About gMG
Mike, a patient living with gMG. Mike has received compensation from Alexion Pharmaceuticals, Inc. and has a relative who works for Alexion.
About generalized myasthenia gravis (gMG)
Understanding pathophysiology, disease education, diagnosis, and treatment approaches
Generalized myasthenia gravis (gMG) is a rare autoimmune disorder that creates a fluctuating weakness of the voluntary muscles due to disrupted neuromuscular transmission. It is the most common type of myasthenia gravis (MG) and typically the most severe.1
A normal functioning complement system plays an important role in helping the body fight off infections. However, in gMG, the complement system attacks the area where your body sends signals to the patient’s muscles. This is known as impaired neuromuscular transmission, which can lead to weakness and fatigue of the muscles.1,2
Mechanism of Disease: In gMG, the complement cascade causes damage at the neuromuscular junction (NMJ)
WATCH: Understanding the Role of Complement in gMG
Learn more about the complement cascade
In gMG, the mechanisms that control the adaptive immune system do not function properly, as the adaptive immune system targets healthy tissues. With gMG, the pathology includes the generation of antibodies against acetylcholine receptors (AChRs). Anti-AChR antibodies disrupt signal transmission at the NMJ by blocking and reducing the number of functioning AChRs.3
- AChRs are found on the surface of muscle cells and are important for the generation of muscle contraction3
- On average, approximately 82% of patients with gMG are anti-AChR antibody positive4-8
At least three pathogenic mechanisms are believed to be responsible for disrupting the NMJ in anti-AChR antibody-positive gMG1,9:
- Complement-mediated postsynaptic membrane destruction
- Antigenic modulation
- Blockage of AChRs
Overview1,9,10: All three mechanisms are believed to be active in anti-AChR antibody-positive gMG, but complement-mediated postsynaptic membrane destruction could be a cause of functional AChR loss and consequent failure of neuromuscular transmission.
Anti-AChR antibody binding to AChRs activates the complement cascade, resulting in the downstream production of terminal complement components, including protein C5a (leading to inflammation) and C5b. C5b is the first protein included in the formation of the membrane attack complex (MAC) at the postsynaptic membrane.
MAC is composed of complement proteins C5b through C9 and damages the postsynaptic membrane and its associated structures, such as the membrane folds and dense clusters of AChRs. This causes a simplification and distortion of the postsynaptic membrane that reduces the overall number of AChRs.
Alteration of folds in the muscle membrane reduces the efficiency of neuromuscular transmission.
Healthy NMJ1,11,12
Complement
cascade leading to NMJ damage
NMJ With Complement-Mediated Damage1,11,13
Healthy NMJ image: Reprinted from Mayo Clin Proc, 52(5), Engel AG, et al. 267-280. © 2009, with permission from Elsevier.
NMJ With Complement-Mediated Damage image: Sahashi K, et al. J Neuropathol Exp Neurol. 1980;39(2):160-172. © 1980 by permission of Oxford University Press
How gMG impacts patients
Prevalence of myasthenia gravis14,15:
Recent studies indicate that ~76% of diagnosed MG cases are generalized.
- Prevalence numbers may underestimate the true number of gMG cases, as the disease is underdiagnosed
- Generally prevalence is higher in females than in males, but reverses in older people; females often have an earlier age of onset compared to males
- The peak of incidence is between 20 and 40 years in females and between 60 and 80 years in males
Muscle damage caused by gMG can make daily activities and physical functions challenging
Some patients may struggle with16:
Vision
Breathing
Speech
Eating
Mobility
Common symptoms of gMG include11,17,18:
- Speech problems
- Difficulty chewing and swallowing
- Double vision/eyelid droop
- Altered facial expressions
- Difficulty holding one’s head up
- Trouble lifting the arms
- Weak grip
- Difficulty walking long distances or climbing stairs
- Trouble in rising from a sitting position
- Difficulty breathing
Diagnosing gMG1,10,14
gMG diagnosis begins with a review of the patient’s medical history and physical examination, but clinicians must be careful as signs and symptoms of gMG can look similar to other conditions. Once gMG is suspected, several tests are used to confirm the diagnosis.
If gMG is suspected, testing for anti-acetylcholine receptor (AChR) antibodies is a key step toward confirming diagnosis
- Blood tests are used to detect the presence of anti-AChR and other antibodies. However, the absence of anti-AChR does not exclude gMG
- Additional tests can include ice pack tests, especially if ptosis is a key symptom (ice packs are placed over the eye for 2 to 5 minutes to assess ptosis improvement). There may be benefit in testing the recovery of muscle strength after rest and the ability to look upward for a sustained period
- Diagnostic testing is preferred to help confirm diagnosis, as unofficial measures/additional testing options may not yield the most accurate results
All patients with confirmed gMG should undergo diagnostic imaging to exclude the presence of a thymoma, as thymomas have been reported in up to 30% of patients with MG.
gMG symptoms in patients are commonly measured by the MG-ADL and QMG scales
The Myasthenia Gravis Activities of Daily Living scale (MG-ADL scale) is a categorical scale that assesses the impact on daily function of 8 signs or symptoms that are typically affected in gMG. Each item is assessed on a 4-point scale where a score of 0 represents normal function and a score of 3 represents loss of ability to perform that function. The total score ranges from 0 to 24, with the higher scores indicating more impairment.17
The Quantitative Myasthenia Gravis scale (QMG scale) is a 13-item categorical scale assessing muscle weakness. Each item is assessed on a 4-point scale where a score of 0 represents no weakness and a score of 3 represents severe weakness. The total score ranges from 0 to 39, where higher scores indicate more severe impairment.19
Alexion has compiled a variety of assessment tools that are available for download below.
Download the Assessment Tool BookletReview a common classification method utilized to group MG patients based on symptom severity20
Per the MGFA Clinical Classification, symptoms are divided into different classes based on severity:
Class I: Any ocular muscle weakness; may have weakness of eye closure. All other muscle strength is normal.
Class II: Mild weakness affecting muscles other than ocular muscles; may also have ocular muscle weakness of any severity.
IIa. Prominently affecting limb, axial muscles, or both. May also have lesser involvement of oropharyngeal muscles.
IIb. Mainly affecting oropharyngeal, respiratory muscles, or both; and potentially lesser or equal involvement of limb, axial muscles, or both.
Class III: Moderate weakness affecting muscles other than ocular muscles; may also have ocular muscle weakness of any severity.
IIIa. Mainly affecting limb, axial muscles, or both; with potentially less impact of oropharyngeal muscles.
IIIb. Mainly affecting oropharyngeal, respiratory muscles, or both; with lesser or equal involvement of limb, axial muscles, or both.
Class IV: Severe weakness affecting muscles other than ocular muscles; may also have ocular muscle weakness of any severity.
IVa. Primarily affecting limb, axial muscles, or both; with lesser impact of oropharyngeal muscles.
IVb. Predominantly affecting oropharyngeal, respiratory muscles, or both; with potentially lesser or equal impact of limb, axial muscles, or both.
Class V: Defined as intubation, with or without mechanical ventilation, except when employed during routine postoperative management. Using a feeding tube without intubation classifies a patient as IVb.
Treating gMG
Today there are several treatment options that allow healthcare providers to individualize treatment plans for gMG patients. These options range from conventional therapies to FDA-approved branded therapies, such as C5 inhibitors.
Treatment goals include21,22:
- Reduced symptomatology, specifically muscle weakness
- Reduced need for medications, ideally allowing patients to reduce/control symptoms with the lowest dose possible
Some strategies include1:
- Symptomatic therapies (eg, AChE inhibitors such as pyridostigmine)
- Long-term immunosuppressive therapies (ISTs) (eg, prednisone and related corticosteroids, azathioprine, and mycophenolate mofetil)
- Short-term ISTs (eg, PLEX and IVIg)
- Surgery (eg, thymectomy)
- Biologic therapies (eg, complement inhibitors, etc.)
AChE, acetylcholinesterase; IVIg, intravenous immunoglobulin; MGFA, Myasthenia Gravis Foundation of America; PLEX, plasma exchange.
See how a treatment option for gMG works
Learn About a Treatment OptionMike, a patient living with gMG. Mike has received compensation from Alexion Pharmaceuticals, Inc. and has a relative who works for Alexion.