Periodic Paralysis

Primary periodic paralyses are inherited ion channelopathies with episodic flaccid weakness.

• Hypokalaemic periodic paralysis (CACNA1S, SCN4A), hyperkalaemic periodic paralysis (SCN4A), paramyotonia congenita, Andersen-Tawil syndrome (KCNJ2: periodic paralysis, ventricular arrhythmia, dysmorphism).

• Secondary: thyrotoxic periodic paralysis (East Asian men), potassium depletion (renal, GI), Bartter/Gitelman, hyperaldosteronism, licorice.

• Attacks last hours to days; permanent myopathy can develop.

• Episodes of proximal limb weakness with preserved consciousness and respiration, onset after rest following exercise, carbohydrate load, stress, cold.

• Reduced or absent reflexes during attacks; ECG changes.

• Interictal myotonia (hyperkalaemic, paramyotonia), cardiac arrhythmia (Andersen-Tawil).

• Serum potassium during attack (low, high or normal), thyroid function, glucose, magnesium, bicarbonate, urinary electrolytes.

• ECG: U waves/QT prolongation (low K), peaked T waves (high K).

• Genetic testing (CACNA1S, SCN4A, KCNJ2).

• Long exercise EMG test (McManis) for decrement in CMAP amplitude.

• Rule out secondary causes (thyrotoxicosis, renal tubular acidosis, aldosteronism).

• Acute attack in hypokalaemic form: potassium chloride 0.5 to 1 mmol/kg orally (20 to 40 mmol), repeat in 30 minutes; avoid IV KCl in dextrose; ECG monitoring.

• Prevention: acetazolamide 125 to 1000 mg daily in divided doses (dichlorphenamide 50 to 150 mg daily), potassium sparing diuretics (triamterene 50 to 150 mg, spironolactone 100 mg), potassium supplements; avoid triggers (high carbohydrate meals, exercise followed by rest, alcohol, cold).

• Hyperkalaemic form: carbohydrate snack, inhaled salbutamol, calcium gluconate 10% 10 to 20 mL IV for severe; hydrochlorothiazide 12.5 to 25 mg or acetazolamide daily for prevention.

• Thyrotoxic periodic paralysis: potassium replacement with propranolol 3 mg/kg/day, treat thyrotoxicosis.

• Andersen-Tawil: flecainide 100 to 150 mg twice daily, ICD for arrhythmia.

• Genetic counselling.