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Q: Doc, doc, got a 35 year old chap in for terrible muscle aches after running a race. He says his urine is black. What could it be?
A: Black urine suggests myoglobin. What’s his CK? Is it elevated together with other muscle enzymes (AST, ALT, Aldolase)? If so, think rhabdo. Now the question is whether he’s got a provoked attack or has underlying genetic susceptibility.
Q: He’s got these episodes 4 times already.
A: Alrighty, let’s first rule out acquired causes. Alcohol, illicit drugs, myotoxic drugs like statins and colchicine, electrolyte disturbances involving sodium, potassium, phosphate, calcium, endocrine, use of diuretics, disturbances of the thyroid, parathyroid, adrenals.
Q: None of these doc. His CK is 100,000, that’s 1 day after the race.
A: Expect CK to peak in 3-5 days. It’s lucky you got him within a day cuz the myoglobinuria usually resolves after a day. Alrighty, it doesn’t sound like inflammatory muscle diseases and he’s got no skin changes to suggest dermatomyositis.
A: Let’s go to genetic causes. Think Mcardles, CPT2, mitochondrial, ryanodine receptor mutations. Mcardles and ryanodine receptor mutations usually have slightly elevated CKs in between episodes.
Q: He says the pain and weakness is worse in his thighs and calves and triggered when he races. If he rests a bit, the symptoms abate and he can race again.
A: Aha! The famous second wind phenomenon. Send for genetic testing for Mcardles - myophosphorylase deficiency.
Q: Got another guy in his 20s who says the pain and weakness is in every muscle when he’s ill or fasting or cold.
A: Think CPT2 deficiency or mitochondrial disorders. Does he have multi-systematic involvement like seizures or heart issues or vision issues?
Q: Just the muscles. And he’s normal in between episodes.
A: More likely to be CPT2 deficiency then. Mitochondrial disorders tend to have weakness inbetween episodes and have systemic phenotypes. Again, genetic testing for CPT2 and muscle biopsy if unrevealing.
A: Now if you thought he had a mitochondrial issue, make sure you do genetic testing on muscle (not blood as usual) cuz of heteroplasmy.
Q: Another guy with the same symptoms got a family history, only affecting the muscles. His dad, granddad all had the same issues.
A: Think an autosomal dominant myopathy like ryanodine receptor mutations causing central core myopathy. This ones important cuz it might cause malignant hyperthermia in a third of cases.
A: Anesthesiologists will thank you if you remind them not to give gas and depolarizing muscle relaxants.
Q: Got a list of blood tests to do then before I send him over to you then?
A: Check CK, aldolase, lactate, pyruvate, acylcarnitine profile, plasma amino acids, electrolytes, thyroid function, urine myoglobin, electrolytes and tox screen. But these may be normal in between episodes.
A: Remember to give him a script to have these immediately repeated at the nearest facility when he gets the next episode. That’s all folks, neuromuscular out. Appreciate thoughts and insights from all. #neurotwitter @AaronLBerkowitz @DxRxEdu more pearls on muscle enzymes?
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