McArdle Disease or Muscle Carnitine? - USMLE Forums
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Old 11-21-2009
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Unhappy McArdle Disease or Muscle Carnitine?

I have been facing some questions that confused me and could not tell the difference between Myophosphorylase deficiency (McArdle) and the other one, Muscle Carnitine deficiency.

Please anybody can simplify the distinction, any quick tip for the exam?

Thanking you,
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Old 11-23-2009
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A good way to differentiate between these two conditions is by remembering that in McArdle you have difficulty in the beginning of the exercise while in myopathic carnitine acyl transferase deficiency you have a problem after a prolonged exercise.

The explanation for this is that glycogen is utilized first by muscles as a source of energy so in McArdle (Muslce glycogen phosphorylase deficiency) you have a problem in the beginning and patients will have muscle cramps after few seconds of starting the exercise. However once the beta oxidation kicks in they will experience the classical "second wind".
While in Muslce carnitine deficiency you have a problem later on when Fatty acids oxidation is needed.

Both conditions may have myoglobinuria

In muscle carnitine you'll see accumulated triglycerides in biopsy while in McArdle (also called Type V glycogen storage disease) you will have accumulated glycogen in muscle biopsy.

Both conditions will not cause hypoglycemia because muscles do not release glucose to the tissues as it lacks the glucose 6 phosphates enzyme.
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Old 04-12-2011
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Quote:
Originally Posted by StepTaker View Post
Both conditions will not cause hypoglycemia because muscles do not release glucose to the tissues as it lacks the glucose 6 phosphates enzyme.
Are you sure about this? First Aid says that Carnitine deficiency causes weakness, hypotonia, and hypoketotic hypoglycemia (page 112). I would think that if you lost the ability for fatty acid oxidation, you would indeed become hypoglycemic when exercising. Would it be possible to clarify (especially regarding the difference, if any, between carnitine deficiency and carnitine acyl transferase deficiency)?

Thanks,
Tig
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Old 04-12-2011
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Carnitine deficiency will cause significant impairment of B-oxidation
why hypoglycemia? --> because B-oxidation provides energy for gluconeogenesis in the liver.
why hypoketosis? --> because the primary source of ketone bodies is acetyl CoA from B-oxidation.

But StepTaker is right, Muscle carnitine deficiency (restricted to muscle) is characterized by depletion of carnitine levels in muscle with normal serum concentrations. Evidence indicates that the causal factor is a defect in the muscle carnitine transporter. in this type blood sugar and Ketone bodies levels will be maintained.

Hope that clarifies it
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Old 04-13-2011
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It does! Thank you kindly.
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