Myasthenia gravis
Myasthenia gravis | |
---|---|
immunosuppressants[1] | |
Frequency | 50 to 200 per million[3][4] |
Myasthenia gravis (MG) is a long-term
Myasthenia gravis is an
MG is generally treated with medications known as
MG affects 50 to 200 people per million.
Signs and symptoms
The initial, main symptom in MG is painless weakness of specific muscles, not fatigue.[15] The muscle weakness becomes progressively worse (fatigue) during periods of physical activity and improves after periods of rest. Typically, the weakness and fatigue are worse toward the end of the day.[16] MG generally starts with ocular (eye) weakness; it might then progress to a more severe generalized form, characterized by weakness in the extremities or in muscles that govern basic life functions.[17]
Eyes
In about two-thirds of individuals, the initial symptom of MG is related to the muscles around the eye.
Eating
The weakness of the muscles involved in swallowing may lead to swallowing difficulty (dysphagia). Typically, this means that some food may be left in the mouth after an attempt to swallow,[19] or food and liquids may regurgitate into the nose rather than go down the throat (velopharyngeal insufficiency).[16] Weakness of the muscles that move the jaw (muscles of mastication) may cause difficulty chewing. In individuals with MG, chewing tends to become more tiring when chewing tough, fibrous foods.[15] Difficulty in swallowing, chewing, and speaking is the first symptom in about one-sixth of individuals.[15]
Speaking
Weakness of the muscles involved in speaking may lead to
Head and neck
Due to weakness of the
Other
The
Pathophysiology
MG is an
Human leukocyte antigen haplotypes are associated with increased susceptibility to myasthenia gravis and other autoimmune disorders. Relatives of people with myasthenia gravis have a higher percentage of other immune disorders.[26][27]
The thymus gland cells form part of the body's immune system. In those with myasthenia gravis, the thymus gland is large and abnormal. It sometimes contains clusters of immune cells that indicate lymphoid hyperplasia, and the thymus gland may give wrong instructions to immune cells.[28]
-
Neuromuscular junction: 1. Axon 2. Muscle cell membrane 3. Synaptic vesicle 4. Nicotinic acetylcholine receptor 5. Mitochondrion
-
A juvenile thymus shrinks with age.
In pregnancy
For women who are pregnant and already have MG, in a third of cases, they have been known to experience an exacerbation of their symptoms, and in those cases, it usually occurs in the
About 10–20% of infants with mothers affected by the condition are born with transient
Diagnosis
MG can be difficult to diagnose, as the symptoms can be subtle and hard to distinguish from both normal variants and other neurological disorders.[19]
Three types of myasthenic symptoms in children can be distinguished:[32]
- Transient neonatal myasthenia occurs in 10 to 15% of babies born to mothers afflicted with the disorder, and disappears after a few weeks.
- Congenital myasthenia, the rarest form, occurs when genes are present from both parents.
- Juvenile myasthenia gravis is most common in females.
Congenital myasthenias cause muscle weakness and fatigability similar to those of MG.[33] The signs of congenital myasthenia usually are present in the first years of childhood, although they may not be recognized until adulthood.[34]
Classification
Class | Description |
---|---|
I | Any eye muscle weakness, possible ptosis, no other evidence of muscle weakness elsewhere |
II | Eye muscle weakness of any severity, mild weakness of other muscles |
IIa | Predominantly limb or axial muscles |
IIb | Predominantly bulbar and/or respiratory muscles |
III | Eye muscle weakness of any severity, moderate weakness of other muscles |
IIIa | Predominantly limb or axial muscles |
IIIb | Predominantly bulbar and/or respiratory muscles |
IV | Eye muscle weakness of any severity, severe weakness of other muscles |
IVa | Predominantly limb or axial muscles |
IVb | Predominantly bulbar and/or respiratory muscles |
V | Intubation needed to maintain airway |
When diagnosed with MG, a person is assessed for his or her neurological status and the level of illness is established. This is usually done using the accepted Myasthenia Gravis Foundation of America Clinical Classification scale.[citation needed]
Physical examination
During a physical examination to check for MG, a doctor might ask the person to perform repetitive movements. For instance, the doctor may ask one to look at a fixed point for 30 seconds and to relax the muscles of the forehead, because a person with MG and ptosis of the eyes might be involuntarily using the forehead muscles to compensate for the weakness in the eyelids.[19] The clinical examiner might also try to elicit the "curtain sign" in a person by holding one of the person's eyes open, which in the case of MG will lead the other eye to close.[19]
Blood tests
If the diagnosis is suspected, serology can be performed:
- One test is for antibodies against the acetylcholine receptor;sensitivityof 80–96%, but in ocular myasthenia, the sensitivity falls to 50%.
- A proportion of the people without antibodies against the acetylcholine receptor have antibodies against the MuSK protein.[36]
- In specific situations, testing is performed for Lambert-Eaton syndrome.[37]
Electrodiagnostics
Muscle fibers of people with MG are easily fatigued, which the repetitive nerve stimulation test can help diagnose. In single-fiber electromyography, which is considered to be the most sensitive (although not the most specific) test for MG,[19] a thin needle electrode is inserted into different areas of a particular muscle to record the action potentials from several samplings of different individual muscle fibers. Two muscle fibers belonging to the same motor unit are identified, and the temporal variability in their firing patterns is measured. Frequency and proportion of particular abnormal action potential patterns, called "jitter" and "blocking", are diagnostic. Jitter refers to the abnormal variation in the time interval between action potentials of adjacent muscle fibers in the same motor unit. Blocking refers to the failure of nerve impulses to elicit action potentials in adjacent muscle fibers of the same motor unit.[38]
Ice test
Applying ice for 2–5 minutes to the muscles reportedly has a sensitivity and specificity of 76.9% and 98.3%, respectively, for the identification of MG. Acetylcholinesterase is thought to be inhibited at the lower temperature, which is the basis for this diagnostic test. This generally is performed on the eyelids when ptosis is present and is deemed positive if a ≥2-mm rise in the eyelid occurs after the ice is removed.[39]
Edrophonium test
This test requires the
Imaging
A
Pulmonary function test
The forced
Differential diagnoses
The muscle weakness that worsens with activity (abnormal
Other diseases that involve abnormal muscle fatigue (which may be described as exercise-induced muscle weakness, reversible muscle weakness, or muscle weakness that improves with rest) include: endocrine myopathies (such as Hoffman syndrome), Tubular aggregate myopathy (TAM), ischemia (such as intermittent claudication, popliteal artery entrapment syndrome, and chronic venous insufficiency), and poor diet or malabsorption diseases that lead to vitamin D deficiency (osteomalic myopathy). Although limb-girdle muscular dystrophies (LGMDs) involve fixed muscle weakness, LGMDR8 also involves muscle fatigue;[53] as do some limb-girdle muscular dystrophy-dystroglycanopathies such as MDDGC3 (a.k.a LGMDR15 and LGMD2O).[52][54] Myofibrillar myopathy 10,[55] dimethylglycine dehydrogenase deficiency,[56] erythrocyte lactate transporter defect,[57] and myopathy with myalgia, increased serum creatine kinase, with or without episodic rhabdomyolysis (MMCKR)[58] also include muscle fatigue.
X-linked episodic muscle weakness (EMWX) includes general muscle weakness, ptosis, and fluctuations in strength. In some individuals, fatiguability was demonstrable, the phenotype having features comparable to congenital myasthenic syndromes and channelopathies.[59]
Signs and symptoms of myasthenia presenting from infancy or childhood may be one of the congenital myasthenic syndromes, which can be inherited in either an autosomal dominant or recessive manner. There are currently over two dozen types of congenital myasthenic syndromes.[60]
Limb–girdle myasthenia gravis is a distinct condition from myasthenia gravis. It is an adult-onset, autoimmune condition affecting the neuromuscular junction. However, it lacks eye abnormalities and is associated with autoimmune conditions such as systemic lupus erythematosus, Hashimoto's thyroiditis, and thymoma.[61]
Lambert–Eaton myasthenic syndrome (LEMS) is an autoimmune condition that attacks the neuromuscular junction, either as a paraneoplastic syndrome (typically older patients) or associated with a non-cancerous primary autoimmune condition (typically younger patients). It usually involves lower limb weakness and exercise-induced fatiguability, although the upper limbs and eyes may also be involved. Lambert's sign is the unusual improvement of grip strength that follows after squeezing the hand at maximum intensity for 2-3 seconds.[62]
Management
Treatment is by medication and/or surgery. Medication consists mainly of acetylcholinesterase inhibitors to directly improve muscle function and
Medication
Worsening may occur with medication such as
Efgartigimod alfa (Vyvgart) was approved for medical use in the United States in December 2021.[68][69][70]
Efgartigimod alfa/hyaluronidase (Vyvgart Hytrulo) was approved for medical use in the United States in June 2023.[71][72]
Rozanolixizumab (Rystiggo) was approved for medical use in the United States in June 2023.[73][74]
Acetylcholinesterase inhibitors
Acetylcholinesterase inhibitors can provide symptomatic benefit and may not fully remove a person's weakness from MG.[75] While they might not fully remove all symptoms of MG, they still may allow a person the ability to perform normal daily activities.[75] Usually, acetylcholinesterase inhibitors are started at a low dose and increased until the desired result is achieved. If taken 30 minutes before a meal, symptoms will be mild during eating, which is helpful for those who have difficulty swallowing due to their illness. Another medication used for MG, atropine, can reduce the muscarinic side effects of acetylcholinesterase inhibitors.[76] Pyridostigmine is a relatively long-acting drug (when compared to other cholinergic agonists), with a half-life around four hours with relatively few side effects.[77] Generally, it is discontinued in those who are being mechanically ventilated, as it is known to increase the amount of salivary secretions.[77] A few high-quality studies have directly compared cholinesterase inhibitors with other treatments (or placebo); their practical benefit may be so significant that conducting studies in which they would be withheld from some people would be difficult.[63]
Immune suppressants
The steroid prednisone might also be used to achieve a better result, but it can lead to the worsening of symptoms and takes weeks to achieve its maximal effectiveness.[77] Research suggests that up to 15% of patients do not positively respond to immune suppressants.[78][79][5] Due to the myriad symptoms that steroid treatments can cause, it is not the preferred method of treatment.[77] Other immune suppressing medications may also be used including rituximab[80] or azathioprine.[1]
Plasmapheresis and IVIG
If the myasthenia is serious (myasthenic crisis),
Surgery
As thymomas are seen in 10% of all people with the MG, they are often given a chest X-ray and CT scan to evaluate their need for surgical removal of their thymus glands and any cancerous tissue that may be present.[22][41] Even if surgery is performed to remove a thymoma, it generally does not lead to the remission of MG.[77] Surgery in the case of MG involves the removal of the thymus, although in 2013, no clear benefit was indicated except in the presence of a thymoma.[82] A 2016 randomized, controlled trial, however, found some benefits.[83]
Physical measures
People with MG should be educated regarding the fluctuating nature of their symptoms, including weakness and exercise-induced fatigue. Exercise participation should be encouraged with frequent rest.[17] In people with generalized MG, some evidence indicates a partial home program including training in diaphragmatic breathing, pursed-lip breathing, and interval-based muscle therapy may improve respiratory muscle strength, chest wall mobility, respiratory pattern, and respiratory endurance.[84]
Medical imaging
In people with myasthenia gravis, older forms of iodinated contrast used for medical imaging have caused an increased risk of exacerbation of the disease, but modern forms have no immediate increased risk.[85]
Prognosis
The prognosis of people with MG is generally good, as is quality of life, when given very good treatment.[86] Monitoring of a person with MG is very important, as at least 20% of people diagnosed with it will experience a myasthenic crisis within two years of their diagnosis, requiring rapid medical intervention.[77] Generally, the most disabling period of MG might be years after the initial diagnosis.[75] Assistive devices may be needed to assist with mobility.[1] In the early 1900s, 70% of detected cases died from lung problems; now, that number is estimated to be around 3–5%, an improvement attributed to increased awareness and medications to manage symptoms.[77]
Epidemiology
MG occurs in all ethnic groups and both sexes. It most commonly affects women under 40 and people from 50 to 70 years old of either sex, but it has been known to occur at any age. Younger people rarely have thymoma. Prevalence in the United States is estimated at between 0.5 and 20.4 cases per 100,000, with an estimated 60,000 Americans affected.[22][87] In the United Kingdom, an estimated 15 cases of MG occur per 100,000 people.[41]
History
The first to write about MG were Thomas Willis, Samuel Wilks, Erb, and Goldflam.[18] The term "myasthenia gravis pseudo-paralytica" was proposed in 1895 by Jolly, a German physician.[18] Mary Walker treated a person with MG with physostigmine in 1934.[18] Simpson and Nastuck detailed the autoimmune nature of the condition.[18] In 1973, Patrick and Lindstrom used rabbits to show that immunization with purified muscle-like acetylcholine receptors caused the development of MG-like symptoms.[18]
Research
Immunomodulating substances, such as drugs that prevent acetylcholine receptor modulation by the immune system, are currently being researched.[88] Some research recently has been on anti-c5 inhibitors for treatment research as they are safe and used in the treatment of other diseases.[13] Ephedrine seems to benefit some people more than other medications, but it has not been properly studied as of 2014.[10][89] In the laboratory, MG is mostly studied in model organisms, such as rodents. In addition, in 2015, scientists developed an in vitro functional, all-human, neuromuscular junction assay from human embryonic stem cells and somatic-muscle stem cells. After the addition of pathogenic antibodies against the acetylcholine receptor and activation of the complement system, the neuromuscular co-culture shows symptoms such as weaker muscle contractions.[90]
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Further reading
- Zhang Z, Guo J, Su G, Li J, Wu H, Xie X (17 November 2014). "Evaluation of the quality of guidelines for myasthenia gravis with the AGREE II instrument". PLOS ONE. 9 (11): e111796. PMID 25402504.
- "Diagnostic: Myasthenia gravis". National Center for Biotechnology Information (NCBI). U.S. National Library of Medicine. Retrieved 11 July 2015.