- As opposed to potassium which is intracellular.
Hyponatraemia is commonly seen both in the community (especially older patients) and in hospital – particularly in ICU – where 30% of patients have hyponatraemia.
Normal sodium level is 135-145mmol/L
Can be considered by severity, and by cause:
- Mild – 125 – 134 mol/L
- Moderate – 120-124 mmol/L
- Severe – <120 mol/L
Typically due to excess sodium loss.
- Diuretics (particularly thiazide & loop diuretics)
- Mineralocorticoid insufficiency (Addison’s)
- Osmotic dieresis (low glucose, urea)
- Excessive sweating
- CNS disturbances – infection, neoplasm, vascular, inflammatory, trauma, psychosis
- Neoplasm – ectopic ADH secretion from SCLC (pancreas, head and neck)
- Pain – post abdominal and thoracic surgery
- Surgery – post transspehnoidal pituitary surgery in 20-35%
- Pulmonary disease – especially pneumonia
- Drugs – SSRI, carbamazepine, cyclophosphamide, opiates, MAOI, ECSTASY (can also be associated with excessive water intake).
Low dietary Na+
Advanced renal failure – inability of the kidneys to excrete free water. Minimum urine osmolality can rise to 200mosm/kg despite no ADH. Low osmolality can be offset by increase urea. However as urea can cross freely across cell membranes, it is an ineffective osmole hence effective osmolality is decreased.
Hormonal insufficiency –
Signs & Symptoms
- Muscular weakness
The speed of onset is a critical factor in the history – rapid onset is more likely to result in significant sequelae. Patients are typically not symptomatic until sodium <125 mmol/L.
- Fluid intake / output – is it related to excess fluid intake ror excessive fluid loss – e.g. due to malignancy, addisons, hypothyroidism
- Nausea / vomiting – vomiting can cause a hypovolaemia hyponatraemia
- Muscular weakness
- Medications – particularly diuretics, SSRI and PPI – especially if any recent changes
- Volume status (see below)
- Moist or dry mucus membranes
- Signs of fluid retention
- Weight loss / gain
A more sensitive method is to look at serum urea and urinary Na+. A low or normal urea in conjunction with elevated urinary Na+ makes normovolaemic hyponatraemia more likely.
- U+E (sodium and renal function)
- Plasma and urinary osmolality (normal plasma level 275-290), normal urine level 3x greater than plasma level
- Urinary Na+ – 15-250mmol/L is normal
- Treat the underlying cause
- Correct sodium level slowly to avoid central pontine myelinolysis
- Common interventions include:
- Fluid restriction to 800mls daily (or at minimum to less than urine output) – Used for oedematous states (heart and liver failure), SIADH, primary polydipsia and advanced renal failure
- Cease any implicated medications
- Isotonic or hypertonic (3%) saline – if true volume depletion (removes stimulus for ADH release) or adrenal insufficiency (replaces Na+ lost from kidneys)
- ADH antagonist
- If acutely seizing:
- Raise sodium by 1-5mmol/L/hour until stops seizing or sodium 125-130mmol/L
- Hypertonic saline (3%)
- Consider frusemide 20mg IV
- If symptomatic but chronic (>48 hours), not actively seizing
- Aim to raise sodium by 10mmol/24hours
- SIADH and asymptomatic
- Fluid restriction
- Frusemide 20-40mg IV daily
- Oral NaCl tablets 3-18g daily
- Urea 30g daily
- Democlocycline 600-1200mg daily