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. 

Flow Chart

1.  Na < 130:

   1.  Osmolality< 275: hypotonic [low Na+ concentration]

   2. Osmolality 275-295: isotonic- hyperlipidaemia, hyperparaproteinaemia

   3. Osmolality > 295: hypertonic- hyperglycaemia, hyperuraemia, post TURP syndrome; [dehydrated]: IVF

2. Hypotonic hyponatraemia, 

   1. Urine osmolality: < 100- appropriate dilution of urine (hypotonic): polydipsia

   2. Urine osmolality: > 100 = hypotonic hyponatraemia, hypertonic urine

Hypotonic hyponatraemia with hypertonic urine. 

1. Is the patient overloaded? 

• Yes -> likely diagnosis heart, liver or kidney failure: manage with diuretics

• No -> Q2

2. Are Kidneys appropriately preserving Na+ by reducing urinary Na 

   • Greater than 20: kidneys leaking Na+, inappropriately

     1. SBP > 120- Retaining water but excreting salt: SIADH - fluid restriction if Na+ PO intake > 3g; otherwise hypertonic saline/ salt tablets @ DW HDU; hypothyroidism, glucocorticoid deficiency; unfortunately these conditions can cause vomiting leading to dehydration. 

     2. SBP < 100- Leaking salt and water: renal wasting syndrome, over diuresis, mineralocorticoid deficiency - give IVF

   • Less than 20: appropriate conservation of Na+

     1. SBP < 100, hypovolaemic: dehydration- Colitis, pancreatitis -> give IVF ++ & kidneys will appropriately preserve Na+

     2. Hydrated: polydipisa, malnutrition, iatrogenic fluids- reduce fluids

Summary

•  Na < 130:

  •  Osmolality< 275: hypotonic

  • Osmolality 275-295: isotonic- hyperlipidaemia, hyperparaproteinaemia

  • Osmolality > 295: hypertonic- hyperglycaemia, hyperuraemia, post TURP syndrome; [dehydrated]: IVF

• Hypotonic hyponatraemia, 

  • Urine osmolality: < 100- hypotonic: polydipsia

  • Urine osmolality: > 100 = hypotonic hyponatraemia, hypertonic urine

Hypotonic hyponatraemia, hypertonic urine

• Urinary Na > 20

  • SBP > 120: SIADH- mx; if dietary Na+ intake:

    •  > 2g /d: fluid restrict 1L 

    • If PO Na+ < 2g /d or H20 intake < 1L: hypertonic saline (not isotonic)

  • SBP < 120: Renal Salt wasting- isotonic saline

• Urinary Na < 20

  • Dehydrated: Colitis, pancreatitis - IVF: isotonic

  • Hydrated: polydipsia, iatrogenic- fluid restrict

Resources and References

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

Written 2025