In acquired myasthenia gravis, the post-synaptic muscle membrane is distorted and simplified, having lost its normal folded shape. The ACh quanta diffuse across the synaptic cleft and bind to receptors on the folded muscle end-plate membrane. The normal neuromuscular junction releases acetylcholine (ACh) from the motor nerve terminal in discrete packages (quanta). The concentration of ACh receptors on the muscle end-plate membrane is reduced, and antibodies are attached to the membrane. Stimulation of the motor nerve releases many ACh quanta that depolarize the muscle end-plate region and then the muscle membrane causing muscle contraction. The post-junctional membrane is less sensitive to applied ACh, and the probability that any nerve impulse will cause a muscle action potential is reduced. ACh is released normally, but its effect on the post-synaptic membrane is reduced.
Different muscles respond differently; with any dose, certain muscles get stronger, others do not change, and still others become weaker. Adverse effects of ChE inhibitors may result from ACh accumulation at muscarinic receptors on smooth muscle and autonomic glands and at nicotinic receptors of skeletal muscle. Symptoms of muscarinic overdosage may indicate that nicotinic overdosage (weakness) is also occurring. ChE inhibitors cause considerable improvement in some patients and little to none in others. Strength rarely returns to normal. ChE inhibitors retard the enzymatic hydrolysis of ACh at cholinergic synapses, so that ACh accumulates at the neuromuscular junction and its effect is prolonged. Gastrointestinal complaints are common; queasiness, loose stools, nausea, vomiting, abdominal cramps, and diarrhea. Pyridostigmine bromide (Mestinon) and neostigmine bromide (Prostigmin) are the most commonly used ChE inhibitors. Central nervous system side effects are rarely seen with the doses used to treat myasthenia gravis. Increased bronchial and oral secretions are a serious problem in patients with swallowing or respiratory insufficiency. No fixed dosage schedule suits all patients. Excessive nicotinic receptor overdosage results in Myasthenic Crisis characterized by severe generalized weakness and respiratory failure. The need for ACh inhibitors varies from day-to-day and during the same day in response to infection, menstruation, emotional stress, and hot weather.
Medications and Myasthenia Gravis (A Reference for Health Care Professionals) (PDF).
AChR binding antibodies concentrations are sometimes increased in patients with systemic lupus erythematosus, inflammatory neuropathy, amyotrophic lateral sclerosis, rheumatoid arthritis taking D-penicillamine, thymoma without myasthenia gravis, and in normal relatives of patients with myasthenia gravis. Approximay 10% of patients who do not have binding antibodies, have other antibodies that modulate the turnover of AChR in tissue culture. The serum concentration of AChR antibody varies widely among patients with similar degrees of weakness and its level cannot predict the severity of disease in individual patients. The concentration of binding antibodies may be low at symptom onset and become elevated later. At least seventy four percent of patients with acquired generalized myasthenia and 54% with ocular myasthenia have serum antibodies that bind human acetylcholine receptor (AChR). In general, an elevated concentration of AChR binding antibodies in a patient with compatible clinical features confirms the diagnosis of myasthenia gravis, but normal antibody concentrations do not exclude the diagnosis. False positive tests are reported when blood is drawn within 48 hours of a surgical procedure involving the use of general anesthesia and muscle relaxants.
To this end, six broad categories of theoretical treatment strategies need to be explored. First, those treatments which target the antigen specific B-cells; Second, those treatments which target the antigen specific CD4+ T-cells; Third, those treatments which interfere with co-stimulatory response for antigen presentation, Fourth, treatments aimed at inducing tolerance or anergy of the CD4+ T-cell to the autoantigen or the CD4+ epitopes; Fifth, those treatments designed to stimulate those immunological circuits which activate CD8+ cells specific for the activation antigens expressed by CD4+ cells and Sixth, those treatments which intervene with cytokine function and discourage autoimmune mediated inflammatory responses. The future of Myasthenia Gravis lies in the elucidation of the molecular immunology of the anti-acetylcholine receptor response with the goal of developing a rational treatment for the illness that will cure the abnormality in the immune system that results in the AChR immune response.
Repetitive nerve stimulation confirms impaired neuromuscular transmission but is not specific to myasthenia gravis and is frequently normal in patients with mild or purely ocular disease. Elevated serum concentrations of AChR binding and probably MuSK antibodies virtually assures the diagnosis of myasthenia gravis, but normal concentrations do not exclude the diagnosis. Intravenous edrophonium chloride is often diagnostic in patients with ptosis or ophthalmoparesis, but is less useful when other muscles are weak. The measurement of jitter by SFEMG is the most sensitive clinical test of neuromuscular transmission and is abnormal in almost all patients with myasthenia gravis. A normal test in a weak muscle excludes the diagnosis of myasthenia gravis, but an abnormal test can occur when other motor unit disorders cause defects in neuromuscular transmission.
The amplitude of the compound muscle action potential (CMAP) elicited by repetitive nerve stimulation is normal or only slightly reduced in patients without MG. A significant decrement to RNS in either a hand or shoulder muscle is found in about 60% of patients with myasthenia gravis. This decrementing response to RNS is seen more often in proximal muscles, such as the facial muscles, biceps, deltoid, and trapezius than in hand muscles. The amplitude of the fourth or fifth response to a train of low frequency nerve stimuli falls at least 10% from the initial value in myasthenic patients.
Ten percent of patients with acquired, presumably immune-mediated MG do not have detectable serum antibodies to AChR or MuSK. Several types of antibodies are found in the majority of patients with MG and include forms directed against the acetylcholine receptor (AChR) and muscle-specific receptor tyrosine kinase (MuSK). In these seronegative patients, the diagnosis is based on the clinical presentation, the response to cholinesterase inhibitors and electrodiagnostic findings.
Spontaneous improvement, even remissions, occur without specific therapy, especially during the early stages of the disease. The response to any form of treatment is difficult to assess because the severity of symptoms fluctuates. All recommended regimens are empirical and experts disagree on treatments of choice. Treatment goals must be individualized according to the severity of disease, the patient's age and sex, and the degree of functional impairment. Treatment decisions should be based on knowledge of the natural history of disease in each patient and the predicted response to a specific form of therapy. A controlled clinical trial has never been reported for any medical or surgical modality used to treat myasthenia gravis.
Marked improvement or complete relief of symptoms occurs in more than 75% of patients treated with prednisone, and some improvement occurs in most of the rest. The most predictable response to prednisone occurs when treatment begins with a daily dose of 1.5 to 2 mg/kg/day. Treatment can be started at low dose to minimize exacerbations; the dose is then slowly increased until improvement occurs. The major disadvantages of chronic corticosteroid therapy are the side effects. Patients with thymoma have an excellent response to prednisone before or after removal of the tumor. Exacerbations may also occur with this approach and the response is less predictable. About one-third of patients become weaker temporarily after starting prednisone, usually within the first 7 to 10 days, and lasting for up to 6 days. The best responses occur in patients with recent onset of symptoms, but patients with chronic disease may also respond. The severity of disease does not predict the ultimate improvement. Much of the improvement occurs in the first 6 to 8 weeks, but strength may increase to total remission in the months that follow.
A wide range of potentially effective treatments are available, many of which have implications for the treatment of other autoimmune disorders. The usual cause is an acquired immunological abnormality, but some cases result from genetic abnormalities at the neuromuscular junction. What was once a relatively obscure condition of interest primarily to neurologists is now the best characterized and understood autoimmune disease. Much has been learned about the pathophysiology and immunopathology of myasthenia gravis during the past 20 years. Myasthenia gravis (MG) is the most common primary disorder of neuromuscular transmission.
Patients with mild or purely ocular muscle weakness may have increased jitter only in facial muscles. Increased jitter is a nonspecific sign of abnormal neuromuscular transmission and can be seen in other motor unit diseases. Normal jitter in a weak muscle excludes abnormal neuromuscular transmission as the cause of weakness. Jitter is greatest in weak muscles but may be abnormal even in muscles with normal strength. SFEMG is the most sensitive clinical test of neuromuscular transmission and shows increased jitter in some muscles in almost all patients with myasthenia gravis.
In some patients, a therapeutic trial of daily oral pyridostigmine may produce improvement that can't be appreciated after a single dose of edrophonium chloride or neostigmine. Some patients who don't respond to intravenous edrophonium chloride may respond to intramuscular neostigmine, because of the longer duration of action. Intramuscular neostigmine is particularly useful in infants and children whose response to intravenous edrophonium chloride may be too brief for adequate observation. Weakness caused by abnormal neuromuscular transmission characteristically improves after intravenous administration of edrophonium chloride.
After achieving the maximal response, the dose is gradually reduced to the minimum that maintains improvement. Many drugs interfere with cyclosporine metabolism and should be avoided or used with caution. Maximum improvement is achieved 6 months or longer after starting treatment. Renal toxicity and hypertension, the important adverse reactions of cyclosporine. Most patients with myasthenia gravis improve 1 to 2 months after starting cyclosporine and improvement is maintained as long as therapeutic doses are given. Cyclosporine inhibits predominantly T-lymphocyte-dependent immune responses and is sometimes beneficial in treating myasthenia gravis.
2006 by Myasthenia Gravis Foundation of America and James F. Howard, Jr., M.D. Reviewed by the MGFA's Medical Advisory Board, June 2010.
A large, international multi-center clinical trial began in 2006 to determine the role of thymectomy in steroid-treated patients. Most reports do not correlate the severity of weakness before surgery and the timing or degree of improvement after thymectomy. Thymectomy is recommended by many physicians for most patients with myasthenia gravis. However, the response is relatively unpredictable and significant impairment may continue for months or years after surgery. Patients with thymomas do not respond as well to thymectomy as do patients without thymoma. Sometimes, improvement is only appreciated in retrospect. The maximal favorable response generally occurs 2 to 5 years after surgery. The best responses to thymectomy are in young people early in the course of their disease, but improvement can occur even after 30 years of symptoms. Patients with disease onset after the age of 60 rarely show substantial improvement from thymectomy.
Emerging data suggests that the patterns of weakness and response to certain treatments may be different from those with AChR "positive" MG. Antibodies to muscle-specific receptor tyrosine kinase (MuSK), a surface membrane component essential in the development of the neuromuscular junction, have recently been identified and are found in up to 50% of MG patients who are seronegative for AChR antibodies.
Clinical Research Standards (PDF).
However, myasthenia gravis is probably under diagnosed and the prevalence is probably higher. Previous studies showed that women are more often affected than men. The prevalence of myasthenia gravis in the United States is estimated at 14 to 20 per 100,000 population, approximay 36,000 to 60,000 cases in the United States. The most common age at onset is the second and third decades in women and the seventh and eighth decades in men. As the population ages, the average age at onset has increased correspondingly, and now males are more often affected than females, and the onset of symptoms is usually after age 50.
Maximum weakness occurs during the first year in two-thirds of patients. In the era before corticosteroids were used for treatment, approximay one-third of patients improved spontaneously, one-third became worse, and one-third died of the disease. After 15 to 20 years, weakness often becomes fixed and the most severely involved muscles are frequently atrophic (burnt-out stage). Factors that worsen myasthenic symptoms are emotional upset, systemic illness (especially viral respiratory infections), hypothyroidism or hyperthyroidism, pregnancy, the menstrual cycle, drugs affecting neuromuscular transmission, and increases in body temperature. The rest have progressive weakness during the first 2 years that involves oropharyngeal and limb muscles. Spontaneous improvement frequently occurred early in the course. Symptoms fluctuated over a relatively short period of time and then became progressively severe for several years (active stage). Weakness is restricted to the ocular muscles in about 10% to 40% of cases. The active stage is followed by an inactive state in which fluctuations in strength still occurred but are attributable to fatigue, intercurrent illness, or other identifiable factors. The course of disease is variable but usually progressive.
StrAbs are rarely, if ever, elevated in MG in the absence of acetylcholine receptor antibodies and are therefore of limited use in confirming the diagnosis. One-third of MG patients without thymoma also have these antibodies; they are more frequent in older patients and in those with more severe disease. They are found in more than 90% of MG patients with thymoma, and in one-third of patients with thymoma who do not have MG. StrAbs are also elevated in other disorders including autoimmune liver disease and infrequently in Lambert-Eaton syndrome and in primary lung cancer. The main clinical value of StrAbs is in predicting thymoma: 60% of patients with MG with onset before age 50 who have elevated StrAbs have thymoma. Anti-striational muscle antibodies (StrAbs), which react with contractile elements of skeletal muscle, are not pathogenic.
The need for plasma exchange, and its frequency of use is determined by the clinical response in the individual patient. Maximum improvement may be reached as early as after the first exchange or as late as the fourteenth. Plasma exchange is used as a short-term intervention for patients with sudden worsening of myasthenic symptoms for any reason, to rapidly improve strength before surgery, and as a chronic intermittent treatment for patients who are refractory to all other treatments. Repeated exchanges do not have a cumulative benefit. Most patients who respond to the first plasma exchange will respond again to subsequent courses. Almost all patients with acquired myasthenia gravis improve temporarily following plasma exchange. Improvement lasts for weeks or months and then the effect is lost unless the exchange is followed by thymectomy or immunosuppressive therapy.
Repetitive Nerve Stimulation (RNS).
Patients with myasthenia gravis come to the physician complaining of specific muscle weakness and not of generalized fatigue. Ocular motor disturbances, ptosis or diplopia, are the initial symptom of myasthenia gravis in two-thirds of patients; almost all had both symptoms within 2 years. Oropharyngeal muscle weakness, difficulty chewing tough, chewy or fibrous foods, swallowing, or talking, is the initial symptom in one-sixth of patients, and limb weakness in only 10%. Initial weakness is rarely limited to single muscle groups such as neck or finger extensors or hip flexors. The severity of weakness fluctuates during the day, usually being least severe in the morning and worse as the day progresses, especially after prolonged use of affected muscles.
Because the response to azathioprine is delayed, both drugs may be started simultaneously with the intent of rapidly tapering prednisone when azathioprine becomes effective. Once improvement begins, it is maintained for as long as the drug is given, but symptoms recur 2 to 3 months after the drug is discontinued or the dose is reduced below therapeutic levels. Patients who fail corticosteroids may respond to azathioprine and the reverse is also true. Azathioprine reverses symptoms in most patients but the effect is delayed by 4 to 8 months. Some respond better to treatment with both drugs than to either alone. Approximay one-third of patients have mild dose-dependent side effects that may require dose reductions but do not require stopping treatment.
Patients with drooping eyelids, double vision and difficulty with speech or swallowing symptoms suggest intracranial pathology and often lead to an evaluation for stroke, brain tumor or multiple sclerosis. The unusual distribution and fluctuating symptoms often suggests psychiatric disease. Patients with anti-MuSK-antibody positive MG may have focal or regional weakness and muscle atrophy that are more suggestive of motor neuron or muscle membrane (myopathy) disease. The diagnosis of MG is often delayed months or even years (in the mildest cases).
Most thymic tumors in patients with myasthenia gravis are benign, well-differentiated and encapsulated, and can be removed compley at surgery. Ten percent of patients with myasthenia gravis have a thymic tumor and 70% have hyperplastic changes (germinal centers) that indicate an active immune response. It is unlikely that thymomas result from chronic thymic hyperactivity because myasthenia gravis can develop years after thymoma removal and the HLA haplotypes that predominate in patients with thymic hyperplasia are different from those with thymomas. Because the thymus is the central organ for immunological self-tolerance, it is reasonable to suspect that thymic abnormalities cause the breakdown in tolerance that causes an immune-mediated attack on AChR in myasthenia gravis. These are areas within lymphoid tissue where B-cells interact with helper T-cells to produce antibodies. Patients with thymoma usually have more severe disease, higher levels of AChR antibodies, and more severe EMG abnormalities than patients without thymoma. However, it is still uncertain whether the role of the thymus in the pathogenesis of myasthenia gravis is primary or secondary. Almost 20% of patients with myasthenia gravis whose symptoms began between the ages of 30 and 60 years have thymoma; the frequency is much lower when symptom onset is after age 60. Thymus tissue from patients with myasthenia gravis produces AChR antibodies when implanted into immunodeficient mice. The thymus contains all the necessary elements for the pathogenesis of myasthenia gravis: myoid cells that express the AChR antigen, antigen presenting cells, and immunocompetent T-cells. Thymic abnormalities are clearly associated with myasthenia gravis but the nature of the association is uncertain.
Case reports, pilot studies and retrospective series have demonstrated a potential role for MMF as a corticosteroid-sparing agent and as adjunctive or primary therapy in refractory MG. More than half of patients become asymptomatic after one year. Cyclophosphamide has been used intravenously and orally for the treatment of myasthenia gravis. The long-term risk of malignancy is not established, but there are no reports of an increased incidence of malignancy in patients with myasthenia gravis receiving immunosuppression. Others are using it as a preferred treatment because of its faster onset of action when compared to azathioprine. Like all immunomodulatory drugs, the potential side effects, costs and potential benefits must be considered wisely. Mycophenolate mofetil (MMF) selectively inhibits the proliferation of activated B and T lymphocytes. Clinical trials are currently underway to establish the role of this drug in the treatment of MG. Life-threatening infections are an important risk in immunosuppressed patients, but in our experience, this risk is limited to patients with invasive thymoma. Side effects are common. It also suppresses the formation of antibodies active in complement-dependent lysis and antibody-dependent, cell-mediated cytotoxicity, a necessary feature to promote many autoimmune diseases.
Improvement occurs in 50 to 100% of patients, usually beginning within 1 week and lasting for several weeks or months. Possible mechanisms of action include down-regulation of antibodies directed against AChR and the introduction of anti-idiotypic antibodies. The common adverse effects of IVIG are related to the rate of infusion. Several groups have reported a favorable response to high-dose (2 grams/kg infused over 2 to 5 days) IVIG. The mechanism of action is not known but is probably non-specific down regulation of antibody production.
Department of Neurology The University of North Carolina at Chapel Hill. MYASTHENIA GRAVIS - A SUMMARY James F. Howard, Jr., M.D.
Myasthenia gravis side effects