The Isolated Forearm Paradox: Why Never a Response to Command in the Completely Unparalyzed?

Sanders et al.¹  have carefully performed an international study by a distinguished consortium that I am sure was not easy to organize. That said, the fact that apparently suitably anesthetized patients move during isolated forearm test (IFT) after induction and tracheal intubation is well established and unsurprising. Long reported, with systematic review showing 31 previous papers with more than 1,300 patients studied,²  a positive response to IFT is easily reproducible by any anesthesiologist, anywhere, at any time. There is some modest interest in the now reported response rate (~5%)¹  being lower than the aggregate of these previous studies (~40%),²  but it is difficult to see what else is novel about this latest report.

Pryor and Veselis³  offer important advice for the direction of future research and I would like to add two suggestions based on paradoxes in the observations. By paradox I mean responses that appear difficult to reconcile, given the stimulus. During the IFT, when we observe the patient moving only to verbal command but not to the obvious, ongoing, and greater stimulus of surgery, we properly regard that as surprising enough to develop sophisticated theories of dysanesthesia,^4,5  connected consciousness,¹  or cognitive unbinding.⁶  Yet, when a patient during IFT fails to move to verbal command, but makes other spontaneous movements that appear purposeless, we dismiss these movements as reflex or light anesthesia. We do not seem equally surprised that a patient light enough to move will not also respond to command. Perhaps it is time to study also this second apparent paradox in more detail, especially if brain imaging coupled with IFT is a way forward, as Pryor and Veselis suggest.³ 

A much more important paradox is why the finding cannot be reproduced in the nonparalyzed (i.e., patients who have received no neuromuscular blockade). I have already reported on the impossibility of eliciting a positive IFT response to verbal command during surgery in these circumstances.⁷  Even when patients retain the ability to move spontaneously to stimuli, they fail to respond to verbal command if unparalyzed, even when they have received the same anesthetic doses and are at similar bispectral index levels as those reported in previous studies. Why this paradox? Why do things change when they are (save the isolated forearm) paralyzed? This distinguished and experienced team has the infrastructure now to explore this paradox more robustly than I previously reported. So, in good spirit I challenge Sanders et al.¹ to harness their international collaboration and report a single case of positive IFT in an apparently suitably anesthetized but unparalyzed patient, anywhere in the world. Or, if they are unable to do so, to explain why this is impossible and how this paradox fits into existing theories of a positive IFT response.

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The author declares no competing interests.