Myasthenia gravis
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Myasthenia gravis
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Preguntas del estilo USMLE Step 1
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External References
First Aid
2024
2023
2022
2021
Diplopia
myasthenia gravis p. 480
Edrophonium p. 239, 565
myasthenia gravis diagnosis p. 479
Myasthenia gravis p. 480
autoantibody p. 113
diagnosis of p. 239
neostigmine for p. 239
as paraneoplastic syndrome p. 219
pyridostigmine for p. 239
restrictive lung disease p. 694
thymoma association p. 96
type II hypersensitivity p. 110
Ptosis (eyelids)
myasthenia gravis p. 479
Pyridostigmine p. 239
myasthenia gravis treatment p. 480
Thymic hyperplasia
myasthenia gravis association p. 479
Thymomas
myasthenia gravis and p. 219, 479
Transcript
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Myasthenia gravis, is a Latin expression that literally means “grave muscle weakness,” which perfectly describes the weakness that happens when this autoimmune disease affects the skeletal muscles.
People with myasthenia gravis might wake up feeling fine, but get progressively weaker as the day goes on and by the end of the day they might feel very weak, sometimes even getting weaker with repetitive movements like chopping vegetables.
Myasthenia gravis can affect the extraocular muscles, which control movement of the eye as well as the eyelids, so individuals might have diplopia or double vision, as well as ptosis, or drooping eyelids.
Myasthenia gravis preferentially affects young women in their 20s and 30s and older men in their 60s and 70s, but the cause of this odd “bimodal” distribution of age-of-onset isn’t quite clear.
To better understand myasthenia gravis, let’s review normal muscle contraction but at the cellular level.
First you’ve got your motor neurons that release the neurotransmitter acetylcholine at the neuromuscular junction, which then binds to nicotinic acetylcholine receptors on muscle cell membranes.
Binding of acetylcholine to its receptor activates a chain reaction in the muscle cell that ultimately results in muscle contraction.
But what happens in myasthenia gravis though? Well remember that it’s an autoimmune disease, specifically one that’s categorized as a type II hypersensitivity which causes cytotoxic injury meaning that it results in the lysis or death of host cells, which is mediated by autoantibodies, which are antibodies that are specific for our own cells or proteins.
Alright, so in this case, the patient’s B cells inappropriately make antibodies that bind to nicotinic acetylcholine receptors on the muscle cells.
Once acetylcholine receptors are bound by the antibody, they are unable to bind to acetylcholine, and therefore don’t respond to the “contract” signal from the central nervous system.
Anti-acetylcholine receptor antibodies can also activate the classical pathway of the complement.
Resumen
Myasthenia gravis is an autoimmune disease, specifically a type II hypersensitivity disorder, which is characterized by autoantibodies against nicotinic acetylcholine receptors on the surface of muscle cells.
The antibodies block the receptors which means the signal to contract isn't received. Those antibodies also activate the complement pathway which leads to muscle cell destruction.
Symptoms can range from mild to severe and may include drooping eyelids, difficulty speaking or swallowing, and muscle weakness in the arms and legs. Treatment involves immunosuppressive drugs like prednisolone, and acetylcholinesterase inhibitors like neostigmine. The purpose is to reduce the immune system's attack on the muscle and to increase muscle strength.
Fuentes
- "Robbins Basic Pathology" Elsevier (2017)
- "Harrison's Principles of Internal Medicine" McGraw Hill Education/ Medical (2018)
- "Pathophysiology of Disease: An Introduction to Clinical Medicine 8E" McGraw-Hill Education / Medical (2018)
- "CURRENT Medical Diagnosis and Treatment 2020" McGraw-Hill Education / Medical (2019)
- "Ephedrine for myasthenia gravis, neonatal myasthenia and the congenital myasthenic syndromes" Cochrane Database of Systematic Reviews (2014)
- "The myasthenic patient in crisis: an update of the management in Neurointensive Care Unit" Arquivos de Neuro-Psiquiatria (2013)
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