Hyperkalaemic Paralysis

Hyperkalaemic paralysis is acute flaccid muscle weakness caused by raised serum potassium, usually over 7.0 mmol/L, or a transient attack in familial hyperkalaemic periodic paralysis with potassium that is normal or only mildly raised. It is a medical emergency because of arrhythmia risk.

Epidemiology. Paralysis from acquired hyperkalaemia is uncommon, since arrhythmia and cardiac arrest usually occur first. Familial hyperkalaemic periodic paralysis is rare, autosomal dominant and caused by SCN4A variants, with onset usually in the first decade.

Causes and associations.

• Acute or chronic kidney injury with reduced potassium excretion.
• Drugs: ACE inhibitors, angiotensin receptor blockers, spironolactone, eplerenone, trimethoprim, NSAIDs, heparin, potassium supplements, ciclosporin, tacrolimus and digoxin toxicity.
• Adrenal insufficiency, hyporeninaemic hypoaldosteronism and type 4 renal tubular acidosis.
• Rhabdomyolysis, tumour lysis, haemolysis, burns and acidosis.
• Familial periodic paralysis: attacks follow rest after exertion, fasting, cold or potassium intake.

Pathophysiology. Raised extracellular potassium depolarises muscle membranes, inactivating sodium channels and causing weakness. In SCN4A disease, defective channel inactivation produces sustained depolarisation and attacks lasting 30 minutes to a few hours.

Presentation is ascending flaccid weakness of the limbs and trunk with reduced or absent reflexes. Cranial nerves and respiratory muscles are usually spared. Paraesthesia and palpitations may occur. ECG may show peaked T waves, flattened P waves, widened QRS, a sine wave pattern, bradycardia or asystole. Familial attacks are shorter, and some patients have myotonia.

Red flags.

• Potassium over 6.5 mmol/L or any ECG change.
• Oliguria or anuria.
• Bradycardia, heart block or wide QRS.
• Respiratory muscle weakness and tissue breakdown (rhabdomyolysis, tumour lysis, burns).

Measure potassium with an urgent blood gas (avoid haemolysed samples), creatinine, urea, glucose, sodium, calcium, magnesium, creatine kinase, cortisol and bicarbonate. Do an ECG at once. Check urinary potassium and the transtubular potassium gradient if the cause is unclear. In suspected familial disease, check potassium during an attack and arrange genetic testing.

Differential diagnosis.

• Hypokalaemic and thyrotoxic periodic paralysis.
• Guillain-Barre syndrome, acute myelopathy and myasthenic crisis.
• Hypermagnesaemia, botulism and tick paralysis.
• Rhabdomyolysis with myopathy.
• Pseudohyperkalaemia from haemolysis or thrombocytosis.

Cardiac protection. For ECG changes or potassium over 6.5 mmol/L, give calcium gluconate 10% 30 mL (3 g) IV over 5 to 10 minutes, repeated after 5 to 10 minutes if ECG changes persist. Calcium chloride 10% 10 mL IV through a central line is an alternative. In digoxin toxicity, give slowly and with caution.

Shift potassium into cells.

• Soluble insulin 10 units IV with glucose 25 g (50 mL of 50% dextrose) over 15 minutes. Use 5 units in renal impairment. Follow with 10% dextrose at 50 mL/h for 5 hours if glucose is under 7 mmol/L. Check glucose hourly for 6 hours.
• Salbutamol 10 to 20 mg nebulised over 10 minutes. Avoid in ischaemic heart disease and tachyarrhythmia.
• Sodium bicarbonate 50 mmol IV (50 mL of 8.4%) over 5 minutes only when there is metabolic acidosis.

Remove potassium.

• Furosemide 40 to 80 mg IV if urine output is present and volume is adequate.
• Sodium zirconium cyclosilicate 10 g orally three times daily for up to 48 hours, then 5 to 15 g daily.
• Patiromer 8.4 g orally daily is slower and used for maintenance.
• Dialysis for refractory hyperkalaemia, anuria, severe acidosis or major tissue breakdown.

Stop contributing drugs. In adrenal insufficiency give hydrocortisone 100 mg IV.

Familial disease. Mild attacks respond to carbohydrate intake or a salbutamol inhaler. Prevention is a low potassium diet, avoidance of fasting and cold, and acetazolamide 125 to 250 mg orally twice daily or dichlorphenamide 50 mg orally twice daily. Monitor potassium and bicarbonate, and avoid in sulphonamide allergy.

Monitoring. Continuous ECG, potassium 1 hour after treatment and then every 2 hours for 6 hours, glucose hourly for 6 hours after insulin, creatinine, urine output and respiratory function. Rebound hyperkalaemia occurs as insulin and salbutamol wear off at 2 to 4 hours.

Referral and escalation. Intensive care and the renal team early for ECG changes with renal failure or refractory hyperkalaemia. Neurology for familial disease.

Special groups. In renal impairment, reduce insulin to 5 units and use dialysis early. In children, calcium gluconate 10% is 0.5 mL/kg IV over 5 to 10 minutes and insulin is 0.1 units/kg with glucose 0.5 g/kg.