Other Metabolic Muscle Diseases

00:00 What are some other metabolic muscle diseases that are important to know about? Well, electrolytes are key. They help to manage that muscle membrane and disruption in certain electrolytes can contribute to myopathic presentations. Magnesium is important. Increased levels of magnesium disrupt calcium influx in the presynaptic terminus and can cause problems with muscle function. This raises the threshold for release of acetylcholine and the muscles become weak. Potassium. Really high potassium and really low potassium can cause a flaccid paralysis. And this is actually on the spectrum of hyperkalemic periodic paralysis, a disease; and hypokalemic periodic paralysis which is also a disease that we’ll touch base with in a few slides. The last is calcium. Calcium is critical for muscle function. It’s critical for that communication between the nerve and the muscle. We can see problems with both hypocalcemia and hypercalcemia, but what I’d like you to remember is hypocalcemia. In this condition we see tetany, continuous activation of the muscles. And this causes spontaneous nerve impulses activating muscles and leading to a tetany. We mentioned periodic paralysis, too rare but really neat conditions that are neat to know about. There’s hypokalemic and hyperkalemic periodic paralysis, and we’re going to walk through each one. So what’s hypokalemic periodic paralysis? These are inherited conditions. They are congenital causes of muscle disorders. The patient doesn’t develop weakness progressively, but intermittently at times when the potassium is either too high or too low. Hypokalemic periodic paralysis is a disorder of the calcium channel. There’s a calcium channel defect. Weakness in these patients is typically present in the morning which is interesting and not seen with other conditions, after awakening and after a large carbohydrate meal. We see in hypokalemic periodic paralysis that the potassium is low. But it’s only low in the acute attacks and if it’s measured before or after the attack you may miss it. You got to look for hyperthyroidism. And hyperthyroidism can precipitate a hypokalemic attack. The management is with dietary potassium, increasing potassium and occasionally we’ll add Diamox and a potassium-sparing diuretic spironolactone for selected patients. What about hyperkalemic periodic paralysis? This is a sodium channel defect. It’s a channelopathy, but here it’s the sodium as opposed to calcium channel. This is inherited in an autosomal dominant fashion with 100% penetrance. It results in episodes of myotonia and occasionally milder attacks of weakness. Again, this is a potassium problem and so we manage it with electrolyte adjustment trying to reduce the amount of potassium that’s in the system, calcium gluconate, insulin glucagon can do that acutely, and long-term treatment is typically with some type of diuretic; Diamox or the thiazide diuretics.