MYASTHENIA
GRAVIS
Classification
The most widely accepted classification of Myasthenia gravis is the Myasthenia Gravis Foundation of America Clinical Classification
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Class I: Any eye muscle weakness, possible ptosis, no other evidence of muscle weakness elsewhere
- Class II: Eye muscle weakness of any severity, mild weakness of other muscles
- Class II a: Predominantly limb or axial muscles
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Class II b: Predominantly bulbar and/or respiratory muscles
- Class III: Eye muscle weakness of any severity, moderate weakness of other muscles
- Class III a: Predominantly limb or axial muscles
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Class III b: Predominantly bulbar and/or respiratory muscles
- Class IV: Eye muscle weakness of any severity, severe weakness of other muscles
- Class IV a: Predominantly limb or axial muscles
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Class IV b: Predominantly bulbar and/or respiratory muscles (Can also include feeding tube without intubation)
- Class V: Intubation needed to maintain airway
Myasthenia gravis can be classified according to which skeletal muscles are affected.
Within a year of onset, approximately 85–90% of patients develop generalized myasthenia gravis, which is characterized by weakness in the trunk, arms, and legs.
About 10–15% of patients have weakness only in muscles that control eye movement. This type is called ocular myasthenia gravis.
Other types of MG include
- Congenital MG. This is a very rare, nonimmune form of MG that is inherited as an autosomal recessive disease. This means that both males and females are equally affected and that two copies of the gene, one inherited from each parent, are necessary to have the condition. Symptoms of congenital MG usually begin in the baby's first year and are lifelong. Congenital MG develops at or shortly after birth and causes generalized symptoms.
- Transient neonatal MG is a temporary condition that develops in 10–20% of infants born to mothers who have MG. Transient neonatal MG is caused by circulation of the mother's antibodies through the placenta and it lasts as long as the mother's antibodies remain in the infant (usually a few weeks after birth).
- Juvenile MG. This autoimmune disorder develops typically in female adolescents, especially white females. It is a lifelong condition that may go in and out of remission. About 10 percent of MG cases are juvenile-onset.
Types of Myasthenia Gravis
Drug-Related Myasthenia Gravis:
There have been many instances in which a patient has experienced an onset of Myasthenia gravis during penicillamine treatment for diseases such as rheumatoid arthritis. This disease onset may be caused by an alteration in the immune system, which allows for the production of anti-AChR antibodies due to the effects of the drug. Aside from penicillamine, several other drugs can heighten the symptoms of Myasthenia gravis in patients that already have the disease. Therefore, even small doses of any drug that acts as a neuromuscular blocking agent should be avoided by patients with Myasthenia gravis.
Bacterial/ Viral Myasthenia gravis
Some forms of myasthenia gravis seem to have been brought on by some sort of bacteria or viral infection, with the patients developing some symptoms of myasthenia gravis shortly afterward. The explanation for this seems to be the theory of molecular mimicry, where the protein amino acid sequence of the foreign invader is similiar to the same sequence of a sequence in the body. The immune system now recognizes both the foreign and the new "self" molecule as foreign. In the case of myasthenia gravis, there is evidence that herpes simplex is similar enough to initiate an immune response against protein components of the acetylcholine receptors.
Types of Myasthenia Gravis That Involve an Autoimmune Response.
Transient Neonatal Myasthenia Gravis:
Neonatal myasthenia gravis, which occurs in infants born to myasthenic mothers, is due to the transplacental exchange of pathogenic immunoglobulins, with or without AChR antibodies, from mother to infant. This disease occurs in about only 10-20% of all children born to myasthenic mothers, but anti-AChR antibodies can be found in most. The infant's symptoms are generally exposed by the third day of life, and without immediate treatment, the myasthenic symptoms may gradually subside over 1-4 weeks. The disease becomes permanent when there is irreversible destruction of AChR by the maternal antibodies, or production of antibodies by the infant; this only occurs on rare occasions. Although this disease is passed along from the mother, there is no relation between the degree of weakness in the infant and the degree of weakness in the mother.
Adult-Onset Myasthenia Gravis:-
This form of myasthenia gravis usually begins in the third decade of life for women, and the fifth decade for men. This form carries with it all of the symptoms that are normally found in patients with Myasthenia gravis. The disease will generally reach its greatest severity within the first three years after onset. Myasthenia gravis may coexist with other autoimmune diseases, and thyroid disease may heighten the myasthenic signs and symptoms.
Experimental Autoimmune Myasthenia Gravis:
This disease model of myasthenia gravis was originally induced in rabbits immunized with AChR from the eel's electric organ. Experimental myasthenia gravis can also be induced by introduction of AChR from many different species, or peptide fragments of the AChR. This disease is divided into an acute and a chronic phase. The acute phase is much like human myasthenia gravis. Two weeks after injecting rats with the immune cells, damage to junctional folds can be seen. Damage to the folds can lead to postsynaptic fragments in the synaptic cleft and the presence of macrophages within the neuromuscular junction. There is quite a bit of literature on this disease, as it is helping to provide information about the treatment of human myasthenia gravis.
Figure 9:- Origin of viral onset Autoimmune response
Figure 10:- Adult-Onset Autoimmune response