorespiratory distress
  • Abnormal and deep somnolence
  • Seizures
  • Coma (Glasgow Coma Scale ≤8)

DO NOT restrict fluid and prescribe salt tablets without identifying the cause first.

Causes of hyponatremia

• Hypovolemic hyponatremia
  • Volume depletion (eg, acute blood loss, burns)
  • Primary adrenal insufficiency (Addison's disease)
  • Gastrointestinal losses (eg, diarrhea, vomiting, acute pancreatitis)
  • Renal losses (eg, diuretics, AKI from any cause including analgesic nephropathy)
• Euvolemic hyponatremia
  • Syndrome of inappropriate antidiuretic hormone (eg, due to drugs, malignancy)
  • Primary (psychogenic) polydipsia
  • Secondary adrenal insufficiency
  • Hypothyroidism
• Hypervolemic hyponatremia
  • Congestive heart failure
  • Cirrhosis
  • Chronic kidney disease or nephrotic syndrome

Common drug causes of low sodium: