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YOUR PATIENT

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An 11-year-old girl with myasthenia gravis presents for thymectomy. The patient has a history of ptosis, dysphagia, and generalized muscle weakness.

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The medications she is using are pyridostigmine and prednisone.

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PREOPERATIVE CONSIDERATIONS

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Myasthenia gravis (MG) is an autoimmune disease in which antibodies are directed against the acetylcholine receptor at the neuromuscular junction. The disease is characterized by weakness and easy fatigability of voluntary muscles. Common symptoms include ptosis, diplopia, dysphagia, and dysarthria. Weakness of respiratory muscles may be severe during a myasthenic crisis. There are three types of myasthenia gravis that present in the pediatric population. Neonatal MG is a transient disorder caused by the passage of antibodies across the placenta from mothers who have MG, which may occur in 20%-30% of infants born to affected mothers. Congenital myasthenia is an autosomal recessive condition affecting the motor endplate. Juvenile MG usually presents in children over the age of 10, has a female predominance, and tends to have a presentation similar to that of the MG seen in adults: autoimmune in nature, often associated with an abnormal thymus gland, and classified as affecting either ocular muscles only or all voluntary muscles. Symptoms of MG worsen with exercise and tend to improve with rest. Medical therapy consists of treatment with cholinesterase inhibitors such as pyridostigmine and neostigmine to increase the available level of acetylcholine at the neuromuscular junction, as well as steroids to suppress the immune response. In a myasthenic crisis, IV immunoglobulin (IVIG) and/or plasmapheresis may be necessary. Surgical treatment with thymectomy can be curative, as antibodies to the acetylcholine receptor are produced by the thymus. Because of potential immunosuppression, thymectomy usually is deferred until the patient is at least 10 years old.

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Preoperative evaluation should include a detailed history of the severity of weakness, specifically regarding bulbar and respiratory muscles. Pulmonary function tests are helpful if the child is cooperative.

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Anticholinesterase medications increase airway secretions, potentiate narcotic effects, increase sensitivity to nondepolarizing muscle relaxants, and inhibit the metabolism of succinylcholine and ester local anesthetics. Ideally, anticholinesterase medications would be discontinued preoperatively, and the patient would be scheduled as the first case. However, discontinuation of anticholinesterase therapy in children with severe MG may cause aspiration or respiratory failure, so in these patients, medications are continued until surgery. Because of the autoimmune nature of MG, there is an association with thyroid dysfunction, which should be evaluated during the preoperative assessment.

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Sedative premedication should be avoided because of possible potentiation effects of anticholinesterase therapy. Consider premedication with an anticholinergic drug such as glycopyrrolate to decrease excess salivation.

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ANESTHETIC MANAGEMENT

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  • Continue any preoperative corticosteroids, and supplement with intraoperative stress dose hydrocortisone.

  • Induction and maintenance can be done with either volatile agents or IV agents such as propofol and remifentanil.

  • Do invasive blood pressure monitoring for the surgery itself, as ...

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