Myasthenia gravis

Myasthenia gravis
Classification and external resources
Specialty	Neurology
ICD-10	G70.0
ICD-9-CM	358.0, 358.01 (in crisis)
OMIM	254200
DiseasesDB	8460
MedlinePlus	000712
eMedicine	neuro/232 emerg/325 (emergency), med/3260 (pregnancy), oph/263 (eye)
Patient UK	Myasthenia gravis
MeSH	D009157

Myasthenia gravis (from Greek μύς "muscle", ἀσθένεια "weakness", and Latin: gravis "serious"; abbreviated MG) is a neuromuscular disease that leads to fluctuating muscle weakness and fatigue. In the most common cases, muscle weakness is caused by circulating antibodies that block acetylcholine receptors at the postsynaptic neuromuscular junction, inhibiting the excitatory effects of the neurotransmitter acetylcholine on nicotinic receptors at neuromuscular junctions. Alternatively, in a much rarer form, muscle weakness is caused by a genetic defect in some portion of the neuromuscular junction that is inherited at birth as opposed to developing through passive transmission from the mother's immune system at birth or through autoimmunity later in life.

Myasthenia is treated with medications such as acetylcholinesterase inhibitors or immunosuppressants, and, in selected cases, thymectomy (surgical removal of the thymus gland). The disease is diagnosed in 3 to 30 people per million per year. Diagnosis is becoming more common due to increased awareness.

Signs and symptoms

The initial, main symptom in MG is painless weakness of specific muscles, not fatigue. The muscle weakness becomes progressively worse during periods of physical activity, and improves after periods of rest. Typically, the weakness and fatigue are worse towards the end of the day. MG generally starts with ocular (eye) weakness; it might then progress to a more severe generalized form, characterized by weakness in the extremities or while performing basic life functions.

Eyes

In about two-thirds of individuals, the initial symptom of MG is related to the muscles around the eye. There may be eyelid drooping (ptosis due to weakness of levator palpebrae superioris) and double vision (diplopia, due to weakness of the extraocular muscles). Eye symptoms tend to get worse when watching television, reading or driving, particularly in bright conditions. Consequently, some affected individuals choose to wear sunglasses. The term "ocular myasthenia gravis" describes a subtype of MG where muscle weakness is confined to the eyes, i.e. extraocular muscles, levator palpebrae superioris and orbicularis oculi. Typically, this subtype evolves into generalized MG, usually after a few years.

Eating

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, or food and liquids may regurgitate into the nose rather than go down the throat (velopharyngeal insufficiency). 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. Difficulty in swallowing, chewing and speaking is the first symptom in about one-sixth of individuals.

Voice

Weakness of the muscles involved in speaking may lead to dysarthria and hypophonia. Speech may be slow and slurred, or have a nasal quality. In some cases a singing hobby or profession must be abandoned.

Head and neck

Due to weakness of the muscles of facial expression and muscles of mastication, there may be facial weakness, manifesting as inability to hold the mouth closed (the "hanging jaw sign"), and a snarling appearance when attempting to smile. Together with drooping eyelids, facial weakness may make the individual appear sleepy or sad. There may be difficulty in holding the head upright.

Other

The muscles that control breathing (dyspnea) and limb movements can also be affected, but rarely do these present as the first symptoms of MG, and they develop over months to years. In a myasthenic crisis, a paralysis of the respiratory muscles occurs, necessitating assisted ventilation to sustain life. Crises may be triggered by various biological stressors such as infection, fever, an adverse reaction to medication, or emotional stress.

Cause

This neuromuscular disease is caused by transmission defects in nerve impulses to muscles. The neuromuscular junction is apparently affected: acetylcholine, which produces muscle contraction under normal conditions no longer produces the contractions necessary to muscle movement.

Pathophysiology

Neuromuscular junction: 1. Axon 2. Muscle cell membrane 3. Synaptic vesicle 4. Nicotinic acetylcholine receptor 5. Mitochondrion

A juvenile thymus shrinks with age.

The nicotinic acetylcholine receptor

Myasthenia gravis is believed to be caused by variations in certain genes. The disorder occurs when the immune system malfunctions and attacks the body's tissues. The antibody in myasthenia gravis attacks normal human protein, targeting a protein called an acetylcholine receptor, or a related protein called a muscle-specific kinase.

Human leukocyte antigens have been associated with MG receptibility. Many of these genes are present among other autoimmune diseases. Relatives of MG patients have a higher percentage of other immune disorders.

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 which indicate lymphoid hyperplasia, and it is believed the thymus gland may give wrong instructions to immune cells.

Associated conditions

Myasthenia gravis is associated with various autoimmune diseases, including:

• Thyroid diseases,
• CNS disease
• Lupus

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 first trimester of pregnancy. Signs and symptoms in pregnant mothers tend to improve during the second and third trimesters. Complete remission can occur in some mothers. Immunosuppressive therapy should be maintained throughout pregnancy, as this reduces the chance of neonatal muscle weakness, as well as controls the mother's myasthenia.

10-20% of infants with mothers affected by the condition are born with Transient Neonatal Myasthenia, which generally produces feeding and respiratory difficulties that develop within 12 hours to several days after birth. A child with TNM typically responds very well to acetylcholinesterase inhibitors, and the condition generally resolves over a period of three weeks as the antibodies degregate and generally does not result in any complications. Very rarely, an infant can be born with arthrogryposis multiplex congenita, secondary to profound intrauterine weakness. This is due to maternal antibodies that target an infant's acetylcholine receptors. In some cases, the mother remains asymptomatic.

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.

Three types of myasthenic symptoms in children can be distinguished:

1. Transient Neonatal: occurs in 10 to 15% of babies born to mothers afflicted with the disorder, and disappears after a few weeks.
2. Congenital: the rarest form; genes are present in both parents.
3. Juvenile myasthenia gravis: most common in females

Congenital myasthenias cause muscle weakness and fatigability similar to those of MG. The signs of congenital myasthenia usually are present in the first years of childhood although they may not be recognized until adulthood.

Classification

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, which is as follows:

Myasthenia Gravis Foundation of America Clinical 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

Physical examination

During a physical examination to check for MG, a doctor might ask the potentially affected person to look at a fixed point for 30 seconds and to relax the muscles of their forehead. This is done because a person with MG and ptosis of their eyes might be involuntarily using their forehead muscles to compensate for the weakness in their eyelids. The clinical examiner might also try to elicit the "curtain sign" in a patient by holding one of the person's eyes open, which in the case of MG will lead the other eye to close.

Blood tests

If the diagnosis is suspected, serology can be performed:

• One test is for antibodies against the acetylcholine receptor, the test has a reasonable sensitivity of 80–96%, but in ocular myasthenia, the sensitivity falls to 50%.
• A proportion of the patients without antibodies against the acetylcholine receptor have antibodies against the MuSK protein.
• In specific situations, testing is performed for Lambert-Eaton syndrome.

Electrodiagnostics

A chest CT-scan showing a thymoma (red circle)

Photograph of a patient showing right partial ptosis (left picture), the left lid shows compensatory pseudo lid retraction because of equal innervation of the levator palpabrae superioris (Hering's law of equal innervation): Right picture: after an edrophonium test, note the improvement in ptosis.

Muscle fibers of patients with MG are easily fatigued, and a test called the repetitive nerve stimulation test can be performed. In single-fiber electromyography, which is considered to be the most sensitive (although not the most specific) test for MG, 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.

Ice test

Applying ice for two to five 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, and this is the basis for this diagnostic test. This generally is performed on the eyelids when a ptosis is present and is deemed positive if there is a ≥2mm raise in the eyelid after the ice is removed.

Edrophonium test

This test requires the intravenous administration of edrophonium chloride or neostigmine, drugs that block the breakdown of acetylcholine by cholinesterase (acetylcholinesterase inhibitors). This test is no longer typically preformed as its use can lead to life-threatening bradycardia (slow heart rate) which requires immediate emergency attention. Production of edrophonium was discontinued in 2008.

Imaging

A chest X-ray may identify widening of the mediastinum suggestive of thymoma, but computed tomography or magnetic resonance imaging (MRI) are more sensitive ways to identify thymomas and are generally done for this reason. MRI of the cranium and orbits may also be performed to exclude compressive and inflammatory lesions of the cranial nerves and ocular muscles.

Pulmonary function test

The forced vital capacity may be monitored at intervals to detect increasing muscular weakness. Acutely, negative inspiratory force may be used to determine adequacy of ventilation; it is performed on those individuals with MG.

Management

Treatment is by medication and/or surgery. Medication consists mainly of acetylcholinesterase inhibitors to directly improve muscle function and immunosuppressant drugs to reduce the autoimmune process. Thymectomy is a surgical method to treat MG.

Medication

Neostigmine, chemical structure

Azathioprine, chemical structure

Acetylcholinesterase inhibitors can provide symptomatic benefit and may not fully remove a person's weakness from MG. While they might not fully remove all symptoms of MG, they still may allow a person the ability to perform normal daily activities. 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 is atropine. Pyridostigmine is a short-lived drug, with a half-life of about four hours with relatively few side effects. Generally, it is discontinued in those who are being mechanically ventilated as it is known to increase the amount of salivary secretions. There have been few high-quality studies directly comparing cholinesterase inhibitors with other treatments (or placebo); it has been suggested that their practical benefit is such that it would be difficult to conduct studies in which they would be withheld from some people. The steroid prednisone might also be utilized to achieve a better result, but it can lead to the worsening of symptoms for 14 days and take 6–8 weeks for it to achieve its maximal effectiveness. Due to the myriad symptoms that steroid treatments can cause, it is not the preferred method of treatment.

Plasmapheresis and IVIG

If the myasthenia is serious (myasthenic crisis), plasmapheresis can be used to remove the putative antibodies from the circulation. Also, intravenous immunoglobulins (IVIGs) can be used to bind the circulating antibodies. Both of these treatments have relatively short-lived benefits, typically measured in weeks, and often are associated with high costs which make them prohibitive; they are generally reserved for when MG requires hospitalization.

Surgery

Main article: Thymectomy

As thymomas are seen in 10% of all people with the MG, patients are often given a chest X-ray and CT scan to evaluate their need for surgical removal of their thymus and any cancerous tissue that may be present. Even if surgery is performed to remove a thymoma, it generally does not lead to the remission of MG. Surgery in the case of MG involves the removal of the thymus although there is no clear consensus that it would be beneficial except in the presence of a thymoma. However, thymectomy should not be done in ocular myasthenia. Currently, there is no literature that gives meaningful conclusions in regard to the benefit of thymectomy in affected individuals. Some observational studies indicate that thymectomy could be prudent in MG.

Physical measures

Patients 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. 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.

Prognosis

The prognosis of MG patients is generally good, as is quality of life, given very good treatment. In the early 1900s the mortality associated with MG was 70%; now that number is estimated to be around 3–5% which is attributed to increased awareness and medications to manage symptoms. 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 emergent medical intervention. Generally, the most disabling period of MG might be years after the initial diagnosis.

Epidemiology

Myasthenia gravis 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 patients rarely have thymoma. The prevalence in the United States is estimated between 0.5–20.4 cases per 100,000, with an estimated 60,000 Americans affected. Within the United Kingdom, it is estimated that there are 15 cases of MG per 100,000 people.

History

The first to write about MG were Thomas Willis, Samuel Wilks, Erb, and Goldflam. The term "myasthenia gravis pseudo-paralytica" was proposed in 1895 by Jolly, a German physician. Mary Walker treated a person with MG with physostigmine in 1934. Simpson and Nastuck detailed the autoimmune nature of the condition. In 1973, Patrick and Lindstrom used rabbits to show that immunization with purified muscle-like acetylcholine receptors caused the development of MG-like symptoms.

Research

Immunomodulating substances, such as drugs that prevent acetylcholine receptor modulation by the immune system, are currently being researched. Some research recently has been on anti-c5 inhibitors for treatment research as they are safe and used in the treatment of other diseases. Ephedrine seems to benefit some people more than other medications, but it has not been properly studied as of 2014.

Notable cases

• Indian actor Amitabh Bachchan was diagnosed with it in 1983.
• Brandon Cox, starting Auburn quarterback from 2005-2007, finished with a record of 29-9.
• Christopher Robin Milne, the son of A. A. Milne, author of the Winnie-the-Pooh books and the person on whom Christopher Robin was based, lived with myasthenia gravis for several years before his death in 1996.
• Aristotle Onassis (1906-1975), Greek shipping magnate and businessman, suffered from the disease during the last years of his life.
• Actor Roger Smith, husband of Ann-Margret, was diagnosed in 1980.
• Days Of Our Lives actress Suzanne Rogers has the condition and so does the character she plays, Maggie Horton.
• Powhatan chief Opchanacanough (Uncle of Pocahontas ) was thought to have the condition.
• Stephen Garrett, also known as Static Major. He made up half of the R&B group "Playa".
• Anselmo Ralph, Angolan singer.

External links

• The Myasthenia Gravis Foundation of America
• The Myasthenia Gravis Association (MGA) for the United Kingdom
• The Myasthenia Gravis Association (MGA) for the Republic of Ireland
• The Myasthenia Gravis Coalition of Canada