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What is Myopathy? - Definition, Causes & Symptoms

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  • 0:33 What is a Myopathy?
  • 1:44 Congenital Myopathies
  • 2:41 Acquired Myopathies
  • 3:37 Diagnosing Myopathies
  • 4:25 Treating Myopathies
  • 5:15 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

This lesson will discuss what a myopathy is. We'll mention the two major classes of myopathies and some examples of each, as well as how they may be diagnosed and treated.

Cramps and Muscle Fatigue

If you have ever played sports, then you have undoubtedly experienced the pain of muscle cramps or the fatigue that comes after a lot of exertion. It may take you days to recover from such episodes in serious cases. In certain other scenarios, however, such issues arise not as a result of massive exertion during a workout, but may come as a result of certain diseases, which this lesson will point out.

What Is A Myopathy?

With that in mind, myopathies are a group of diseases that cause muscle weakness, cramps, and spasms due to a primary defect of the muscle fiber. The more technical term for a muscle fiber is a myocyte, or muscle cell, or myofiber. They're all synonymous.

The key thing to note here is that these diseases are not primary nerve diseases that can cause muscle weakness. For instance, ALS, more commonly known as Lou Gehrig's disease, is a condition that destroys the nerves that innervate muscles of voluntary movement. If the nerves, the electrical signal to the muscle, are damaged, then the muscle cannot function either. However, in the case of a primary myopathy, the nerves are just fine, and the disease lies within the myocyte itself.

You can liken this to our body being like a remote control car. If the batteries that provide power for the electricity run out, that's like the failure of our nerves. But, if the wheels on the car are damaged, even if there's plenty of battery power, then it's a myopathy.

Congenital Myopathies

Some of the myopathies a person may have are genetic in their origin. Such myopathies, arising as a result of genetic mutations, are called congenital myopathies. One famous example of a congenital myopathy is a group of conditions known as glycogen storage diseases of muscles, where the muscle is unable to process sugar molecules such as glycogen. This leads to a buildup of glycogen within the cell, causing its dysfunction.

It's not hard to imagine why. A cell is made up of a lot of moving parts. If it gets stuffed full of molecules it's not supposed to be stuck with, those moving parts will fail, causing the cell to stop functioning properly. If the muscle cell can't work normally, then the muscle cannot contract, leading to muscle weakness. It's like if you were to stuff a moving car engine full of little pieces of metal or something; eventually, that engine wouldn't be able to move properly, and the car wouldn't be able to move either.

Acquired Myopathies

Regardless, congenital myopathies are slightly different in their original cause when compared to acquired myopathies. These are myopathies that are generated as a result of external factors, such as drugs or infectious causes. These acquired myopathies include rhabdomyolysis, or the breakdown of skeletal muscle cells that releases myoglobin into the body, resulting in myoglobinuria, or the presence of myoglobin in urine, and even kidney damage due to the myoglobin itself.

They also include inflammatory conditions, such as dermatomyositis, polymyositis, and inclusion body myositis. On a test, the way you'll know all of these are inflammatory conditions affecting the muscles is because you can easily spot the prefix 'myo-' in them, referring to muscle, and the suffix '-itis,' referring to the inflammation of something.

Diagnosing Myopathies

Well, regardless of whether a condition is acquired or congenital, the way by which we diagnose them definitively is many times the same. We do it through a procedure called a muscle biopsy, where we take a little sample of muscle tissue, slice it into really thin pieces, and look at it under the microscope in order to tell exactly what it is that we are dealing with.

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