USMLE MSK 5: Muscle Pathology (Myasthenia Gravis And More!)

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  / lymed   Welcome to LY Med, where I go over everything you need to know for the USMLE STEP 1, with new videos every day.

Follow along with First Aid, or with my notes which can be found here: https://www.dropbox.com/sh/ces0j7obod...

After our discussion on the anatomy of the muscles, including the sarcomere, we now turn our focus onto muscular pathology. Path today will start with a group of conditions called muscular dystrophy. These are conditions that cause your muscles to waste away. The first one i Duchenne Muscular Dystrophy. This is a mutation in dystrophin, in particular a frameshift mutation. Symptoms include difficulty walking, pseudo-hypertrophy of the calves, and the classic Gowers' sign. Labs include elevated CK and aldolase, and on biopsy will show fatty replacement. Western blot will show the mutated dystrophin protein.

Now what happens if you have a milder mutation? That leads to Becker's Muscular Dystrophy and it's just a milder form.

Our next topic is myotonic dystrophy. Myotonic dystrophy is an inherited trinucleotide repeat disorder. There is an increase in CTG repeats and causes a mutation in a gene called myotonic dystrophy protein kinase. Symptoms include frontal balding, cataracts, testicular atrophy, and myotonia. Cardiac muscle involvement leads to arrhythmia and heart block.

Our last topic includes neuromuscular junction disorders. Now the main function of the muscles is to contract. The signal for contraction comes from a nerve, which releases acetylcholine to stimulate depolarization. Without this release of acetylcholine, there is no contraction.

Disorders here include myasthenia gravis, an autoimmune condition in which antibodies bind the Ach receptors, destroying them. With a decrease in receptor, the muscles can't contract and it leads to fatigue. Symptoms include diplopia, ptosis, and symptoms often get worse after use. We can test for this with edrophonium, an acetylcholine esterase inhibitor. Or we can just look for those antibodies. Many times the antibodies come from the thymus and a thymoma should always be excluded. Treatment includes immunosuppresants and long acting Ach-esterase inhibitors. Pharm aims to slow down progression.

The last disorder is Lambert Eaton Myasthenic Syndrome, which is due to antibodies against presynaptic calcium channels. By binding this, Ach is not released and thus contraction cannot happen. Symptoms here include proximal weakness and the big difference is that symptoms get better with use. Antibodies can come from a neoplasm, and thus Lambert Eaton can be paraneoplastic, often from small cell lung cancer.

That does it for this video!