Management of anesthesia and pregnancy in the myasthenic patient Surgical procedures of any type are often sufficiently stressful to produce decompensation of the disease. If the patient is unable to take medications orally, anticholinesterase agents may be given intramuscularly (approximately one-thirtieth of the oral dose of pyridostigmine and one-tenth the oral dose of neostigmine listed in Table 46-1). If corticosteroids were being used, they may be continued and the dose generally left unchanged; large “stress” doses are generally unnecessary, as mentioned earlier in the discussion of thymectomy. Neuromuscular blocking agents of the noncompetitive type may have a very prolonged effect in these patients and should be avoided as part of the anesthetic regimen. If they are necessary for some reason, a period of mechanical ventilation should be anticipated. In contrast, the dose of succinylcholine (which is not recommended) required to produce muscle relaxation may be larger than usual. Any drug, the use of which is contemplated in anesthetic and postsurgical management, should be checked against the list of agents that are capable of exaggerating myasthenic weakness (see further on).
Pregnancy is usually uncomplicated in patients with myasthenia but some women who are partially treated for myasthenia and have generalized weakness may have difficulty in assisting with vaginal delivery. However, the use of intravenous cholinesterase inhibitors is contraindicated because of the possibility of inducing uterine contractions, and cytotoxic drugs are generally avoided during pregnancy because of the potential for fetal abnormalities. Also, magnesium is not recommended for the treatment of eclampsia because its neuromuscular blocking effects may worsen myasthenic weakness. Delivery usually proceeds normally, and breast-feeding is not thought to be a problem with regard to the transmission of AChR antibodies. Almost half of women with myasthenia have an exacerbation of varying degree in the several weeks postpartum. A rapidly dropping level of alpha-fetoprotein has been implicated as this protein inhibits binding of antiacetylcholine antibodies to postsynaptic receptors. The issues of neonatal myasthenia and of reduced intrauterine movements with arthrogryposis are considered later.
Considered here are several disorders of neuromuscular transmission characterized clinically by muscular weakness and fatigability but differing in mechanism from autoimmune myasthenia gravis. The Lambert-Eaton myasthenic syndrome, neonatal myasthenia, the congenital myasthenic syndromes, and the myasthenic syndromes induced by drugs and toxins are the main disorders in this group. Two additional important diseases—botulism and organophosphate poisoning—are described elsewhere in the book.
The Myasthenic-Myopathic Syndrome of
This special form of neuromuscular disorder that in a few ways simulates myasthenia gravis, is observed most often in patients with small cell carcinoma of the lung, was first described by Lambert, Eaton, and Rooke in 1956 and further by Eaton and Lambert in 1957. Unlike myasthenia gravis, the muscles of the trunk, shoulder girdle, pelvic girdle, and lower extremities are the ones that become weak and fatigable. The first symptoms are difficulty in arising from a chair, climbing stairs, and walking; the shoulder muscles are usually affected later. Although ptosis, diplopia, dysarthria, and dysphagia may occur, presentation with these symptoms is distinctly unusual. Increasing weakness after exertion stamps the condition as myasthenic, but in direct contrast to myasthenia gravis, there may be a temporary increase in muscle power during the first few contractions. The tendon reflexes are often diminished but complete abolition of the reflexes should raise the question of an associated carcinomatous polyneuropathy. Fasciculations are not seen.