June 1997

Antagonism of suxamethonium-induced jaw muscle contraction in rats.

Shi Y, Storella RJ, Keykhah MM, Rosenberg H;

British Journal of Anaesthesia 1997; 78: 332-333

[ see abstract below ]

Shi et al, in an animal model have demonstrated that a low dose of vecuronium or dantrolene will decrease the masseter muscle rigidity induced by succinylcholine. They performed the experiment with an elevated jaw muscle temperature to exaggerate the normal response to succinylcholine. They speculate that pretreatment with small doses of vecuronium might decrease succinylcholine induced masseter muscle rigidity in the clinical situation as well.

This is an interesting paper and one that gets back to the very practical issue of using succinylcholine and being selective about that use. Succinylcholine should only be used to treat laryngospasm, for emergency securing of the airway as in a rapid sequence induction, and intramuscularly "when a convenient vein is not available". The latter was added to protect the infant who needs to have their airway secured even for an elective operation but has difficult IV access.

In that situation securing the airway takes precedence over the minor possibility of acute rhabdomyolysis. One should never delay administration of succinylcholine. It should be recalled that the vast majority of acute rhabdomyolysis occurs in males, therefore administration to females is very safe.

Malignant hyperthermia is a different issue from acute rhabdomyolysis and obviously both genders are susceptible. One must initiate treatment for hyperkalemia if cardiac arrest soon follows succinylcholine and not confuse this with malignant hyperthermia. It is certainly reasonable to avoid succinylcholine if other relaxants with a better safety profile are available.

This paper brings up other concerns: How do we translate 0.03 mg/kg in the rat to the human? Also at what age (and weight) do we use this dose? Since fasciculations are virtually absent in infants do we give vecuronium in low dose to infants? In an emergency do we really want to wait for a defasciculating dose or does it matter if we don't wait? Finally and most practically there is a major difference between the small increase in masseter muscle tone observed following succinylcholine and true masseter muscle spasm (which I would call masseter muscle tetany).

I have had the good fortune to observe this two times in my career. On both occasions the patient was actively biting down so that I could not get the mouth open despite no peripheral twitches (and in one case a second dose of succinylcholine before I realized what was happening--it was this child's 8th anesthetic--a former tracheoesophageal fistula patient with a piece of hot dog stuck in the esophagus).

This patient had a CPK of 26,000 the next day the other patient had a CPK of just 750. The former we considered a potential candidate for malignant hyperthermia and suggested that that child not receive succinylcholine again. The latter patient was more difficult because it had the "Whistling Face Syndrome" which has since been associated with malignant hyperthermia but this child did not have any evidence of even minor rhabdomyolysis. The bottom line is be very selective about the use of succinylcholine and we don't know if giving low dose vecuronium is any different than the older defasciculating dose of curare that I grew up with.

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ABSTRACT