A 53-year-old man is being treated for hypertension and diabetes. His medications include insulin and propranolol. He presents at his physician's office complaining of muscle weakness. Blood tests reveal hyperkalemia (elevated serum potassium) as well as elevated BUN (blood urea nitrogen). Propranolol is gradually eliminated and his insulin dosage is adjusted. His serum potassium normalizes and his muscle weakness is alleviated. What probably caused his muscle weakness?

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Quizplus · Accepted Answer

The muscle weakness was likely due to skeletal muscle depolarization caused by hyperkalemia, which leads to inactivation of sodium channels and impaired muscle contraction. Elevated serum potassium levels cause membrane depolarization with a resulting Nachannel inactivation. Fibers are thus less able to fire action potentials, leading to impaired excitation contraction coupling, with muscle weakness. Hyperkalemia in this patient is probably due to multiple factors. Since insulin promotes potassium uptake into cells, too low an insulin dosage in the diabetic can lead to hyperkalemia. In addition, propranolol can cause a shift of potassium from cell to blood. Finally, the elevated BUN indicates some renal failure, and failing kidneys cannot efficiently excrete potassium into the urine.

A 53-Year-Old Man Is Being Treated for Hypertension and Diabetes