Hyponatraemia

Exam Cram

• Hyponatraemia is a sign defined as low sodium, the cause is required for a diagnosis

• Presentation: asymptomatic; confusion, headaches, seizure

• Diagnosis: serum Na+ < 135

• Management: urinary Na+ & osmolality and serum Na+ & osmolality; treat as per underlying cause

Outline

Assessment

History

• Asymptomatic

• Headaches, confusion, seizures, coma

Examination

• Fluid status: hyper, eu or hypovolaemia

Investigations

• Serum Na+ & osmolality

• Urinary Na+ & osmolality

Diagnosis

Diagnostic criteria: serum Na+ < 135

Classification

• Severity

  • Asymptomatic vs symptomatic

  • Levels (serum Na+)

    • Mild: 125-135

    • Moderate: 120-124

    • Severe: < 120 

• Aetiology

  • Hypovolemia- loss of Na+ & H20

    • Lack of intake

      • Dehydration

    • Increased losses

      • Renal losses: diuretics

      • Vomiting: bowel obstruction

      • Diarrhoea: Gastroenteritis, Colitis 

      • Sweating

      • Polyuria- Diabetes Mellitus, Diabetes Insipidus, HyperCalcaemia

  • Euvolemia

    • SIADH

      • Lung Cancer, Carbamazepine, SSRIs

    • Psychogenic polydipsia: low urinary osmolality

    • HypoThyroidism 

    • Alcohol Abuse

  • Hypervolemia

    • Transudative overload: CHF, Liver Cirrhosis, nephrotic syndrome

    • Pregnancy

    • HypoThyroidism 

Management

Treat underlying cause

As per severity: 

• Seizures/ coma

  1. Hypertonic saline 2.7% NaCl via central line

  2. Requires HDU/ ICU admission

• Severe: < 120: 

  1. Admission to HDU / ICU

  2. Six hourly serum Na+

• Non severe

Med Reg Notes

SIADH vs Hypovolaemia

For myself this can often be a difficult diagnostic dilemna. An individual with hyponatreamia of unclear cause appears chronically mildly dehydrated, should they be fluid restricted of given IVF. I often found with bed rest, good meals and drink to thirst to a mild fluid restriction, the hyponatraemia resolves. 

Resources and References

1. https://litfl.com/hyponatraemia/

Written 2025