Anaesthesia and myasthenia gravis

Myasthenia gravis is an autoimmune disease, resulting from the production of antibodies against the acetylcholine receptors of the endplate. These antibodies reduce the number of active receptors, brought about either by functional block of the receptors, by increased rate of receptor degradation, or by complement-mediated lysis. In myasthenic muscles, the miniature endplate potential amplitude is decreased, and a large proportion of the endplate potentials are subthreshold. Repetitive nerve stimulation results in a decremental response. The disease is frequently associated with morphological abnormalities of the thymus. In young patients, thymic hyperplasia is common while thymoma is more frequent in elderly patients. Medical treatment of myasthenia gravis aims at improving of neuromuscular transmission by anticholinesterases, suppressing the immune system by corticosteroids and immunosuppressants, or by decreasing the circulating antibodies by plasmapheresis. Adults with generalized myasthenia should have a trans-sternal thymectomy. A balanced technique of general anaesthesia which includes the use of muscle relaxants can be safely used, provided neuromuscular transmission is monitored. Myasthenic patients are sensitive to nondepolarizing relaxants but intermediate-acting nondepolarizing relaxants such as atracurium and vecuronium are eliminated rapidly, and can be titrated to achieve the required neuromuscular block that can be completely reversed at the end of surgery. Postoperatively, ventilatory support may be required in high-risk patients. Also, medical treatment may be maintained, tapered or discontinued depending on the outcome of surgery. Thymectomy benefits nearly 96% of patients, 46% develop complete remission and 50% are asymptomatic or improve on therapy.