General MedicineTier 1Disease (DEADMAN)

Metabolic emergencies - hyponatraemia

Description

Serum Na+ <135 mmol/L; severity - mild (130-134), moderate (125-129), severe (<125)
Classified by volume status (hypo-/eu-/hypervolaemic) and tonicity (true hypotonic hyponatraemia vs pseudohyponatraemia)

Commonest electrolyte disorder in hospitalised patients
More common in the elderly (multiple contributing drugs/comorbidities), postoperative patients, and heart failure/cirrhosis

Hypovolaemic: renal (diuretics, Addison's, salt-wasting) or extra-renal (vomiting, diarrhoea, burns) losses replaced by hypotonic fluid
Euvolaemic: SIADH (malignancy, CNS/pulmonary pathology, drugs - SSRIs, carbamazepine, MDMA), hypothyroidism, cortisol deficiency, primary polydipsia
Hypervolaemic: heart failure, cirrhosis, nephrotic syndrome - impaired free water excretion despite total body Na+ excess
Symptoms arise from cerebral oedema as extracellular osmolality falls relative to intracellular - severity depends on absolute level and rate of fall (acute worse tolerated than chronic)

Assess volume status clinically, serum/urine osmolality, urine sodium
Urine osmolality >100 mOsm/kg with clinical euvolaemia and urine Na+ >30 supports SIADH (diagnosis of exclusion - check TSH, cortisol first)
Severity/urgency determined by symptoms (headache, nausea -> confusion, seizures, coma) more than absolute number

1. Severe symptomatic hyponatraemia (seizure, coma, severe confusion) - emergency

Hypertonic saline (3%) bolus 100-150mL, repeated to achieve 4-6 mmol/L rise within the first few hours to reverse cerebral oedema
Do not exceed correction limits: <=10 mmol/L in the first 24h, <=18 mmol/L in 48h - risk of osmotic demyelination syndrome with faster correction

2. Chronic/asymptomatic hyponatraemia

Treat the underlying cause: fluid restriction (SIADH), stop causative drug, treat volume depletion with isotonic saline (hypovolaemic), treat heart failure/cirrhosis (hypervolaemic)
Tolvaptan (vasopressin receptor antagonist) for SIADH refractory to fluid restriction - specialist-initiated, requires inpatient initiation and close monitoring (rapid overcorrection risk)

3. Monitoring

Frequent sodium recheck (every 2-4h initially in severe/symptomatic cases) to avoid overshoot
If overcorrection occurs, relowering with hypotonic fluid/desmopressin ("DDAVP clamp") can mitigate ODS risk

Osmotic demyelination syndrome (central pontine myelinolysis) - overcorrection, esp. in alcoholics/malnourished/hypokalaemic patients (higher risk group)
Falls and fractures even with mild chronic hyponatraemia in the elderly
Underlying malignancy (SIADH as paraneoplastic presentation)

Acute severe hyponatraemia carries significant mortality/morbidity risk from cerebral oedema if untreated
Chronic hyponatraemia is better tolerated but still associated with increased falls, fractures, and mortality even when "mild"
Osmotic demyelination, once established, can cause permanent neurological deficit - prevention (controlled correction rate) is the key management principle

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