Hyperkalemic Periodic Paralysis

Hyperkalaemic periodic paralysis (HyperPP) is an autosomal dominant sodium channelopathy (SCN4A, T704M, M1592V) with attacks of weakness and normal to high potassium.

• Onset in first decade; attacks shorter (30 minutes to 3 hours) and more frequent than hypokalaemic.

• Often myotonia and paramyotonia between attacks.

• Triggers: rest after exercise, fasting, potassium-rich foods, cold, stress.

• Late fixed weakness in some.

• Episodes of flaccid weakness with myotonia (lid lag, grip), usually preserved bulbar function.

• Warming of muscles after mild exercise may avert attacks.

• Paramyotonia (stiffness worsening with exercise and cold) in overlapping phenotypes.

• Potassium normal or raised (up to 5 to 6.5 mmol/L) during attack; rises in 2 to 3 hours after potassium challenge (not necessary).

• ECG: peaked T waves if high K.

• EMG: myotonic discharges.

• Genetic testing of SCN4A.

• Exclude hyperkalaemia from renal or adrenal disease, hypoaldosteronism.

• Mild attacks: light exercise, carbohydrate snacks (glucose 30 g), inhaled salbutamol 2 puffs.

• Severe attacks: calcium gluconate 10% 10 to 20 mL IV, insulin 10 units with glucose, salbutamol nebulised.

• Prevention: frequent carbohydrate-rich low potassium meals; avoid fasting, cold and rest after exertion.

• Medication: acetazolamide 125 to 250 mg twice daily, dichlorphenamide 50 mg twice daily, hydrochlorothiazide 12.5 to 25 mg daily (monitor potassium).

• Myotonia: mexiletine 150 to 200 mg two to three times daily.

• Caution with anaesthesia (depolarising relaxants).

• Genetic counselling.