AKT · Renal & urology/Fluid, electrolyte & acid-base

Sodium & water balance disorders

Disorders of water relative to sodium — hyponatraemia (water excess) / hypernatraemia (water deficit)

Overview

Disorders of plasma sodium, which reflect WATER balance relative to sodium rather than total sodium content. Hyponatraemia is assessed by volume status (hypo-, eu-, hypervolaemic) — the key exam algorithm. The danger of correcting either too fast is osmotic demyelination (central pontine myelinolysis) when over-correcting hyponatraemia, and cerebral oedema when over-correcting hypernatraemia.

Recognise

Hyponatraemia: nausea, headache, confusion, seizures (severe/acute); assess fluid status — hypovolaemic (D&V, diuretics, Addison's), euvolaemic (SIADH, hypothyroid), hypervolaemic (heart/liver/renal failure)
Hypernatraemia: thirst, lethargy, irritability, seizures; usually water loss (D&V, diabetes insipidus, inadequate intake in the dependent elderly)
Symptoms relate to the RATE of change as much as the absolute level

Red flags

Acute severe symptomatic hyponatraemia (seizures, reduced GCS) → hypertonic saline in a controlled setting
Over-rapid correction of hyponatraemia → osmotic demyelination (central pontine myelinolysis); over-rapid correction of hypernatraemia → cerebral oedema

Differentials & how to tell them apart

SIADHeuvolaemic hyponatraemia, concentrated urine (high urine osmolality/Na), no oedema/dehydration — exclude hypothyroid/adrenal; cross-ref endocrine

Hypovolaemic hyponatraemiaclinically dry, low urine Na (extra-renal loss) or high urine Na (diuretics/Addison's)

Diabetes insipidus (hypernatraemia)large volumes of dilute urine; cranial vs nephrogenic — cross-ref endocrine

Pseudohyponatraemiahyperglycaemia/hyperlipidaemia/paraprotein — correct for glucose

Investigations

Plasma + urine sodium and osmolality, volume-status assessment (the key step in hyponatraemia); TFTs and 9 am cortisol (euvolaemic — exclude hypothyroid/adrenal insufficiency); glucose (pseudohyponatraemia/hyperglycaemia); review diuretics/drugs; SIADH is a diagnosis of exclusion (euvolaemic, concentrated urine, low urine output, no other cause).

Management

Assess volume status → treat accordingly; correct sodium SLOWLY (avoid osmotic demyelination / cerebral oedema)

1For hyponatraemia, ASSESS VOLUME STATUS first (hypo/eu/hypervolaemic) and send paired plasma/urine osmolality and sodium ± TFTs/cortisol. Treat by category: saline (hypovolaemic), fluid restriction (SIADH), treat-the-cause + restriction (hypervolaemic).Gate: Acute severe symptomatic hyponatraemia (seizures/coma) → hypertonic saline; but cap the correction rate — over-rapid correction causes osmotic demyelination (central pontine myelinolysis).
2Hypernatraemia → replace the water deficit slowly (avoid cerebral oedema) and treat the cause (DI, hypodipsia, losses). Stop culprit drugs; SIADH/DI specifics cross-ref endocrine.

Treat by volume status: hypovolaemic → isotonic saline; euvolaemic (SIADH) → fluid restriction; hypervolaemic → treat the cause + fluid/salt restriction — the volume-status algorithm drives treatment

Hypertonic saline for severe symptomatic hyponatraemia — controlled setting; cap the correction rate (≤8–10 mmol/L per 24 h)

Hypernatraemia: replace the water deficit slowly + treat the cause — oral/IV water; correct slowly to avoid cerebral oedema; desmopressin for cranial DI

Treat the underlying cause / stop culprit drugs — diuretics, SSRIs, carbamazepine for hyponatraemia; address DI/hypodipsia for hypernatraemia

Key points

Sodium = WATER balance. Hyponatraemia → assess VOLUME STATUS (hypo/eu/hyper) — that algorithm is the answer. SIADH = euvolaemic + concentrated urine (diagnosis of exclusion). Correct sodium SLOWLY: too fast up = central pontine myelinolysis; too fast down (in hypernatraemia) = cerebral oedema.

Monitor & prognosis

Serial sodium (correction RATE), fluid status, neurological status; urine osmolality/output.

Good if corrected at a safe rate; osmotic demyelination is devastating and avoidable.

Source: NICE CKS; cross-ref endocrine (SIADH/DI)