Write a short note on hyponatraemia.

Definition

Hyponatraemia is a serum sodium below 135 mEq/L — the commonest electrolyte abnormality in clinical practice. It is classified as mild (130–135), moderate (125–129) and severe (< 125 mEq/L), and as acute (< 48 hours) or chronic, a distinction that determines management.

Hyponatraemia is fundamentally a disorder of water excess relative to sodium, not usually of sodium deficiency.

Classification and Causes

Step 1 — measure serum osmolality:

• Normal osmolality (pseudohyponatraemia): severe hyperlipidaemia, hyperproteinaemia (myeloma) — a laboratory artefact.
• High osmolality (translocational): hyperglycaemia (serum sodium falls approximately 1.6 mEq/L for every 100 mg/dL rise in glucose), mannitol.
• Low osmolality — true hypotonic hyponatraemia: proceed to assess volume status.

Step 2 — assess volume status:

Siadh — Diagnostic Criteria

• Hypotonic hyponatraemia (serum osmolality < 275 mOsm/kg)
• Inappropriately concentrated urine (urine osmolality > 100 mOsm/kg)
• Urinary sodium > 30–40 mEq/L on a normal salt intake
• Clinical euvolaemia — no oedema, no signs of volume depletion
• Normal thyroid, adrenal and renal function, and no recent diuretic use

Causes of SIADH: malignancy (small cell lung carcinoma classically), pulmonary disease (pneumonia, tuberculosis), CNS disease (meningitis, stroke, head injury, tumour), and drugs — carbamazepine, SSRIs, antipsychotics, NSAIDs, vincristine, cyclophosphamide, desmopressin.

Clinical Features

Determined by the rate of fall more than the absolute value, because the brain adapts to slowly developing hyponatraemia by extruding osmolytes.

• Mild/chronic: often asymptomatic; nausea, headache, lethargy, poor concentration, unsteadiness and falls in the elderly.
• Moderate: confusion, disorientation, muscle cramps, vomiting.
• Severe/acute: seizures, obtundation, coma, respiratory arrest from cerebral oedema and brainstem herniation — a medical emergency.

Management

The two governing questions are: is the patient symptomatic, and is it acute or chronic?

Severe symptoms (seizures, coma) — regardless of chronicity:

• Hypertonic (3%) saline as small boluses, aiming to raise serum sodium by about 4–6 mEq/L over the first few hours, which is sufficient to relieve cerebral oedema.
• Manage in a monitored setting with hourly sodium measurement initially.

Asymptomatic or mildly symptomatic — treat by cause:

• Hypovolaemic: isotonic (0.9%) saline to restore volume; the ADH stimulus switches off as volume is restored, and a brisk water diuresis may then cause an abrupt rise in sodium — anticipate and monitor this.
• Euvolaemic (SIADH): fluid restriction (typically 800–1000 mL/day) is first-line; add salt and protein intake or urea; tolvaptan in selected refractory cases; treat the underlying cause and stop causative drugs.
• Hypervolaemic: sodium and fluid restriction with loop diuretics; treat the heart, liver or kidney disease.