Hypokalemic Periodic Paralysis

Hypokalaemic periodic paralysis (HypoKPP) is an autosomal dominant channelopathy with attacks of weakness and low serum potassium.

• CACNA1S (60%, R528H) and SCN4A (20%, R672); penetrance reduced in women, male predominance.

• Onset in adolescence; attacks every few weeks, fewer with age.

• Progressive proximal myopathy with vacuoles in later life.

• Sporadic thyrotoxic form is common in Asian men aged 20 to 40.

• Attacks of flaccid proximal weakness lasting 2 to 24 hours (up to days), often on waking after exercise, high carbohydrate meal or alcohol.

• Preserved respiratory muscles in most; respiratory and arrhythmic complications are rare.

• Interictal normal strength early; fixed weakness later.

• Serum potassium below 3.0 mmol/L during attack (between attacks normal), low phosphate and magnesium.

• ECG: sinus bradycardia, U waves, ST depression.

• Thyroid function tests, aldosterone/renin, urine potassium (low in periodic paralysis, high in renal loss), bicarbonate.

• Genetic testing; long exercise test.

• Exclude thyrotoxic periodic paralysis and renal potassium loss states.

• Acute: oral potassium chloride 20 to 40 mmol, repeated every 30 minutes up to 100 mmol total while monitoring potassium and ECG; IV potassium 10 mmol/h via central line in severe weakness (avoid dextrose-based fluids, rebound hyperkalaemia).

• Prevention: acetazolamide 125 mg twice daily increasing to 500 mg twice daily (some SCN4A patients worsen), dichlorphenamide 50 mg twice daily; potassium sparing diuretics (amiloride 5 to 10 mg, triamterene); oral potassium 10 to 20 mmol three times daily.

• Dietary: low carbohydrate, low sodium diet; avoid strenuous exercise followed by rest, alcohol.

• Thyrotoxic form: propranolol 40 mg every 6 hours plus potassium, then antithyroid therapy.

• Monitor kidney stones with acetazolamide.

• Genetic counselling.