Myasthenia gravis – causes, symptoms, treatment, pathology

Myasthenia gravis – causes, symptoms, treatment, pathology


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. The complement system is a family of small
proteins that work in an enzymatic cascade to fight off bacterial infections. The activation of complement causes inflammation
and destruction of the muscle cells and reduces the amount of acetylcholine receptors on the
surface. Now a minority of people with myasthenia gravis
produce another type of harmful antibody called muscle specific receptor tyrosine kinase antibodies,
which attack proteins inside of muscle cells instead of nicotinic acetylcholine receptors
on the muscle cell surface, but they similarly leads to destruction of healthy cells. In very rare cases, myasthenia gravis can
present as a paraneoplastic syndrome, which means that an underlying cancer like bronchogenic
carcinoma or thymic neoplasm (also known as thymoma) generates an immune response which
results in generation of autoantibodies. In some cases, if certain muscles are affected,
myasthenic patients can experience a myasthenic crisis, which is a life-threatening manifestation
of the disease. An example of this might be a decreased function
of the muscles that control breathing. The good news is is that treatment has been
able to reduce mortality from myasthenia gravis. One type of medication is an acetylcholinesterase
inhibitor, like neostigmine or pyridostigmine. Acetylcholinesterase degrades acetylcholine,
so acetylcholinesterase inhibitors stop acetylcholinesterase from breaking down acetylcholine, which ends
up increasing the concentration of acetylcholine around muscle cells and helps counteract the
effects of acetylcholine receptor antibodies. Myasthenia gravis is also treated with immunosuppressive
drugs (like prednisone), which reduce the production of the harmful antibodies. Finally, a lot of people with myasthenia gravis
undergo surgical removal of the thymus. This seems to reduce the muscle weakness symptoms
even in people that don’t appear to have any identifiable problems with the thymus
gland itself. It’s possible that this is because helper
T cells, which originate in the thymus, help B cells make antibodies to the acetylcholine
receptor. Alright, as a quick review—myasthenia gravis
is a type II hypersensitivity disorder where the body produces antibodies that most commonly
target 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. Myasthenia gravis preferentially affects young
women or older men and commonly affects the extraocular muscles leading to eyelid drooping
or double vision, but it might affect any skeletal muscle. Thanks for watching, you can help support
us by donating on patreon, subscribing to our channel, or following us on social media.

4 comments

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