SCE Renal Medicine: Mastering Hyponatraemia Guidelines

Hyponatraemia is the most common electrolyte disturbance you will encounter in hospital practice, affecting 15–30% of inpatients, and it is examined relentlessly in the Specialty Certificate Examination (SCE) — whether you are sitting the Renal Medicine SCE, Acute Internal Medicine SCE, Geriatric Medicine SCE or Endocrinology SCE. Examiners love it because a single vignette can test physiology, drug prescribing, guideline application and patient safety all at once.

This guide walks you through the UK-relevant guidelines (NICE CKS and the European Clinical Practice Guideline) with the depth, structure and exam-focus you need for SCE success.

1. Why Hyponatraemia Is SCE Gold

Every SCE blueprint includes disorders of water balance. Expect questions that:

• Give you a sodium value, a drug list and a vague symptom profile, then ask for the next best investigation (usually paired serum and urine osmolality with urine sodium)

• Ask for the immediate management of a seizing patient with sodium of 108 mmol/L (hypertonic saline bolus)

• Test recognition of thiazide-induced hyponatraemia in an elderly woman

• Ask about correction limits and osmotic demyelination syndrome (ODS) risk

• Present overcorrection and ask what to do next (DDAVP ± 5% dextrose)

2. Definitions and Classification You Must State in Any Answer

Severity (UK convention)

Onset

• Acute: <48 hours (e.g., perioperative, MDMA, exercise-associated, desmopressin) — higher risk of cerebral oedema; correct promptly

• Chronic: ≥48 hours (most community-acquired and hospital cases) — higher risk of ODS if corrected too quickly

Symptoms track severity and speed of fall

Headache, nausea, lethargy, confusion, falls in older people (a classic SCE presentation — always check sodium in the confused or fallen elderly patient), vomiting, seizures, coma, respiratory arrest.

SCE pearl: A sodium of 120 mmol/L reached over 3 weeks may be asymptomatic; a sodium of 125 reached over 6 hours post-operatively can convulse. Speed beats depth.

3. Step 1 — Measure Serum Osmolality Before Anything Else

Hyponatraemia is only true hyponatraemia when the serum is hypotonic. This is the single highest-yield diagnostic step.

Correcting sodium for glucose (know a formula)

A practical UK-exam convention (Hillier): sodium falls by approximately 1.6 mmol/L for every 5.5 mmol/L rise in glucose between 5.5 and 11 mmol/L, and by about 2.4 mmol/L per 5.5 mmol/L rise above 11 mmol/L.

SCE pearl: In DKA/HHS, always state the corrected sodium before commenting on tonicity — an exam favourite.

4. Step 2 — Assess Volume Status with Urine Sodium and Urine Osmolality

For hypotonic hyponatraemia, classify by volume status and confirm with urine indices:

Urine osmolality >100 mOsm/kg means ADH is active — this is the key discriminator from polydipsia.

5. SIADH — Diagnostic Criteria (Learn Them Verbatim)

Essential criteria

1. Hypotonic hyponatraemia (serum osmolality <275)

2. Inappropriately concentrated urine (urine osmolality >100)

3. Clinical euvolaemia

4. Urine sodium >30 mmol/L with normal dietary salt and water intake

5. Normal thyroid function (TSH)

6. Normal adrenal function (no glucocorticoid deficiency)

7. Absence of recent diuretic use

Supplemental (supporting) criteria

• Low serum uric acid (often <0.24 mmol/L) and low fractional excretion of urate

• Low serum urea and creatinine

• Failure to correct with isotonic saline (sodium falls further)

• Fractional sodium excretion >0.5%

Causes — the exam table

SCE pearl: Before you ever label SIADH, you must have excluded glucocorticoid deficiency — secondary adrenal failure mimics SIADH biochemically and identically. If the question includes weight loss, hyperpigmentation or hypotension, think Addison's and go for a short Synacthen test.

6. Management — The Guidelines That Examiners Want

A. Severe symptomatic hyponatraemia (seizures, coma, respiratory arrest)

This is a medical emergency — cerebral oedema kills faster than ODS.

UK (NICE CKS-aligned) approach:

1. Give a single IV bolus of 100 mL of 3% sodium chloride (513 mmol/L) — or 1.8% — over 10–15 minutes

2. Recheck serum sodium and symptoms

3. Repeat bolus up to a maximum of 3 boluses, aiming for symptom improvement (a rise of approximately 5 mmol/L is usually enough to stop seizures)

4. Aim: ≤5 mmol/L rise in the first 24 hours for symptom relief; never exceed 10 mmol/L in 24 hours (8 mmol/L in high-risk patients)

5. Monitor sodium frequently (every 2–4 hours) during active correction, ideally in a high-dependency setting

B. Correction limits and osmotic demyelination syndrome

• Maximum rise: 10 mmol/L per 24 hours (standard-risk); 8 mmol/L per 24 hours if high risk

• High-risk patients for ODS: alcohol use disorder, malnutrition/cachexia, hypokalaemia, advanced liver disease, elderly women on thiazides, sodium ≤105

• ODS presents 2–6 days after over-rapid correction: dysarthria, dysphagia, spastic quadriparesis, pseudobulbar palsy, behavioural change, locked-in syndrome; MRI may initially be normal

• Rescue therapy if overcorrected (increasingly examined): give DDAVP (desmopressin 1–2 micrograms IV) with 5% dextrose to relower sodium — know this reflex

C. Chronic or asymptomatic hyponatraemia — a stepwise ladder

1. Treat the underlying cause — stop thiazides, SSRIs, PPIs (where implicated); treat pneumonia, heart failure, hypothyroidism

2. Fluid restriction (1–1.5 L/24 h) — first line for SIADH and hypervolaemic causes; more likely to succeed when urine (Na + K) / serum Na ratio <1

3. Second line (SIADH, specialist input):

   • Loop diuretic + oral sodium chloride — useful when the sum of urine sodium and potassium exceeds serum sodium

   • Tolvaptan (V2-receptor antagonist): must be started in hospital with sodium monitoring every 6–12 hours; do not fluid restrict during initiation; beware rapid overcorrection and hypernatraemia; avoid in cirrhosis, use caution in heart failure (no outcome benefit in EVEREST), avoid with strong CYP3A4 inhibitors and in hypovolaemia; hepatic monitoring considerations from the ADPKD experience

   • Oral urea — a European guideline option, unlicensed/limited UK availability

   • Demeclocycline — largely historical

4. Hypovolaemic hyponatraemia: volume expansion with 0.9% sodium chloride, plus potassium replacement

5. Adrenal insufficiency: hydrocortisone 100 mg IV (do not delay in the acutely unwell; Synacthen test can be taken first)

6. Hypothyroidism: levothyroxine

D. Cerebral salt wasting — the SIADH mimic

In neurosurgical/neuro-ICU patients (SAH especially): hypovolaemic hyponatraemia with high urine sodium. Management is the opposite of SIADH — saline volume replacement (± salt, ± fludrocortisone). Fluid restriction is contraindicated. A beloved SCE discriminator.

7. High-Yield Drug Causes — Commit This Table to Memory

SCE pearl: Thiazide-induced hyponatraemia classically affects older women, weeks after initiation or dose increase, often with hypokalaemia. Stop the drug, check cortisol and TSH, and never re-challenge.

8. Worked SCE-Style Questions

Question 1 — A 79-year-old woman on bendroflumethiazide and sertraline presents after two falls. She is clinically euvolaemic. Na 117 mmol/L, serum osmolality 252, urine osmolality 420, urine sodium 45 mmol/L, TSH normal. What is the most appropriate initial management?

Answer: Stop the thiazide (and review the SSRI) with close sodium monitoring and mild fluid restriction. She is asymptomatic and chronic — no hypertonic saline. Note that recent diuretics make a formal SIADH diagnosis unsafe; re-assess after washout.

Question 2 — A 68-year-old man post hip surgery develops a generalised seizure. Na 106 mmol/L. What is the immediate management?

Answer: IV bolus of 100 mL of 3% sodium chloride over 10–15 minutes, recheck sodium, repeat up to 3 boluses. Not fluid restriction, not tolvaptan, not 0.9% saline.

Question 3 — A malnourished 58-year-old man with alcohol dependence is treated for symptomatic hyponatraemia (Na 104). Twenty-four hours later his sodium is 126 and he is dysarthric with new arm weakness. What is the most appropriate action?

Answer: This is overcorrection threatening ODS. Give DDAVP 1–2 micrograms IV with 5% dextrose to relower serum sodium toward a safe trajectory, and involve critical care/nephrology.

9. Source Guidelines Worth Reading Before the Exam

• NICE CKS — Hyponatraemia (the UK gold standard and the safest authority to quote in SCE answers)

• European Clinical Practice Guideline on Hyponatraemia (Spasovski et al., 2014; ESICM/ERBP/EAU) — the hypertonic saline bolus and correction-limit evidence base

• US Expert Panel Recommendations (Verbalis et al.) — alternative bolus regimens and vaptan guidance

• UK Kidney Association / Renal Association curriculum statements on water balance

• BNF/MHRA safety advice on tolvaptan

10. Ten Take-Homes for Exam Day

1. Osmolality first — confirm hypotonic before anything else

2. Correct for glucose in hypertonic hyponatraemia

3. Urine osmolality <100 = no ADH = polydipsia or low solute

4. Urine Na >30 + euvolaemia = think SIADH — but exclude cortisol and TSH first

5. Seizing = 100 mL 3% saline bolus, up to 3 boluses, recheck between

6. Correct no faster than 10 mmol/L per 24 h (8 if high risk)

7. Overcorrected = DDAVP + 5% dextrose

8. Fluid restriction 1–1.5 L is first line in chronic SIADH; tolvaptan is specialist, inpatient-only initiation

9. Cerebral salt wasting gets saline, not restriction — hypovolaemia is the discriminator

10. Elderly woman + thiazide + fall = stop the drug

Hyponatraemia rewards a systematic, guideline-anchored approach — exactly what SCE examiners are sampling. Master the algorithm above, rehearse the three worked stems, and you will convert one of the most feared SCE topics into guaranteed marks. Good luck!