Overview
Hyponatraemia is an imbalance between sodium and water balance in the body, producing a reduced serum sodium concentration of <135 mmol/L.
Pathogenesis
Causes of SIADH
- Malignancy - lung cancers, pancreatic cancer, colorectal cancer, bladder cancer, lymphoma, sarcoma
- CNS - tumours, demyelination, ischaemic stroke, haemorrhage, infection, vasculitis, traumatic brain injury
- Pulmonary - pneumonia, tuberculosis, abscess, vasculitis
- Drugs - desmopressin, SSRIs, TCAs, carbemazepine, levetiracetam, haloperidol, cyclophosphamide, vincristine
- Postoperative state
Manifestations
Mild or gradual onset hyponatraemia generally presents with few symptoms, while severe or rapid onset hyponatraemia can be life-threatening.
Acute hyponatraemia may present with nausea, vomiting, headaches or confusion; as the sodium level falls patients may present with seizures (including status epilepticus) or coma.
Chronic hyponatraemia may also present with nausea, vomiting or headaches. Patients may also report polydipsia, lethargy or muscle cramps.
Diagnosis
In order to further investigate the cause of hyponatraemia, first check the serum and urine sodium and osmolality at the same time.
Normal serum osmolality (280 - 285 mOsm/kg) indicates either a spurious result (pseudohyponatraemia) or excessive administration of isotonic solutes like isotonic mannitol. Pseudohyponatraemia is the false laboratory measurement of sodium due to an increased solid phase of plasma, such as with hyperproteinaemia or hyperlipidaemia.
Elevated serum osmolality (>285mOsm/kg) indicates movement of water from cells to the interstitium and intravascularly, driven by extracellular solutes such as sugars. This can occur with hyperglycaemia and hypertonic mannitol. A sodium correction should be performed to account for serum glucose - see the Calculators section for this tool.
Low serum osmolality (<280 mOsm/kg) suggests excess total body water in relation to total body sodium, due to sodium loss or water retention. Examine the patient's fluid status to further delineate a cause.
Low serum osmolality with dehydration suggests salt and water loss from the kidneys or elsewhere. Low urinary sodium (<20 mEq/L) in these patients is suggestive of GI or sequestrational loss, such as due to vomiting, diarrhoea or third spacing. Normal urinary sodium (>20 mEq/L) is suggestive of renal loss, such as due to diuretics, mineralocorticoid deficiency, renal tubular acidosis, cerebral salt wasting or salt wasting nephropathy.
Low serum osmolality with euvolaemia is suggestive of redistribution. Urine osmolality <100 mOsm/kg may be due to primary polydipsia or beer potomania syndrome; urine osmolality >100mOsm/kg suggest SIADH, hypothyroidism or glucocorticoid deficiency.
Low serum osmolality with hypervolaemia is suggestive of water retention. Low urinary sodium (<20 mEq/L) in this case is suggestive of renal failure as a cause, while normal urinary sodium (>20 mEq/L) suggests other causes of fluid overload such as heart failure, liver cirrhosis, nephrotic syndrome or hypoalbuminaemia.
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