To the Editor:
We found the article by Theroux et al.  interesting but are confused by an inconsistency in their proposed mechanism. They first invoke the hypothesis of up-regulation of acetylcholine receptors (AChRs), but, in their discussion, they equate spasticity with exercise. In fact, exercise might lead to down-regulation of AChRs , with sensitivity to nondepolarizing muscle relaxants (NDMRs) and theoretic (but unproven) resistance to succinylcholine (SCh).
We found sensitivity (prolonged duration) to SCh in patients receiving anticonvulsants , presumably due to modest up-regulation of AChRs. These patients have NDMR resistance , as do cerebral palsy patients . The literature suggests that up-regulation of AChRs produces agonist sensitivity/antagonist resistance, while down-regulation produces agonist resistance/antagonist sensitivity . We hypothesize different degrees of up-regulation . In burn patients, there could be marked spread of AChRs over the muscle, causing hyperkalemia after SCh; NDMR resistance occurs because of the increased AChR number at the neuromuscular junction. In mild up-regulation (anticonvulsant therapy, cerebral palsy), AChRs might be increased only perijunctionally, so that NDMR resistance occurs, but not SCh-induced hyperkalemia, since the overall AChR number is only mildly increased. Confirmation will require determination of AChR numbers and location. Until then, we rely upon studies like that of Theroux et al. .
Joseph F. Antognini, MD
Gerald A. Gronert, MD
Department of Anesthesiology, School of Medicine, University of California, Davis, Davis, CA 95616
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