Anesthesia for Periodic Paralysis

PERIODIC PARALYSIS

Periodic paralysis is a group of disorders character-ized by spontaneous episodes of transient muscle weakness or paralysis. Symptoms usually begin in childhood, with episodes lasting a few hours and typically sparing respiratory muscle involvement. The weakness usually lasts less than 1 hour but can last several days, and frequent attacks may lead to progressive, long-term weakness in some patients. Hypothermia exacerbates the frequency and severity of episodes. Muscle strength and serum potassium concentrations are usually normal between attacks. The episodes of weakness are due to a loss of muscle fiber excitability secondary to partial depolarization of the resting potential. This partial depolarization prevents the generation of action potentials and thereby precipitates weakness. Periodic paralysis is classified into primary genetic channelopathies and secondary acquired forms. The genetic types are due to dominantly inherited mutations in the voltage-gated sodium, calcium, or potassium ion channels. Classifications have been based on clinical differences, but these have not been shown to relate to specific ion chan-nels. Different defects in the same channel can cause different clinical pictures, whereas mutations in dif-ferent channels may have similar clinical pictures. However, the clinical classifications remain useful as guides to prognosis and therapy.

Hypokalemic periodic paralysis is typicallyassociated with low serum potassium levels, and hyperkalemic periodic paralysis with elevated serumpotassium levels, during episodes of weakness. In these defects, muscle membranes are inexcitable to both direct and indirect stimulation due to either decreased potassium conductance or increased sodium conductance, respectively. Both defects are associated with fluid and electrolyte shifts.

Th yrotoxicosis is associated with a secondary form of hypokalemic periodic paralysis. It resembles the primary form but is much more common in men than women, particularly in persons of Asian descent and in young adults. Once the thyroid condition is treated, the episodes usually cease. The disorder can develop in 10–25% of hyperthyroid Asian men. The metabolic sequelae and fluid and electrolyte shifts seen in the primary form are also seen in secondary hypokalemic periodic paralysis. Treatment involves management of the hyperthy-roidism, avoidance of high carbohydrate and low potassium meals, and administration of potassium chloride for acute attacks.

Secondary hypokalemic paralysis can also develop if there are marked losses of potassium through the kidneys or the gastrointestinal tract. The associated weakness is, at times, episodic and potas-sium levels are much lower than in other variants of hypokalemic periodic paralysis. Management of the primary disease with potassium replacement, and treatment of acidosis or alkalosis, is important in preventing attacks.

Patients who consume large amounts of bar-ium salts, which block potassium channels, can also develop hypokalemic periodic paralysis. This condition is treated by stopping the barium salts and administering oral potassium.

Potassium levels that exceed 7 mEq/L between episodes of weakness suggest a secondary form of hyperkalemic periodic paralysis. Treatment is tar-geted toward the primary disease and involves restriction of potassium.

Anesthetic Considerations

Anesthetic management of patients with peri-odic paralysis is directed toward preventingattacks. Intraoperative management should include frequent determinations of plasma potassium con-centration and careful electrocardiographic moni-toring to detect arrhythmias. Because of the potential for glucose-containing intravenous solutions to lower plasma potassium concentration, they should not be used in patients with hypokalemic paralysis, whereas they may benefit patients

with hyperkalemic paralysis. The response to NMBs is unpre-dictable, and neuromuscular function shouldbe carefully monitored during their use. Increased sensitivity to nondepolarizing NMBs is particularly apt to be encountered in patients with hypokalemic periodic paralysis. Succinylcholine is contraindi-cated in hyperkalemic paralysis and perhaps other variants as well because of the risk of hyperkalemia. Intraoperative maintenance of core temperature is important because shivering and hypothermia may trigger or exacerbate episodes of periodic paralysis.