A 17-year-old patient with third-degree burns over 30% of body area is 12 days post-injury for débridement and grafting. Which statement about muscle relaxants is true?

A 17-year-old patient with third-degree burns over 30% of body area is 12 days post-injury for débridement and grafting. Which statement about muscle relaxants is true?

• A. Sensitivity to both depolarizing and nondepolarizing relaxants is increased
• B. Sensitivity to both is decreased
• C. Sensitivity to depolarizing relaxants is increased while nondepolarizing is decreased
• D. Sensitivity to depolarizing is decreased while nondepolarizing is increased

Answer: (C)

Burn injuries trigger upregulation of extrajunctional acetylcholine receptors on skeletal muscle. This means more nicotinic receptors are present not only at the normal motor endplate but also along the muscle membrane. With a depolarizing blocker like succinylcholine, these extra receptors allow widespread and prolonged depolarization, leading to an exaggerated response and, importantly, a dangerous release of potassium from muscle cells (hyperkalemia). That’s why sensitivity to depolarizing agents is increased after burns. Conversely, nondepolarizing (competitive) blockers work by occupying these receptors to prevent ACh from activating them. With more receptors around, it takes a higher dose to achieve the same level of blockade, so sensitivity to nondepolarizing agents is decreased. In a patient 12 days after a large burn, this receptor upregulation is present, so the statement that depolarizing relaxants are more potent and nondepolarizing relaxants are less potent best fits the physiology. This is also why succinylcholine is typically avoided in burn patients due to the risk of life-threatening hyperkalemia.

Burn injuries trigger upregulation of extrajunctional acetylcholine receptors on skeletal muscle. This means more nicotinic receptors are present not only at the normal motor endplate but also along the muscle membrane. With a depolarizing blocker like succinylcholine, these extra receptors allow widespread and prolonged depolarization, leading to an exaggerated response and, importantly, a dangerous release of potassium from muscle cells (hyperkalemia). That’s why sensitivity to depolarizing agents is increased after burns.

Conversely, nondepolarizing (competitive) blockers work by occupying these receptors to prevent ACh from activating them. With more receptors around, it takes a higher dose to achieve the same level of blockade, so sensitivity to nondepolarizing agents is decreased.

In a patient 12 days after a large burn, this receptor upregulation is present, so the statement that depolarizing relaxants are more potent and nondepolarizing relaxants are less potent best fits the physiology. This is also why succinylcholine is typically avoided in burn patients due to the risk of life-threatening hyperkalemia.