Physiology
Endocrine
Regarding hyponatraemia, which of the following statements is CORRECT:
Answer:
Hyponatraemia is defined as a serum sodium concentration < 135 mmol/L. Acute hyponatremia can be safely corrected more quickly than chronic hyponatremia; correction should not be too rapid, especially in chronic hyponatraemia, as this may result in central pontine myelinolysis. Hyponatraemia of any cause is associated with a low plasma osmolality. Normally if serum osmolality is low, urine osmolality should also be low – this is because the kidneys should be trying to retain solute. In SIADH, the excess ADH causes water retention, but not solute retention. As a result, concentrated urine relatively high in sodium is produced, despite low the low serum sodium (urine osmolality > 100 mosmol/kg).Hyponatraemia
Physiology / Endocrine / Pituitary Function
Last Updated: 16th July 2020
Hyponatraemia is defined as a serum sodium concentration < 135 mmol/L. A level < 125 mmol/L is considered severe. Hyponatraemia results from a relative excess of body water to sodium.
Clinical Features
The rate of change of sodium is more important than the absolute sodium value so patients with chronic hyponatraemia can be asymptomatic, while patients with a sudden drop can be very unwell.
Early symptoms are headache, nausea, vomiting and general malaise. Later signs are confusion, agitation and drowsiness. Acute severe hyponatraemia leads to seizures, respiratory depression, coma and death. This is due to swelling of brain cells when water moves from the extracellular to the intracellular compartment because of differences in the osmolality between brain and plasma, resulting in cerebral oedema and raised intracranial pressure.
Diagnostic Approach
- Drug history
- Hydration status
- Plasma osmolality
- Urine osmolality
- Urine sodium
- Thyroid function
- Assessment of cortisol reserve
Plasma osmolality
Hyponatraemia of any cause is associated with a low plasma osmolality.
If the plasma osmolality is normal, then the possibility of pseudohyponatraemia should be considered; this is an artifactual result due to a reduction in plasma water caused by marked hyperlipidaemia or hyperproteinaemia e.g. in multiple myeloma.
If the plasma osmolality is high, then the possibility of hyperosmolar hyponatraemia should be considered; this may be due to hyperglycaemia or administration of mannitol amongst other causes and reflects the shift of water out of cells into the extracellular fluid in response to osmotic effects.
Urine osmolality
Once hypotonic hyponatraemia has been confirmed, urine osmolality should be checked.
Low urine osmolality (< 100 mosmol/kg):
- Primary polydipsia
- Inappropriate administration of IV fluids
- Malnutrition - low salt diet
Normal urine osmolality (> 100 mosmol/kg):
- Urinary Na+ < 30 mmol/L
- Patient hypovolaemic (true volume depletion or third space loss)
- Gastrointestinal salt loss - severe diarrhoea and vomiting
- Transdermal salt loss - sweating, extensive skin burns
- Third space loss - pancreatitis, bowel obstruction, sepsis
- Diuretics
- Patient hypervolaemic (intravascular depletion in fluid overloaded state)
- Congestive cardiac failure
- Cirrhosis
- Nephrotic syndrome
- Patient hypovolaemic (true volume depletion or third space loss)
- Urinary Na+ > 30 mmol/L
- Patient euvolaemic (normal circulating volume)
- SIADH
- ACTH deficiency
- Severe hypothyroidism
- Patient hypovolaemic (low circulating volume)
- Addison's disease
- Cerebral salt-wasting
- Renal salt-wasting
- Vomiting (causes loss of hydrogen ions and a metabolic alkalosis which is corrected by the renal excretion of sodium bicarbonate)
- Patient euvolaemic (normal circulating volume)
However, if the individual is using diuretics, or there is evidence of kidney disease, all causes should be considered, as these can cause a low or a high urinary sodium concentration.
Management
Treatment is cause-specific. Appropriate fluid replacement in patients with hypovolaemic hyponatraemia with normal saline typically leads to improvement. In patients with hypervolaemic hyponatraemia, specialist treatment of the underlying condition is indicated.
In acute severe hyponatraemia with neurological compromise, the use of hypertonic saline should be considered whatever the cause, with specialist input and careful monitoring. Correction should not be too rapid, especially in chronic hyponatraemia, as this may result in central pontine myelinolysis.
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- Biochemistry
- Blood Gases
- Haematology
| Biochemistry | Normal Value |
|---|---|
| Sodium | 135 – 145 mmol/l |
| Potassium | 3.0 – 4.5 mmol/l |
| Urea | 2.5 – 7.5 mmol/l |
| Glucose | 3.5 – 5.0 mmol/l |
| Creatinine | 35 – 135 μmol/l |
| Alanine Aminotransferase (ALT) | 5 – 35 U/l |
| Gamma-glutamyl Transferase (GGT) | < 65 U/l |
| Alkaline Phosphatase (ALP) | 30 – 135 U/l |
| Aspartate Aminotransferase (AST) | < 40 U/l |
| Total Protein | 60 – 80 g/l |
| Albumin | 35 – 50 g/l |
| Globulin | 2.4 – 3.5 g/dl |
| Amylase | < 70 U/l |
| Total Bilirubin | 3 – 17 μmol/l |
| Calcium | 2.1 – 2.5 mmol/l |
| Chloride | 95 – 105 mmol/l |
| Phosphate | 0.8 – 1.4 mmol/l |
| Haematology | Normal Value |
|---|---|
| Haemoglobin | 11.5 – 16.6 g/dl |
| White Blood Cells | 4.0 – 11.0 x 109/l |
| Platelets | 150 – 450 x 109/l |
| MCV | 80 – 96 fl |
| MCHC | 32 – 36 g/dl |
| Neutrophils | 2.0 – 7.5 x 109/l |
| Lymphocytes | 1.5 – 4.0 x 109/l |
| Monocytes | 0.3 – 1.0 x 109/l |
| Eosinophils | 0.1 – 0.5 x 109/l |
| Basophils | < 0.2 x 109/l |
| Reticulocytes | < 2% |
| Haematocrit | 0.35 – 0.49 |
| Red Cell Distribution Width | 11 – 15% |
| Blood Gases | Normal Value |
|---|---|
| pH | 7.35 – 7.45 |
| pO2 | 11 – 14 kPa |
| pCO2 | 4.5 – 6.0 kPa |
| Base Excess | -2 – +2 mmol/l |
| Bicarbonate | 24 – 30 mmol/l |
| Lactate | < 2 mmol/l |
