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Excerpt
We thank Dr. Kaiser et al. for their attention to our study examining the relationship between paresthesia and nerve stimulator for axillary brachial plexus block. We would like to respond to several points raised by Dr. Kaiser.
First, the end point of paresthesia in our study is the same as that used in clinical practice. Patients were instructed to report as soon as they experienced sensation that was “like an electric shock,” “pins and needles,” and “hitting the funny bone.” The needle-seeking paresthesia was advanced slowly in a controlled manner. We were fully aware that quick needle advance might lead to intraneural impingement and uncontrolled withdrawal reflex of the arm. This did not happen in our study.
Second, like Dr. Kaiser, we were surprised by the finding that 23% of the patients who experienced paresthesia required a stimulating current of greater than 0.5 mA. We believe that arm movement away from the needle is not a likely answer because if this were the case, it would be impossible to get nerve stimulation as low as 0.03 mA. A more plausible explanation is the topography of nerves in the axilla. Motor and sensory nerve fibers are known to occupy variable locations (periphery v core) within the peripheral nerves. Thus, stimulation of the sensory nerve fibers (paresthesia) does not necessarily mean the same degree of stimulation to the motor nerve fibers (muscle twitch). This may account for the poor correlation of the sensory and motor responses in some of the cases we observed.
Third, withdrawal reflex to avoid injury happens with any type of peripheral nerve block. When the block needle becomes truly “noxious,” limb withdrawal will happen and is just as likely with sciatic nerve block as with axillary brachial plexus. If unwanted needle movement is more likely to happen with axillary block due to lack of “secure muscle tissue fixation,” would we not expect a lower rate of block success with the paresthesia technique at the axilla? This is clearly not the case in our clinical experience. We therefore believe that the axillary block is as good a model to test paresthesia as any other.
