Intercostal muscle biopsy studies in myasthenia gravis: clinical correlations and the direct effects of drugs and myasthenic serum

Complement and myasthenia gravis

Myasthenia gravis is a neuromuscular disease associated with antibodies against components of the neuromuscular junction, most often against the acetylcholine receptor (AChR). Although several mechanisms have been postulated to explain how these autoantibodies can lead to the pathology of the disease, convincing evidence suggests that destruction of the receptor-bearing postsynaptic membrane by complement membrane attack complex is of central importance. In this review, evidence for the importance of complement, and possible relationships between autoantigen, autoantibodies, complement activation, and the destruction of the membrane are discussed. More recent insights from the results of the complement-inhibiting therapeutic antibody eculizumab are also described, and the mechanisms connecting antibody binding to complement activation are considered from a structural viewpoint.

Thymoma removal in a cat with acquired myasthenia gravis: a case report and literature review of anesthetic techniques

HISTORY AND PRESENTATION: A 12 year old, 4.2 kg, domestic long hair, castrated male cat was presented with regurgitation, inability to retract the claws, general weakness, cervical ventroflexion and weight loss. A thymic mass was evident on radiographs. Acetylcholine receptor antibody titer was positive for acquired myasthenia gravis (MG). Thymectomy via midline sternotomy was scheduled. ANESTHETIC MANAGEMENT:  Oxymorphone and atropine were administered subcutaneously as premedication, and anesthesia was induced with etomidate and diazepam given intravenously to effect. The cat's trachea was intubated and anesthesia was maintained with isoflurane in oxygen, and continuous infusions of remifentanil and ketamine. Epidural analgesia with preservative-free morphine was administered prior to surgery. Postoperative analgesia was provided by oxymorphone subcutaneously, interpleural bupivacaine, and fentanyl infusion. Postoperative complications included airway obstruction, hypoxemia and hypercapnia.

FOLLOW-UP

The cat was discharged 3 days after surgery. Discharge medications included pyridostigmine and prednisone. Nine days after surgery, the cat had a significant increase in its activity level, and medications were discontinued. Histopathologically, the mass was consistent with a thymoma. Approximately 6 weeks later the cat became weak again and pyridostigmine and prednisone administration was resumed.

CONCLUSION

The perioperative management of patients with MG for transsternal thymectomy is a complex task. The increased potential for respiratory compromise requires the anesthesiologist to be familiar with the underlying disease state, and the interaction of anesthetic and non-anesthetic drugs with MG. Careful monitoring of ventilation and oxygenation is indicated postoperatively.

[Endotracheal intubation without the use of muscle relaxants in patients with myasthenia gravis]

INTRODUCTION

Although muscle relaxants have been widely used to facilitate endotracheal intubation, the administration of these drugs in myasthenic patients may be associated with adverse events.

MATERIAL AND METHODS

After obtaining Institutional Reviewing Board approval and informed, patient consent, 30 patients with myasthenia gravis were enrolled in a prospective, double-blind, randomized clinical trial. We compared intubating conditions (ease of laryngoscopy, vocal cords, cough, jaw relaxation, limb movement) following fentanyl 2 mg/kg and propofol 2 mg/kg (group PRO, n = 15) vs fentanyl 2 mg/kg and sevoflurane 5% in a 1:2 mixture of oxygen and nitrous oxide (group SEVO, n = 15). The statistical analysis was performed using Student's t test and Chi-quadrate test, p < 0.05 being regarded as significant.

RESULTS

The overall intubating conditions were excellent in 67% of patients in the group PRO vs 80% of patients in the group SEVO (p > 0.05). One patient in each group had clinically unacceptable conditions for intubation. The mean intubation score was 5.7 +/- 1.0 in the group PRO vs 5.9 +/- 0.9 in the group SEVO (p > 0.05). Three patients receiving propofol and one patient receiving sevoflurane had mild hoarseness after the surgery (p > 0.05).

CONCLUSION

Both propofol and sevoflurane, supplemented with fentanyl, provide good intubating conditions without the use of muscle relaxants in patients with myasthenia gravis.

[Anaesthesia of the myasthenic patients during thymectomy]

Myasthenia is a rare autoimmune disease characterized by fluctuating muscle weakness and fatigability due to a reduction in available acetylcholine receptors at the neuromuscular junction. Data of 186 patients suffering from myasthenia were collected retrospectively. All patients underwent thymectomy over a 23 years period from 1981 to 2006 without surgical mortality. Postoperative ventilation was required for more than 24 hours in seven patients and one patient needed postoperative ventilatory support more than seven days. Thymectomy for myasthenia was performed using promethazine and atropine in general anaesthesia. Introduction could be facilitated with propofol, etomidate or thiopental and sevoflurane, avoiding use of any muscle relaxants. Non-depolarizing muscle relaxants were not used during the procedures. Adequate surgical conditions were provided by short-acting inhaled anaesthetics (sevoflurane) and small doses of opiates. 95% of the narcotized patients were immediately extubated after the procedure in the operating room. Length of stay in intensive care unit could have been reduced without any postoperative ventilatory support. Nonsteroid analgesics and nalbuphine were used for pain relief. Anaesthesia of thymectomy is based on volatile gases. Airway complications can be prevented with use of small amounts of anticholinergic drugs, perhaps steroids.

Myasthenia gravis: an autoimmune response against the acetylcholine receptor

Myasthenia gravis (MG) is an organ-specific autoimmune disease caused by an antibody-mediated assault on the muscle nicotinic acetylcholine receptor (AChR) at the neuromuscular junction. Binding of antibodies to the AChR leads to loss of functional AChRs and impairs the neuromuscular signal transmission, resulting in muscular weakness. Although a great deal of information on the immunopathological mechanisms involved in AChR destruction exists due to well-characterized animal models, it is not known which etiological factors determine the susceptibility for the disease. This review gives an overview of the literature on the AChR, MG and experimental models for this autoimmune disease.

Lambert-Eaton myasthenic syndrome: the lack of short-term in vitro effects of serum factors on neuromuscular transmission

Serum was obtained from 7 patients with the Lambert-Eaton myasthenic syndrome (LES), 3 patients with small-cell carcinoma of the lung (SCCL), and 9 healthy control subjects. Serum samples were applied in vitro to the rat neuromuscular junction (for 1-3 h for control LES sera; 4 h for SCCL sera), following which the pre- and postjunctional physiological effects of serum factors were studied in the presence of 10 mM [Mg2+]o. All sera produced a marked reduction in the frequency of spontaneous miniature end-plate potentials (MEPPs), while causing slight to moderate changes in MEPP amplitude. There were no consistent changes in the quantum content of the impulse-evoked end-plate potentials, though the serum from one LES patient significantly and reversibly inhibited the evoked quantal release. No significant effect was found when a human intercostal muscle was exposed to serum from another LES patient for 2 h. Therefore, when applied in vitro on a short-term basis, the putative LES autoantibodies do not consistently react with voltage-dependent calcium channels in the motor nerve terminal and thus fail to reproduce the physiologic abnormality of the syndrome. We suggest that the pathogenic IgG molecules may require more than 3h of incubation in order to gain access to, and inhibit the function of, the prejunctional Ca2+ channels.

Drugs that may exacerbate myasthenia gravis

Myasthenia gravis is an uncommon disease. The emergency physician should be cautious when prescribing medications to myasthenics for problems not related to myasthenia gravis. We have discussed some of those agents (Figure 3) that are recognized to cause exacerbation of MG or that may have the potential to exacerbate MG. We recommend that management of any medical or surgical problem of the myasthenic be done in consultation with a managing neurologist, and that either early follow-up or admission is necessary when these agents are used in the patient with myasthenia gravis.

A computer system for real-time data acquisition and analysis of biopotentials and quantal content at the neuromuscular junction

A computerized data acquisition system for on-line analysis of the parameters of neuromuscular transmission is described. Both hardware usage and software methodologies are discussed with regard to sampling in real-time and analyzing miniature end-plate potentials (MEPPs), end-plate potentials (EPPs) and quantal content of the evoked transmitter release. Significant features of the program include: (1) automatic threshold determination for MEPP detection; (2) the use of a circular buffer to give pre-trigger information; (3) real-time noise spike rejection; (4) an automatic procedure for EPP failure detection; (5) rapid quantal content determinations by several methods as well as complete MEPP and EPP waveform analysis. The system has proven both accurate and reliable during more than two years of use. Advantages of the system over conventional methods include: (1) increased accuracy and efficiency in data analysis; (2) immediate availability of results; (3) conventional data storage; (4) flexibility to meet changing requirements.

Neuromuscular blocking properties of suxamethonium and decamethonium in normal and myasthenic rat muscle

Patients with myasthenia gravis (MG) have increased tolerance to the neuromuscular blocking properties of suxamethonium (SCh) and decamethonium (C10) and exhibit a reversal of the C10-induced block by neostigmine. The effects of these drugs were compared in forelimb flexor digitorum longus muscle from normal rats and from rats with experimental autoimmune myasthenia gravis (EAMG) to investigate the similarity of EAMG to MG. The depolarization induced by 1, 5, 10 and 25 microM SCh or C10 at the motor end-plates was significantly higher in normal than in EAMG muscle. However, both normal and EAMG end-plates responded in a similar qualitative manner to each drug. The depolarization produced by SCh was typically maintained until the drug was washed from the bath. The depolarization produced by C10 tended to decrease after reaching its peak despite continued application of the drug. With both drugs, miniature end-plate potential (MEPP) amplitude reduction is maintained until a saline wash. Neostigmine interaction with SCh and C10 in normal and EAMG muscle was compared by measuring isometric twitch tension in vitro. Neostigmine potentiated the neuromuscular block produced by either SCh or C10 in both normal and EAMG muscle. Thus muscle from rats with EAMG shares with MG an increased tolerance to SCh and C10 when compared to normal muscle but does not exhibit the qualitatively different interaction of C10 affected muscle with neostigmine that is found in MG patients. This and other studies comparing EAMG and MG indicate that EAMG is an appropriate model of MG but differences such as we have noted should be considered when extrapolating data from EAMG to the human disease.