Definition of Myasthenia Gravis (MG)
It is a neuromuscular disorder of autoimmune origin in which the acetylcholine receptors (AchR) in the motor end-plates of the muscles are damaged.
- MG is characterized by weakness and fatigability of skeletal muscles (resulting from a failure of neuromuscular transmission).
- The term ‘Myasthenia‘ means ‘Muscle weakness‘ and ‘Gravis‘ implies ‘Serious‘, thus both together denote the clinical characteristics of the disease.
Causes – Autoimmune disease
Types of Myasthenia Gravis (MG)
It is typically characterized into five types :-
Congenital myasthenia gravis – It is not autoimmune disease but a defect in the genes which result in abnormal proteins, inherited as an autosomal recessive disease.
- Transient neonatal myasthenia gravis – Relatively rare form of myasthenia gravis may develop in a child immediately after birth. The reason for this are the mmother’s antibodies, suffering from a disease transmitted through the placenta. It is necessary to distinguish the nenonatal form of another type of myasthenia gravis associated with a genetic mutation COLQ, and inherited.
- Juvenile myasthenia gravis – Develops in childhood and adolscence, usually girls, occurs in appoximately 10% of myasthenia gravis cases.
- Generalized myasthenia gravis – The most common form of disease. The most characteristic occurrence of the disease in women aged 20-30 and men aged 50-60 years.
- Ocular myasthenia gravis – Around 10% – 15% of people only experience problems with the muscles the control eye.
Mechanism of Myasthenia Gravis (MG) [Pathophysiology]
Normally, acetylcholine is synthesised in the motor nerve terminal and stored in vesicles that are released spontanesoly when an action potential reaches the nerve terminal. Acetylcholine from released vesicles combines with AChRs, initiating an action potential which is propagted along the muscle fibres triggering muscle contraction. When ACh combines with the binding sites on the AChR, the channel in the AChR opens, permitting the rapid entry of cations, chiefly sodium, which produces depolarization at the end-plate region of the muscle fiber. If the depolarization is sufficiently large, it initiates an action potential that is propagated along the muscle fiber, triggering muscle contraction. This process is rapidly terminated by hydrolysis of ACh by acetylcholinesterase (AChE), which is present within the synaptic folds, and by diffusion of ACh away from the receptor.
In MG, the basic defect is reduction in the number of available AChRs at the postsynaptic muscle membrane. In addition, the postsynaptic folds are flattened. These changes result in decreased neuromuscular transmission leading to failure to trigger muscle action potentials and consequent weakened muscle contraction. The neuromuscular abnormalities in MG are mediated by autoimmune response. These antibodies reduce the number of available AChRs either by blocking the active sites of the receptors or by damaging the post-synaptic muscle membrane in collaboration with complement. The exact mechanism how autoimmune response is initiated is not completely understood but the thymus appears to play a role in this process. Majority of patients of MG may have either thymoma or thymic hyperplasia; thymectomy is helpful in ameliorating the condition. The thymus possibly sensitises B cells to produce anti-AChR antibodies.
Flow chart of mechanism of Myasthenia Gravis (MG)
Diagnosis and Evaluation of Myasthenia Gravis (MG)
There are various tests for diagnosis and evaluation of Myasthenia Gravis (MG) such as :
- Ameliorative Test using Edrophonium or Neostigmine
- Electrodianostic Tests
- Antiacetlcholine Receptor antibody
- Provocative test using d-tubocurarine
Symptoms of Myasthenia Gravis (MG)
There are various symptoms of Myasthenia Gravis, due to it’s auto-immune cause, it may be difficult to know it’s presence. Look out for possible symptoms and signs below and consult your doctor:
- Muscle weakness
- trouble talking
- problems walking up stairs or lifting objects
- Difficulty in holding up head for long
- facial paralysis
- difficulty breathing due to muscle weakness
- difficulty swallowing or chewing
- hoarse voice
- drooping of eyelids
- double vision
Treatment of Myasthenia Gravis (MG)
- Thymectomy (removal of thymus gland)
- Immunosuppresssion (using glucocorticoids, azathioprine and other drugs)
- Anticolinesterase Medications (using atropine pyridostigmine)
- Plasmaphersis (plasma exchange)
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