Limb-Girdle Muscular Dystrophies and Channelopathies

by Carlo Raj, MD

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    00:01 Limb-Girdle Muscular Dystrophies.

    00:04 Shoulder and hip, as exactly the name implies.

    00:08 Shoulder and hip, shoulder and hip girdle weakness.

    00:12 Relative sparing, once again, of the ocular and bulbar.

    00:15 And here, cardiomyopathy is less frequent.

    00:19 So if you get a situation where you’re suspecting muscular dystrophies, and it has nothing to do with dystrophin, and in fact, if you’re looking for dystrophin on your band and you find it, you know how to interpret a band.

    00:36 A band.

    00:37 Literally, you find a band and then you find coloration, and if you find coloration for that particular gene, dystrophin, then it exists.

    00:46 If the band if blank, that means your dystrophin is not there.

    00:50 So, if you find the dystrophin is present and you find issues with the shoulder and the hip and the girdle, then you’re diagnosed with limb-girdle muscular dystrophy.

    01:01 Cardiomyopathy, much less common.

    01:05 Pathophys here.

    01:06 Heterogeneous abnormalities of muscle related proteins.

    01:11 It is going to be hereditary because this is a dystrophy.

    01:15 Labs here, once again, elevated CK, creatinine kinase as you can imagine.

    01:19 May require muscle biopsy, however, for definitive diagnosis because the genetics here with dystrophin is not going to be clear cut.

    01:28 Management here will be supportive.

    01:30 Be familiar with LG muscular dystrophy, please.

    01:36 In this illustration, what is being highlighted for you is the dystrophin.

    01:41 And with the dystrophin, you’ll notice here that this is the anchor for much of your muscle.

    01:46 And if for whatever reason that dystrophin is not present as in Duchenne muscular dystrophy, your skeletal muscle could be affected and your heart muscle could be affected.

    01:55 And please keep in mind that your respiratory system could be at serious risk of arrest.

    02:03 Here, we’ll take a look at myotonic dystrophy.

    02:07 That’s the most common adult onset of muscular dystrophy.

    02:10 So once again, it’s inheritance, right? That’s what dystrophy means by definition.

    02:15 We have type 1 MD, which is muscular dystrophy, and this is called Steinert disease.

    02:22 And if it’s type 2, it’s called proximal myotonic myopathy, P-R-O-M-M.

    02:28 Distal weakness predominates, and that’s important for you to pay attention to, huh? Distal weakness.

    02:34 So we already begin out here.

    02:36 The myotonia, state of increased muscle contraction and impaired relaxation.

    02:41 It’s extremely important for you to understand, myotonic, meaning to say that the muscle is in a sustained state of contraction, impaired relaxation.

    02:52 There is going to be frontal balding.

    02:54 That is very, very important as a clinical feature.

    02:57 Cataracts, cardiac conduction defect, and multisystem disease with myotonic dystrophy.

    03:04 We’ll talk a little bit more with myotonic dystrophy.

    03:08 We call it hatchet facies.

    03:09 At this point, frontal balding is good enough for you to know.

    03:15 Pathophys here, it’s a trinucleotide expansion.

    03:19 So here, the trinucleotide expansion that you hopefully have memorized is CTG myotonic dystrophy.

    03:29 Say it with me, C-T-G, myotonic dystrophy.

    03:34 You’ll never forget it.

    03:36 C-A-G, Huntington, right? And there’s a few more that you’ve learned about in genetics for sure, such as CGG.

    03:47 I don’t want to get ahead of myself.

    03:48 At this point, let’s focus on myotonic dystrophy.

    03:52 What does trinucleotide expansion mean to you? Anticipation.

    03:57 On your labs, normal, or perhaps, mildly elevated CK because you begin distally.

    04:02 DNA testing is available.

    04:04 And on your EMG, myotonic discharges are what you’re looking for.

    04:09 What does that mean to you? Increased muscle contraction with decreased ability to relax.

    04:16 Channelopathies.

    04:18 Think about all the different channels that are responsible for contraction, huh? Sodium, obviously, will play a huge role.

    04:24 It’s episodic weakness due to muscle ion channel defect.

    04:27 Now, there are some famous athletes.

    04:32 In baseball, there was a man from Orlando.

    04:36 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.

    04:47 But eventually, he came to be diagnosed with a channelopathy.

    04:50 So, it does exist, and actually, it’s more common than what we think.

    04:55 All known are autosomal dominant.

    04:57 And apart from sodium, you’ll also want to keep in mind hyperkalemic periodic paralysis due to a defect in the sodium channel.

    05:06 And these two, obviously, will play a role always.

    05:09 Potassium, obviously, is going to play a role in establishing resting membrane potential.

    05:13 And if for whatever reason there’s a potassium channel type of defect, and if there’s hyperkalemia, well, you know that you’re going to raise your resting membrane potentially towards your threshold, which you’re messing up everything in terms of your sodium channel.

    05:26 That has to be, that’s the basic physiology.

    05:29 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.

    05:46 What about hypokalemic? If it’s hypokalemic, can I ask you? What happens to resting membrane potential? Good.

    05:54 It becomes hyperpolarized.

    05:56 It gets farther away from your threshold.

    06:00 So, guess what happens here? Periodic paralysis, once again, due to a defect in sodium or your calcium channel.

    06:07 So, all of this must play a role with channelopathies, and whenever you deal with contraction, obviously, you’re dealing with potassium, sodium, and calcium.

    06:15 You have to.

    06:16 That’s how the body functions.

    About the Lecture

    The lecture Limb-Girdle Muscular Dystrophies and Channelopathies by Carlo Raj, MD is from the course Polyneuropathy. It contains the following chapters:

    • Myotonic Dystrophy
    • Channelopathies

    Included Quiz Questions

    1. Conduction defects in the heart
    2. Hypertrophic cardiomyopathy
    3. Restrictive cardiomyopathy
    4. Myocardial infarctions
    5. Myocarditis
    1. Autosomal dominant
    2. Autosomal recessive
    3. X-linked dominant
    4. X-linked recessive
    5. Y-linked dominant
    1. Pupil involvement
    2. Frontal balding
    3. Cataracts
    4. Arrhythmias
    5. Weakness of muscles
    1. Sodium channel defect
    2. Potassium channel defect
    3. High voltage gated calcium channel defect
    4. Low voltage gated calcium channel defect
    5. Chloride channel defect
    1. Myotonic dystrophy
    2. Limb-girdle muscular dystrophy
    3. Duchenne muscular dystrophy
    4. Becker muscular dystrophy
    5. Channelopathies
    1. CTG
    2. CAG
    3. GAA
    4. CGG
    5. GAG
    1. Chitin
    2. Fukutin
    3. Dystroglycan
    4. Syntrophin
    5. Dystrobrevin

    Author of lecture Limb-Girdle Muscular Dystrophies and Channelopathies

     Carlo Raj, MD

    Carlo Raj, MD

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