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Department of Anesthesiology, Vak
f Gureba Hospital, Istanbul, Turkey
To the Editor:
We read with interest the article by Itoh et al. (1) and we agree that patients with myasthenia gravis are hypersensitive to nondepolarizing neuromuscular blocking drugs. In this letter, we report a case of a symptomless myasthenic patient who responded abnormally to neuromuscular blockade.
A 24-yr-old woman, weight 80 kg, was admitted to our hospital for urgent Cesarean delivery. Myasthenia gravis was diagnosed 11 years ago and treated by pyridostigmine, followed 2 yr later by thymectomy. She was only followed-up 1 yr after the operation. Her exercise tolerance had been excellent until the operation. The patient was fit and well on admission. She was not receiving medication. Immediate neurologic examination was normal. Anesthesia was induced with thiopental 5mg/kg, and rocuronium 0.25 mg/kg to produce neuromuscular blockade. Neuromuscular blockade was monitored train-of-four (TOF). Her electrocardiogram, oxygen saturation, and noninvasive arterial blood pressure were monitored throughout the anesthetic with no abnormalities. The newborn baby did not have symptoms of myasthenia. Anesthesia was maintained with oxygen, nitrous oxide, and 2% sevoflurane. Surgery was completed 45 min after induction, but response to TOF was absent. The patient was accepted to intensive care unit (ICU) with midazolam sedation. Pressure-controlled ventilation was started in ICU. One hundred ten minutes after induction, respiratory effort began. She did well against verbal stimulus and was able to cough. The trachea was extubated 128 min later. She was monitored in the ICU for 24 h and sent to the ward after normal physical examination findings.
Myasthenia gravis is an important disease to anesthesiologists, because it affects the neuromuscular junction. A number of case reports have described the safe use of intermediate acting muscle relaxants in myasthenic patients. Doses ranging from 10% to 50% of those required in nonmyasthenic patients were used, and prompt extubation was performed after spontaneous on pharmacologic reversal (2). Recently, it was reported that sevoflurane is suitable as a sole anesthetic for myasthenic patients undergoing sternal split thymectomy, implying that sevoflurane alone provided adequate muscle relaxation (3). An abnormal sensitivity to nondepolarizing muscle relaxant is the future of myasthenic patients, but there is only one report about the response of myasthenic patients in remission (4). Cured myasthenic patients should be assumed to be sensitive to affect neuromuscular blocking drugs and volatile anesthetics in the urgent anesthetic approach.
References
Department of Anesthesiology and Intensive Care Medicine Kanazawa University School of Medicine, Kanazawa, Japan
In Response:
We would like to thank Basaranoglu et al. for their comments on our article. As is often mentioned, patients with myasthenia gravis (MG) are hypersensitive to nondepolarizing neuromuscular blocking drugs (NMBDs). Little has been reported, however, about the response of MG patients in remission. We have also performed anesthetic management of an MG patient who had been thymectomized 13 months previously and was symptomless without medication (data not published). We calculated the effective doses of vecuronium in the adductor pollicis and orbicularis oculi muscles. Effective doses of 50% (ED50) in these two muscles under sevoflurane-nitrous oxide anesthesia were 12.1 and 3.7 µg/kg, respectively. Although ED50 in the adductor pollicis muscle was almost normal (1), the value in the orbicularis oculi muscle was very small. We have often observed this tendency for such a difference to occur between these muscles (2).
As pointed out by Basaranoglu et al., even cured MG patients should be assumed to remain sensitive to the effect of NMBDs. We agree with this point. Sensitivity to NMBDs in MG patients varies greatly between patients (3,4). This tendency is thought to apply to MG patients in remission. Monitoring of neuromuscular function is therefore especially important, regardless of the state of MG.
References
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