Limb-Girdle Muscular Dystrophies and Channelopathies

00:00 Limb-girdle muscular dystrophies, shoulder and hip as exactly the name implies.

00:08 Shoulder and hip, shoulder and hip girdle weakness. Well, relative sparing once again of the oculobulbar and here cardiomyopathy is less frequent. So, if you get a situation where you're suspecting muscular dystrophies and has nothing to do with dystrophin.

00:28 So if you find the dystrophin is present and you find issues with the shoulder and the hip and the girdle, then your diagnosis is limb-girdle muscular dystrophy. Cardiomyopathy, much less common. The pathophys here, heterogenous abnormalities of muscle related proteins. It is going to be hereditary because this is a dystrophy. Labs here, once again, elevated CK, creatinine kinase as you can imagine, may require muscle biopsy. However, definitive diagnosis because the genetics here with dystrophin is not going to be clear cut. Management here will be supportive. Be familiar with LG muscular dystrophy please. In this illustration, what is being highlighted for you is the dystrophin.

01:19 And with the dystrophin, you'll notice here that this is the anchor for much of your muscle. And if for whatever reason that dystrophin is not present as in Duchenne muscular dystrophy, their skeletal muscle could be affected and your heart muscle could be affected. And please keep in mind that the respiratory system could be at serious risk of arrest. Here, we'll take a look at myotonic dystrophy. It's the most common adult onset of muscular dystrophy. So once again, it's inheritance. Right? That's what dystrophy means by definition. We have type 1 MD, which is muscular dystrophy. And this is called Steiner disease. And if it's type 2, it's called proximal myotonic myopathy, PROMM. The distal weakness predominates and that's important for you to pay attention to huh, the distal weakness. It's already begin out here.

02:14 The myotonia, state of increased muscle contraction and impaired relaxation, is extremely important for you to understand. Myotonic, meaning to say that the muscle is in a sustained state of contraction, impaired relaxation. There is going to be frontal balding. That is very very important as a clinical feature. Cataracts, cardiac conduction defect, and multisystem disease with myotonic dystrophy.

02:42 We'll talk a little bit more with myotonic dystrophy. Hatchet facies. Hatchet facies at this point frontal balding is good enough for you to know. The pathophys here, it's a trinucleotide expansion. Let's have a closer look at the pathophysiology of type 1 myotonic dystrophy. So here, the trinucleotide expansion that you hopefully have memorized is CTG, myotonic dystrophy. Say it with me, CTG, myotonic dystrophy, you'll never get it. CAG, Huntington. Right? And there is a few more that you've learned about in genetics for sure such as CGG. I don't want to get ahead of myself.

03:32 At this point, let's focus on myotonic dystrophy. What does trinucleotide expansion mean to you, anticipation? On your labs, normal or perhaps mildly elevated CK. DNA testing is available and on your EMG, myotonic discharges are what you're looking for.

03:53 What does that mean to you? Increased muscle contraction with decreased ability to relax. Channelopathies. Think about all of the different channels that are responsible for contraction. Huh. So here obviously will play a huge role. It's episodic weakness due to muscle on channel defect. Now, there are some famous athletes in baseball.

04:18 There was a man from Orlando. I forgot his name but he had weakness and at some point in time the management got upset with him because they just thought that he was making stuff up but eventually came to be diagnosed with a channelopathy.

04:35 So it does exist and actually it's more common than what we think. All known are autosomal dominant and apart from sodium you also want to keep in mind hyperkalemic periodic paralysis due to a defect in the sodium channel and these 2 obviously will play a role always. Potassium obviously is going to play a role in establishing resting membrane potential and if for whatever reason there is a sodium channel defect and if there's hypokalemia well you know that you're going to raise your resting membrane potential towards your threshold. But you're messing up everything in terms of your sodium channel. That has to be, that's basic physiology. I can't really help you out there right now. Right? So, if you're unfamiliar how to interpret hyperkalemia and what it does to resting membrane potential and how that then affects the sodium channel. Guess what? Go back and review now so that what I'm talking to you makes perfect sense. What about hypokalemic? If it's hypokalemic, can I ask you? What happens to resting membrane potential? Good. It becomes hyperpolarized. It gets further away from your threshold. So guess what happens here? Periodic paralysis once again due to a defect in sodium or your calcium channel. So all of these must play a role with channelopathies and whenever you deal with contraction, obviously dealing with potassium, sodium, and calcium. You have to. That's how the body functions.