Key Facts

Electrolyte disorders are ubiquitous in hospital medicine and require systematic assessment of volume status, underlying cause, and severity to guide safe correction.

Epidemiology

• Hyponatraemia: 20-30% of hospitalised patients
• Hyperkalaemia: 10% of hospitalised patients
• Hypokalaemia: 10-20% of hospitalised patients
• Electrolyte disorders are associated with increased length of stay, ICU admission, and mortality

Classification of Hyponatraemia by Volume Status

Hypovolaemic (low total body water + low Na): diarrhoea, vomiting, Addison disease, diuretics Euvolaemic (normal total body water): SIADH (most common euvolaemic cause), hypothyroidism, cortisol deficiency Hypervolaemic (high total body water): heart failure, cirrhosis, nephrotic syndrome, CKD

Pathophysiology

• Sodium determines plasma osmolality and ECF volume
• Potassium maintains resting membrane potential – critical for cardiac and neuromuscular function
• Calcium: 50% protein-bound (correct for albumin); ionised calcium is physiologically active
• Magnesium: cofactor for Na-K-ATPase; deficiency causes refractory hypokalaemia and hypocalcaemia

Hyponatraemia

• Mild (130-134): often asymptomatic
• Moderate (125-129): nausea, headache, confusion
• Severe (<120): seizures, coma, respiratory arrest
• Chronic hyponatraemia: may be surprisingly well-tolerated due to brain adaptation

Hypokalaemia

• Muscle weakness, cramps, paralysis
• U-wave on ECG, flattened T waves, ST depression
• Cardiac arrhythmias (especially with digoxin)
• Constipation, ileus

Hypercalcaemia

• "Bones, stones, groans, and psychic moans"
• Polyuria, polydipsia, dehydration
• Short QT on ECG
• Confusion, depression, coma in severe cases

Hypocalcaemia

• Perioral and distal paraesthesiae
• Muscle cramps, tetany, carpopedal spasm
• Chvostek and Trousseau signs
• Prolonged QT → risk of torsades de pointes
• Seizures in severe cases

Red Flags

• Na <120 with neurological symptoms → hypertonic saline (3% NaCl)
• K+ >6.5 with ECG changes → emergency protocol
• Corrected calcium >3.5 mmol/L → IV saline + zoledronic acid/calcitonin
• Symptomatic hypocalcaemia → IV calcium gluconate 10% 10-20mL over 10 minutes

For All Electrolyte Disorders

• Serum electrolytes: Na, K, Ca (adjusted for albumin), Mg, PO4
• Renal function: U&Es
• Serum osmolality: key for hyponatraemia workup
• Urine osmolality and urine Na: essential for hyponatraemia classification
• ECG: K+ and Ca²⁺ abnormalities
• ABG/VBG: acid-base status

Specific

• Hyponatraemia: serum/urine osmolality, urine Na, TFTs, cortisol, volume status assessment
• Hyperkalaemia: ECG, exclude pseudohyperkalaemia, CK (rhabdomyolysis), cortisol
• Hypercalcaemia: PTH, vitamin D, myeloma screen, PTHrP, CXR
• Hypocalcaemia: PTH, vitamin D, magnesium, phosphate
• Hypomagnesaemia: serum Mg, urinary Mg (renal vs GI loss)

Hyponatraemia

• Acute symptomatic (<48h or seizures): 3% hypertonic saline 100-150mL IV over 20 min (repeat ×2 if needed); aim ↑Na by 4-6 mmol/L in first 6 hours
• Chronic: correct by ≤10 mmol/L per 24 hours (risk of ODS if too fast)
• Hypovolaemic: IV 0.9% NaCl
• SIADH: fluid restriction (750-1000mL/day); tolvaptan 15mg OD if refractory; demeclocycline (less used)
• Hypervolaemic: fluid restriction + treat underlying cause (diuretics for HF)

Hyperkalaemia

• See dedicated hyperkalaemia topic for emergency protocol

Hypercalcaemia

• Mild (<3.0): oral hydration, avoid thiazides
• Moderate-severe (>3.0): IV 0.9% NaCl 3-4L/24h (rehydration) + IV zoledronic acid 4mg over 15 min (onset 2-4 days)
• Life-threatening (>3.5): add calcitonin 4IU/kg SC/IM 12-hourly (rapid onset but tachyphylaxis)
• Denosumab: if refractory to bisphosphonates
• Treat underlying cause: parathyroidectomy (PHPT), chemo/radio (malignancy)

Hypocalcaemia

• Symptomatic: 10% calcium gluconate 10-20mL IV over 10 minutes (with cardiac monitoring); may need continuous infusion
• Chronic: oral calcium (Calcichew 1-2 tablets TDS) + alfacalcidol/calcitriol
• Correct magnesium if low (IV MgSO4 8mmol over 15-20 min)

Hypomagnesaemia

• IV MgSO4 8-20mmol over 2-4 hours (severe/symptomatic)
• Oral Mg glycerophosphate 1-2 tablets TDS (chronic)
• Must correct before hypokalaemia/hypocalcaemia will respond to treatment

Referral Criteria

• Severe/symptomatic electrolyte disorder → acute medicine/ICU
• Recurrent hyponatraemia → endocrinology (SIADH workup)
• Hypercalcaemia with raised PTH → endocrine surgery
• Refractory hypokalaemia → nephrology (Bartter/Gitelman)

• Severe hyponatraemia (<120): mortality 10-25% if acute; osmotic demyelination syndrome if corrected too rapidly (irreversible brain damage)
• Severe hyperkalaemia (>7.0): mortality >50% if untreated
• Severe hypercalcaemia (>3.5): can cause cardiac arrest; good prognosis if treated promptly
• Chronic electrolyte disorders: prognosis depends on underlying cause
• ODS (from rapid sodium correction): devastating neurological outcome; classically pontine myelinolysis

Hyponatraemia Management Algorithm

ECG Changes in Electrolyte Disorders