Myasthenia gravis: a personal view of pathogenesis and mechanism, part 1

Treatment of Acquired Autoimmune Myasthenia Gravis: a Topic Review

We propose a new approach to staging the disease based on clinical and immunological response to treatment. We oppose clinical remission to immunological remission and define total clinical remission as the goal of therapy. We describe the use, side effects and indications of established therapies. Acetycholine esterase inhibitors are only a symptomatic treatment as is plasma exchange. Usefulness and limits of thymectomy, corticosteroids and immunosuppressants are described here. Their goal is to reduce the auto-immune process. Long-term hazards from these medications are described and methods to reduce their potential risks are suggested. We suggest the number of patients having life threatening complications while undergoing aggressive immunosuppression can be reduced by a systematic approach to follow-up. In the second part of this review article, adapting management to specific situations is emphasized in refractory disease, respiratory failure, neonatal and juvenile forms of the disease. The special situation of seronegative myasthenia is discussed.

Myasthenia gravis and Lambert-Eaton syndrome

Myasthenia gravis is a common autoimmune disorder characterized by the presence of pathogenic antibodies directed against the acetylcholine receptor. Patients present with variable degrees and distribution of fluctuating weakness at times life threatening. Clinical manifestations, establishment of diagnosis, the natural history of myasthenia gravis, and therapeutic options are herein reviewed. Far less common is Lambert-Eaton syndrome (the myasthenic syndrome), another autoimmune disorder due to the presence of antibodies directed against the PQ-type voltage-gated calcium channels. Clinical features and treatment options are summarized.

Diseases and disorders of muscle

Muscle may suffer from a number of diseases or disorders, some being fatal to humans and animals. Their management or treatment depends on correct diagnosis. Although no single method may be used to identify all diseases, recognition depends on the following diagnostic procedures: (1) history and clinical examination, (2) blood biochemistry, (3) electromyography, (4) muscle biopsy, (5) nuclear magnetic resonance, (6) measurement of muscle cross-sectional area, (7) tests of muscle function, (8) provocation tests, and (9) studies on protein turnover. One or all of these procedures may prove helpful in diagnosis, but even then identification of the disorder may not be possible. Nevertheless, each of these procedures can provide useful information. Among the most common diseases in muscle are the muscular dystrophies, in which the newly identified muscle protein dystrophin is either absent or present at less than normal amounts in both Duchenne and Becker's muscular dystrophy. Although the identification of dystrophin represents a major breakthrough, treatment has not progressed to the experimental stage. Other major diseases of muscle include the inflammatory myopathies and neuropathies. Atrophy and hypertrophy of muscle and the relationship of aging, exercise, and fatigue all add to our understanding of the behavior of normal and abnormal muscle. Some other interesting related diseases and disorders of muscle include myasthenia gravis, muscular dysgenesis, and myclonus. Disorders of energy metabolism include those caused by abnormal glycolysis (Von Gierke's, Pompe's, Cori-Forbes, Andersen's, McArdle's, Hers', and Tauri's diseases) and by the acquired diseases of glycolysis (disorders of mitochondrial oxidation). Still other diseases associated with abnormal energy metabolism include lipid-related disorders (carnitine and carnitine palmitoyl-transferase deficiencies) and myotonic syndromes (myotonia congenita, paramyotonia congenita, hypokalemic and hyperkalemic periodic paralysis, and malignant hyperexia). Diseases of the connective tissues discussed include those of nutritional origin (scurvy, lathyrism, starvation, and protein deficiency), the genetic diseases (dermatosparaxis, Ehlers-Danlos syndrome, osteogenesis imperfecta, Marfan syndrome, homocystinuria, alcaptonuria, epidermolysis bullosa, rheumatoid arthritis in humans, polyarthritis in swine, Aleutian disease of mink, and the several types of systemic lupus erythematosus) and the acquired diseases of connective tissues (abnormal calcification, systemic sclerosis, interstitial lung disease, hepatic fibrosis, and carcinomas of the connective tissues). Several of the diseases of connective tissues may prove to be useful models for determining the relationship of collagen to meat tenderness and its other physical properties. Several other promising models for studying the nutrition-related disorders and the quality-related characteristics of meat are also reviewed.

Adult-onset muscle weakness. How to identify the underlying cause

When adults present with muscle weakness as the predominant symptom and sign, the causes can usually be narrowed down to four disorders: amyotrophic lateral sclerosis, Guillain-Barré syndrome, myasthenia gravis, and inflammatory myopathy. Perhaps the most difficult task for the clinician is identifying which of these patients are actually weak and which are describing generalized fatigue. This task should be made easier by keeping in mind the definition of muscle weakness--a loss of power resulting in reduction of motor function--and by performing careful functional muscle testing.

[Myasthenia gravis induced by D-penicillamine in a patient with progressive systemic sclerosis]

The development of autoimmune diseases in some patients treated with D-penicillamine (DPA) suggests that the reported occurrence of a conduction disorder at the neuromuscular junction and the development of a reversible myasthenia gravis in rheumatoid disease, progressive systemic sclerosis or Wilson's disease after the use of DPA are part of a general predisposition for autoimmune disease related to DPA therapy. The case reported is an example. The DPA- induced myasthenia gravis (MG) is similar to the spontaneous MG clinically and electrophysiologically, though ocular signs prevail in the former. Antibodies to acetylcholine receptor have been demonstrated and thymic hyperplasia also has been formed. Regarding the onset of myasthenic manifestations the duration of the treatment with DPA varies from 6 to 10 months. The action of DPA on the neuromuscular junction is different from that occurring in spontaneous MG. The pathogenesis of the DPA induced MG is still obscure. The chemical properties of DPA permit it to react with many proteins and some alteration of proteins may appear, with structural changes in the composition and antigenicity of the collagen fibers. In vitro DPA causes disorder of acetylcholine receptor bridges to alpha, beta, gamma sub-units with reduction of the S-S bridges in the gamma-subunit. This decreases the linkage of high affinity and abolishes its positive cooperative system, reducing the S-S connection in the alpha-unit near the acetylcholine linkage. The interaction between DPA and receptor may induce antigenic alteration in this latter, starting the autoimmune phenomena. The other possibility is the stimulation of prostaglandin E-1 synthesis by DPA may fill the allosteric place of ACh receptor, interfering on the neuromuscular junction.

Long-term corticosteroid treatment of myasthenia gravis: report of 116 patients

One hundred sixteen patients, aged 8 to 82 years, with myasthenia gravis were treated with prednisone, 60 to 80 mg daily, until the onset of improvement, followed by lower-dose alternate-day therapy of several years' duration. Of all patients, 80.2% achieved either remission (27.6%) or marked improvement (52.6%). Moderate improvement occurred in 14.7%, and 5.2% showed no improvement. Increasing age correlated with a favorable outcome, but sex, duration of illness prior to treatment, severity and distribution of weakness at the time of onset of treatment, and presence of thymoma were not factors in the response to therapy.

Thymectomy in myasthenia gravis. A review

The role of thymectomy in the management of myasthenia gravis is reviewed in the light of the published data and of a personal series. The patients in whom the operation is most successful are non thymomatous patients aged between 10 and 40 years with an MG history of less than 3 years. There is no sex prevalence. Lasting improvement may be expected. There are no proven correlations between biological indices like the germinal centers in the thymus and/or AChR antibody titers and the postoperative course of the disease. Complete removal of the thymus seems to be crucial and hence the transsternal approach is preferred. The operation, less effective in patients with thymoma than in those with an active thymus, is nonetheless necessary to in these patients prevent putative damage to surrounding organs from thymoma infiltration. Why thymectomy should be effective in patients with an active thymus and not in those with a thymoma may be revealed by in vitro studies of the interactions between thymic cells and peripheral B cells, now in progress.

Prediction of the need for postoperative mechanical ventilation in myasthenia gravis: thymectomy compared to other surgical procedures

In a recent report Leventhal, Orkin, and Hirsh described a scoring system felt to be of value in predicting the need for postoperative mechanical ventilation in patients with myasthenia gravis undergoing thymectomy. Leventhal, et al. identified four risk factors felt to have predictive value, namely: (1) duration of myasthenia gravis greater than or equal to 6 years, (2) chronic respiratory disease, (3) dose of pyridostigmine greater than or equal to 750 mg per day, and (4) vital capacity less than or equal to 2.9 litres. Forty-six patients with myasthenia gravis who received 68 general anaesthetics were studied retrospectively. They represented the past 10 years' anaesthetic experience with myasthenia gravis at the Vancouver General Hospital. The patients were divided into two groups: (1) those who underwent thymectomy, and (2) those who underwent procedures other than thymectomy. Using the risk factors of Leventhal, et al., a predictive score was assessed for each patient; the time of postoperative tracheal extubation was also noted for each patient. From this study it was concluded that the scoring system proposed by Leventhal, et al. may have been of some value in predicting whether or not a particular patient undergoing thymectomy was likely to need ventilation postoperatively. In 41 myasthenics who had procedures other than thymectomy, however, this scoring system was found to be of no value.

Myasthenia gravis. A survey

Myasthenia gravis is a chronic disease characterized by a fluctuating weakness of voluntary muscle, with a preference for the muscles innervated by cranial nerves. The pathophysiological mechanism is a loss of postsynaptic acetylcholine receptors to less than 20-30% so that the safety margin of neuromuscular transmission is lost. It is probable that the function of the remaining acetylcholine receptors is impaired by antibodies against receptor proteïn, which can be demonstrated in the serum in 80-90% of the patients, and which are highly specific for the disease. An experimental autoimmune myasthenia can be induced in many animal species by immunization with purified receptor proteïn and this disease is remarkably similar to the human myasthenia with exception of the fluctuating course. The human disease has to be considered as an autoimmune disease, although the initiating mechanism is unknown. The occurrence of tumors of the thymus in 10-15% and the presence of germinal centres in about 70% of the thymus glands removed by operation are highly suggestive of the importance of the thymus in the pathogenesis, but the definite mechanism (harbouring of an abnormal antigen in myoid cells, or/and false instruction of thymocytes with lack of suppressor cells) is essentially unknown. In most patients the disease tends ot have a favourable course from 5-10 years after onset and complete remission occur in about 20% after 10-20 years. Therapy with anticholinesterases, providing an increase in acetylcholine, is of partial benefit in most patients. Thymectomy has an excellent effect in about 30% of the patients without thymoma under the age of 40 during the first three years of the disease, and is of benefit in still another 30-40%. The use of prednisone and immuno-suppressive drugs has improved the prognosis of the 20% of the patients with severe life threatening symptoms, half of whom have a thymoma.

Facilitatory effects of 4-aminopyridine on neuromuscular transmission in disease states

The in vitro effects of 4-aminopyridine (4-AP) on neuromuscular transmission were determined by microelectrode techniques in intercostal muscles from patients with myasthenia gravis (MG) and the Eaton-Lambert syndrome (ELS), and in forelimb muscles from rats with experimental autoimmune myasthenia gravis (EAMG). In MG and EAMG, the amplitudes of miniature endplate potentials (MEPPs) and endplate potentials (EPPs) were reduced, and there was increased sensitivity to the blocking action of d-tubocurarine (dTc). In ELS, MEPP amplitude was normal but the average number of acetylcholine quanta released by nerve impulses was reduced, causing subthreshold EPPs. In EAMG muscle, 4-AP produced dose-dependent increases in EPP amplitude and in the duration of indirectly elicited muscle action potentials but no changes in MEPP amplitude and resting membrane potential. 4-AP completely reversed the postsynaptic blockade produced by dTc and EAMG. 4-AP appears to facilitate neuromuscular transmission in EAMG, MG, and ELS by increasing the neurally evoked transmitter release, thus overcoming either the pre- or the postsynaptic neuromuscular blockade.

Treatment of myasthenia gravis. Report on 139 patients

In the treatment of myasthenia gravis (MG) considerable progress has recently been achieved. Our experience is based on the observation of 139 patients with an average follow-up of 3 years and 4 months. A treatment plan and results are presented. Indications for thymectomy: all cases of MG in adult life, apart from ocular myasthenia without radiological thymoma and without electrophysiological and pharmacological signs of generalization; before puberty only cases with radiological thymoma and severely incapacitating or life-threatening signs. Median sternotomy is preferable for thymoma, the transcervical approach with a sternal split for non-neoplastic thymus. Mediastinal radiotherapy is indicated after removal of an invasive or adhesive thymoma. Indications for corticosteroids: 1) before thymectomy: respiratory weakness; 2) soon after thymectomy: life-threatening signs; 3) later after thymectomy: incapacitating or life-threatening signs; 4) as an alternative to thymectomy: when surgery cannot be performed or it is not indicated. Oral Prednisone was nearly always preferred: alternate-day high single dose (75 to 115 mg) has given good results in most cases even if in some cases a small dose was required in the "off day"; inversely a lower alternate-day or daily dose was often sufficient. Long-term results: following this schedule for adult patients good results were scored in 67% of thymomas, in 94% of hyperplasias, and in 62% of unthymectomized patients: in prepuberal life the few cases of severe MG have all shown a favorable evolution.

Short-term effects of prednisolone on neuromuscular transmission in normal rats and those with experimental autoimmune myasthenia gravis

Electrophysiological investigations of the effects of bath-applied prednisolone at the neuromuscular junction were performed in muscles from normal rats and rats with experimental autoimmune myasthenia gravis (EAMG). In muscles from both groups, prednisolone reversible and significantly depressed the amplitudes of minature end-plate potentials (MEPPs), end-plate potentials (EPPs) and indirectly elicited action potentials (APs) without affecting resting membrane potentials. Prednisolone also caused a significant reduction in EPP rise time to peak and half-decay time while markedly increasing MEPP frequency and AP rise time to peak and duration. These effects were shown to be dose-dependent. The percentage decrease in amplitude after prednisolone perfusion was similar for EPPs and MEPPs, indicating that the depressive effect of prednisolone at the junction is postsynaptic. In all of the parameters studied, the percentage effect of prednisolone was the same in EAMG and normal preparations. No stimulus-linked repetitive EPPs or APs were observed after prednisolone. It is concluded that prednisolone has a depressive effect on neuromuscular transmission, but that this occurs only at high concentrations of the drug which are not achieved during the treatment of myasthenia gravis.

Assessment of the value of thymic scan in myasthenia gravis

Fifty-three patients were investigated by Se methionine scan in order to evaluate the role of the thymus gland in myasthenia gravis. The thymus of 28 of the above patients was examined histologically. Five out of six thymomas were detected with this technique and eight out of twelve hyperplasias were revealed. The above data suggests that Se methionine scan is a useful, innocuous tool for diagnosing neoplasms and abnormal germinal centers in thymus glands.

Myasthenia gravis: review of diagnosis and management

Although the cause of myasthenia gravis is still unknown, its pathogenesis appears clear: immunologic attack on synaptic receptors in muscle causes receptor deficiency, decreased miniature endplate potentials, and decrements in the compound action potentials evoked from muscles on repetitive stimulation of peripheral nerves. In addition to the involvement of skeletal muscle, some MG patients may manifest subtle alterations of the function of heart, lung, smooth muscle, and CNS, indicating that this is truly a systemic disorder. Modern therapy involves adjusting treatment to the needs of individual patients. Anticholinesterases, calcium, ephedrine, potassium, and germine partially correct the defect in neuromuscular transmission; prednisone, ACTH, cytotoxic drugs, antilymphocyte serums, gamma globulin, thoracic duct drainage, plasmapheresis, and thymectomy partially modify the abnormalities of the immune system.