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Myasthenia Gravis
Causes:
Autoimmune destruction or inactivation of postsynaptic acetylcholine receptors at the neuromuscular junction, leading to reduced number of receptors and degradation of their function.
Clinical signs and symptoms:
The muscle weakness can be asymmetric, confined to one group of muscles, or generalized. Ocular muscles are most commonly affected, resulting in fluctuating ptosis and diplopia. With bulbar involvement, laryngeal and pharyngeal muscle weakness can result in dysarthria, difficulty in chewing and swallowing, problems clearing secretions, or pulmonary aspiration. Severe disease is usually also associated with proximal muscle weakness (primarily in the neck and shoulders) and involvement of respiratory muscles. Muscle strength characteristically improves with rest but deteriorates rapidly with exertion.
Treatment: Anticholinesterase drugs are the most commonly used agents to treat muscle weakness. Pyridostigmine is the agent used most often. Excessive administration of an anticholinesterase may precipitate cholinergic crisis.
Myasthenia gravis crisis is an exacerbation requiring mechanical ventilation and should be suspected in any patient with respiratory failure of unclear etiology.
• Evaluation should focus on the course of the disease, muscle groups affected, drug therapy, and coexisting illnesses. Myasthenic patients with respiratory and oropharyngeal weakness should be treated aggressively preoperatively with intravenous (IV) immunoglobulin or plasmapheresis.
• Adjustments in anticholinesterase medication, immunosuppressants, or steroid therapy in the perioperative period may be necessary.
• Patients who have myasthenia gravis with respiratory muscle or bulbar involvement are at increased risk for pulmonary aspiration. Premedication with metoclopramide or an H2 blocker or proton pump inhibitor may decrease this risk.
• Because patients with myasthenia are often very sensitive to the respiratory depressant effect of opioids and benzodiazepines, premedication with these drugs should be done with caution, if at all.
Intraoperative Management
With the exception of neuromuscular blocking agents (NMBAs), standard anesthetic agents may be used in patients with myasthenia gravis.
• Regarding depolarizing NMBAs, the response to succinylcholine is unpredictable. Patients may manifest a relative resistance, a prolonged effect, or an unusual response (phase II block). The dose of succinylcholine may be increased to 2 mg/kg to overcome any resistance, but a prolonged effect should be anticipated.
• Many patients with myasthenia gravis are exquisitely sensitive to nondepolarizing NMBAs. Even a defasciculating dose in some patients may result in nearly complete paralysis. If NMBAs are necessary, small doses of a relatively short-acting nondepolarizing agent is preferred.
Paraneoplastic Neuromuscular Syndromes
Paraneoplastic syndromes are immune-mediated diseases associated with an underlying cancer. Myasthenia gravis is often considered a paraneoplastic syndrome because it is an autoimmune disorder associated with thymic hyperplasia, including thymoma. Other neurologic or neuromuscular paraneoplastic syndromes include Lambert-Eaton myasthenic syndrome (LEMS), limbic encephalitis, neuromyotonia, stiff person syndrome, myotonic dystrophy, and polymyositis.
LEMS is a paraneoplastic syndrome characterized by proximal muscle weakness that typically begins in the lower extremities but may spread to involve upper limb, bulbar, and respiratory muscles. Dry mouth, male impotence, and other manifestations of autonomic dysfunction are also very common. LEMS is usually associated with small cell carcinoma of the lung but may also be seen with other malignancies or as an idiopathic autoimmune disease. The disorder results from a presynaptic defect of neuromuscular transmission in which antibodies to voltage-gated calcium channels on the nerve terminal markedly reduce the quantal release of acetylcholine at the motor end plate. In contrast to myasthenia gravis, muscle weakness associated with LEMS improves with repeated effort and is improved less dramatically by anticholinesterase drugs. Guanidine hydro-chloride and diaminopyridine, which increase the presynaptic release of acetylcholine, often produce significant improvement in LEMS.
Limbic encephalitis is a degenerative central nervous system disorder characterized by personality changes, hallucinations, seizures, autonomic dysfunction, varying degrees of dementia, and asymmetric loss of sensation in the extremities. Approximately 60% of cases are paraneoplastic, with a strong association with small cell lung carcinoma.
Neuromyotonia is a condition of peripheral nerve hyperexcitability that is frequently associated with an underlying cancer but may also be inherited or associated with diabetic, drug- or toxin-induced, or other acquired neuropathies. Its features include myokymia (a continuous undulating movement of muscles similar to a “bag of worms”), stiffness, impaired muscle relaxation, painful muscle cramping, hyperhidrosis, and muscle hypertrophy.
Stiff person syndrome is a progressive disorder characterized by axial stiffness and rigidity that may subsequently involve the proximal limb muscles. In advanced cases, paraspinal rigidity may cause marked spinal deformities, and the patient may have difficulty with ambulation and a history of frequently falling.
Polymyositis is an inflammatory myopathy of skeletal musculature, especially proximal limb muscles, characterized by weakness and easy fatigability. Patients are prone to aspiration and frequent pneumonias because of thoracic muscle weakness and because of dysphagia secondary to oropharyngeal muscle involvement. They may also exhibit cardiac dysrhythmias caused by conduction defects.
Anesthetic management: Patients with LEMS and other neuromuscular paraneoplastic syndromes are very sensitive to both depolarizing and nondepolarizing NMBAs. Volatile agents alone are often sufficient to provide muscle relaxation for both intubation and most surgical procedures. NMBAs should be given only in small increments and with careful neuromuscular monitoring. Because these patients frequently exhibit marked debility, benzodiazepines, opioids, and other medications with sedative effects should be administered with caution.
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