Practice Essentials
Myasthenia gravis (MG) is a relatively rare acquired, autoimmune disorder caused by an antibody-mediated blockade of neuromuscular transmission resulting in skeletal muscle weakness and rapid muscle fatigue. The autoimmune attack occurs when autoantibodies form against the nicotinic acetylcholine postsynaptic receptors at the neuromuscular junction of skeletal muscles (see the image below).
Although the chief target of the autoimmune attack in most cases is the skeletal muscle nicotinic acetylcholine receptor (nAChR), other antigenic targets that are components of the neuromuscular junction (NMJ) have also been implicated.
Normal neuromuscular junction showing a presynaptic terminal with a motor nerve ending in an enlargement (bouton terminale): Synaptic cleft and postsynaptic membrane with multiple folds and embedded with several acetylcholine receptors.
Signs and symptoms
The presentation of MG has the following characteristics:
The clinical hallmark of MG is the presence of fluctuating fatigable muscle weakness that worsens with activity and improves on rest
50% to 85% of patients with MG present with ocular symptoms with or without generalized weakness
50% to 60% of patients with MG who initially present with isolated ocular involvement go on to develop generalized weakness, often within 3 years after onset of symptoms
The disease remains exclusively ocular in only 15% to 25% of patients throughout their course
Approximately 20% of patients with MG may present with prominent bulbar symptoms
Bulbar muscle weakness is also common, along with weakness of head extension and flexion
Limb weakness may be more severe proximally than distally
Isolated limb muscle weakness is the presenting symptom in about 5% of patients
Weakness progresses from mild to more severe over weeks or months, with exacerbations and remissions
The following factors may trigger or worsen exacerbations:
Warm weather
Surgery
Immunization
Emotional stress
Menstruation
Intercurrent illness (eg, viral infection)
Tapering of immunosuppression
Pregnancy and postpartum period
Worsening of chronic medical illnesses (cardiac, renal, autoimmune, etc.)
Medication (eg, aminoglycosides, ciprofloxacin, telithromycin. clindamycin, phenothiazines, chlorpromazine, diazepam, halothane, ketamine, lidocaine, procain, non-depolarizing neuromuscular blocking agents, chloroquine, procaine, lithium, phenytoin, beta-blockers, procainamide, propafenone, bretylium, qunidine, calcium channel blockers, d-penicillamine, high-dose prednisone, magneisum)
The Myasthenia Gravis Foundation of America Clinical Classification divides MG into 5 main classes and several subclasses
:
Class I: Any ocular muscle weakness; may have weakness of eye closure; all other muscle strength is normal
Class II: Mild weakness affecting other than ocular muscles; may also have ocular muscle weakness of any severity
Class IIa: Predominantly affecting limb, axial muscles, or both; may also have lesser involvement of oropharyngeal muscles
Class IIb: Predominantly affecting oropharyngeal, respiratory muscles, or both; may also have lesser or equal involvement of limb, axial muscles, or both
Class III: Moderate weakness affecting other than ocular muscles; may also have ocular muscle weakness of any severity
Class IIIa: Predominantly affecting limb, axial muscles, or both; may also have lesser involvement of oropharyngeal muscles
Class IIIb: Predominantly affecting oropharyngeal, respiratory muscles, or both; may also have lesser or equal involvement of limb, axial muscles, or both
Class IV: Severe weakness affecting other than ocular muscles; may also have ocular muscle weakness of any severity
Class IVa: Predominantly affecting limb, axial muscles, or both; may also have lesser involvement of oropharyngeal muscles
Class IVb: Predominantly affecting oropharyngeal, respiratory muscles, or both; may also have lesser or equal involvement of limb, axial muscles, or both
Class V: Defined by the need for intubation, with or without mechanical ventilation, except when used during routine postoperative management. The use of a feeding tube without intubation places the patient in class IVb.
See Clinical Presentation for more detail.
Diagnosis
The anti–acetylcholine receptor (AChR) antibody test for diagnosing MG has the following characteristics:
High specificity (up to 100%
)
Positive in as many as 85% of patients who have generalized MG
Positive in only 50% of patients who have purely ocular MG
False-positive anti-AChR antibody test results have been reported in patients with the following:
Thymoma without MG
Lambert-Eaton myasthenic syndrome
Small cell lung cancer
Rheumatoid arthritis treated with penicillamine
1-3% of the population older than 70 years
Assays for the following antibodies may also be useful:
Anti-MuSK antibody (present in about half of patients with negative results for anti-AChR antibody)
Anti-lipoprotein-related protein 4 (LRP4) antibody
Anti-agrin antibody
Antistriational antibody (present in almost all patients with thymoma and MG, as well as in half of MG patients with onset of MG at 50 years or older)
Anti-cortactin antibody
Other studies are as follows:
Plain chest radiographs may identify a thymoma as an anterior mediastinal mass
Chest computed tomography is important to identify or rule out thymoma or thymic enlargement in all cases of MG
In strictly ocular MG, magnetic resonance imaging of the brain and orbit is helpful to evaluate for mass lesions compressing the cranial nerves or a brainstem lesion that may masquerade as ocular MG
Electrodiagnostic studies (repetitive nerve stimulation and single-fiber electromyography)
See Workup for more detail.
Management
Based on recent advances in understanding the various underlying antibodies that cause myasthenia gravis and differences in how they present clinically and their response to various therapies, it is suggested that patients with myasthenia gravis should be classified into subgroups. Subgroups are based on the profile of serum autantibodies, the age of onset, the presence or absence of thymic pathology, and the distribution of clinical weakness.
Therapy for MG includes the following:
Symptomatic therapy
Anticholinesterase (AchE) inhibitors
Pyridostigmine is used for maintenance therapy
Neostigmine is generally used only when pyridostigmine is unavailable
Rapidly acting or short-term immunomodulating agents:
Intravenous immune globulin (IVIg)
Plasmapheresis
Long-term immunosuppression
Prednisone is the most important immunosuppressant and provides short- and long-term benefit.
Azathioprine (AZA) is a first-line steroid-sparing agent
Mycophenolate mofetil (MMF) is also a first-line steroid-sparing agent. It is widely used and less toxic than azathioprine.
Cyclosporine (CyA) is used as a steroid-sparing agent in patients who are intolerant to azathioprine and mycophenolate mofetil.
Tacrolimus is used as a steroid-sparing agent in patients intolerant or unresponsive to AZA, MMF, or CyA.
Methotrexate is used as a steroid-sparing agent and has similar efficacy and tolerability to AZA.
Rituximab may be an effective intervention in refractory MG, both for anti-AChR and anti-MuSK-positive disease.{ref5
Cyclophosphamide is used in refractory/severe MG.
Emerging novel therapies targeting immunopathogenesis of MG
Eculizumab is a humanized monoclonal IgG antibody against terminal complement activity (C5b-9) preventing its depositing at the NMJ in adult patients with anti-AchR+ve gMG.
Investigational agents
Ocrelizumab, a second-generation, humanized monoclonal antibody against B-cell (anti-CD20), which appears to be better tolerated (over first-generation rituximab), is a potential consideration.
Abatacept is a cytotoxic T-lymphocyte protein 4 (CTLA-4) monoclonal antibody, which binds to CD80/CD86 and blocks both activating (CD28) and inhibitory signals (CTLA-4).
Daclizumab is a monoclonal antibody that binds to CD25 thereby antagonizing the activating effects of IL-2 on T cells. This drug was pulled from the worldwide market in 2018 owing to reports of severe liver damage and immune-related conditions, including encephalitis.
Belimumab binds to soluble B-cell activating factor (BAFF) and reduces B-cell activation and differentiation into antibody-producing plasma cells.
Bortezomib is a protease inhibitor that has shown experimental benefit in MG. It reduced anti-AChR antibody titers, inhibited damage to the postsynaptic muscle membrane, and resulted in clinical improvement.
Fc receptor modulators (FcRn–IgG) in the form of a monoclonal antibody is being studied in passive and active models of rat experimental MG and shown to ameliorate symptoms and lower autoantibody levels.
Several human monoclonal antibodies directed against IL-17 are in development, including brodalumab (AMG 827), ixekizumab (LY2439821), and secukinumab.
Tocilizumab, a humanized monoclonal antibody targeting the IL-6 receptor, is being studied as a potential therapy.
Autologous stem cell transplantation has been proposed as a therapeutic approach for refractory autoimmune disease and is based on the idea that the transplanted immune cells will be “reset” and free of autoimmune reactivity.
3,4-Diaminopyridine is another symptomatic therapy with potential application in MuSK MG. Exploring use of this drug, which enhances AChR release at the motor nerve terminal, in MuSK MG is supported by preclinical models, experience with congenital MG with MuSK mutations, and case reports.
Tirasemtiv is a selective fast skeletal muscle troponin activator. This small molecule binds to skeletal muscle troponin, thereby sensitizing the muscle to calcium and ultimately improving muscle strength under submaximal stimulation. A study of tirasemtiv in experimental MG improved grip strength and muscle force after a single dose.
Thymectomy
The Randomized Trial of Thymectomy in Myasthenia Gravis, a landmark study, has shown that trans-sternal thymectomy improves MG and even induces remission, or lessens the requirements or perhaps removes the need for prednisone and other immunosuppressants over a period of 3 years.
See Treatment and Medication for more detail.
What is myasthenia gravis? Myasthenia gravis is an autoimmune disease that’s categorized as a type II hypersensitivity that involves autoantibodies binding acetylcholine receptors on skeletal muscle cells. Courtesy of Osmosis.org (https://www.osmosis.org/).