Anesthesia for Myotonias

MYOTONIAS

Myotonia Congenita & Paramyotonia Congenita

Myotonia congenita is a disorder manifested early in life with generalized myotonia. Both autosomal dominant (Thomsen’s) and recessive (Becker’s) forms exist. The disease is confined to skeletal muscle, and weakness is minimal or absent. Many patients have very well developed musculature due to near constant muscle contraction. Antimyotonic therapy includes phenytoin, mexiletine, quinine sul-fate, or procainamide. Other medications that have been used include tocainide, dantrolene, predni-sone, acetazolamide, and taurine. There is no cardiac involvement in myotonia congenita, and a normal life span is expected.Paramyotonia congenita is a very rare autoso-mal dominant disorder characterized by transient stiffness (myotonia) and, occasionally, weakness after exposure to cold temperatures. The stiffness worsens with activity, in contrast to true myoto-nia, thus the term paramyotonia. Serum potassium concentration may rise following an attack simi-lar to hyperkalemic periodic paralysis . Medications that have been used to block the cold response include mexiletine and tocainide.

Anesthetic management of patients with myo-tonia congenita and paramyotonia is complicated by an abnormal response to succinylcholine, intraop-erative myotonic contractions, and the need to avoid hypothermia. NMBs may paradoxically cause gen-eralized muscle spasms, including trismus, leading to difficulty with intubation and ventilation.

Infiltration of muscles in the operative field with a dilute local anesthetic may alleviate refrac-tory myotonic contraction. Among patients with these types of myotonia, none have been reported with positive in vitro tests for malignant hyperther-mia. Excised muscle in these patients does, however, display a prolonged myotonic contraction when exposed to succinylcholine. Excessive muscle con-traction during anesthesia, therefore, likely repre-sents aggravation of myotonia and not malignant hyperthermia.