The myotonias are characterized by delayed relaxation of skeletal muscle after voluntary contraction owing to dysfunction of ion channels in the muscle membrane.
- Clinical features include diabetes mellitus, thyroid dysfunction, adrenal insufficiency, and cardiac abnormalities (e.g., conduction delays, heart block [sudden death], tachydysrhythmias, cardiomyopathy).
- Pulmonary function studies demonstrate a restrictive lung disease pattern, mild arterial hypoxemia, and diminished ventilatory responses to hypoxia and hypercapnia.
- Pregnancy may produce an exacerbation of myotonic dystrophy, and congestive heart failure is more likely to occur during pregnancy.
- Management of Anesthesia
- SCh produces myotonia and should not be administered to these patients. The response of these patients to nondepolarizing muscle relaxants may be enhanced. Reversal with neostigmine may provoke myotonia. The response to the peripheral nerve stimulator must be carefully interpreted because muscle stimulation may produce myotonia (misinterpreted as sustained tetanus when significant neuromuscular block still exists).
- Patients with myotonia are very sensitive to the ventilatory depressant effects of opioids, barbiturates, benzodiazepines, and volatile anesthetics.
- No specific anesthetic technique has been shown to be superior for patients with myotonic dystrophy. Propofol infusions may be acceptable. Inhaled anesthetics may be used, but close monitoring of cardiac rhythm and cardiovascular function is indicated.