The Intensity Of The Current At Which Sciatic Nerve Stimulation Is Achieved Is More Important Factor In Determining The Quality Of Nerve Block Than The Type Of Motor Response Obtained

In a recent report in Anesthesiology, Benzon et al. attempted to determine whether there is a correlation between the type of observed motor response and the ability to block all divisions of the sciatic nerve.[1] The authors defined a successful sciatic nerve localization as a motor response to nerve stimulation using a Braun Stimuplex DIG peripheral nerve stimulator (B. Braun Medical, Bethlehem, PA) when the stimulating current was

What was the exact current at which the response was obtained for every one of the four elicited responses? The Braun Stimuplex DIG peripheral nerve stimulator is a constant current generator with a built-in LCD display allowing current adjustment in 0.01 mA increments for precise current delivery. The authors should report the exact current at which every one of the 4 different responses was obtained. In the absence of this information, the differences in the number of sciatic nerve branches that were blocked could simply be a function of different needle to nerve distances at which the local anesthetics were injected.

Confirmation of the placement of the needle in the vicinity of the nerve by observing the disappearance of the motor response to nerve stimulation as used by Benzon et al. is unreliable. [2,3] Since an immediate cessation of this response can not be the result of LA uptake by the nerve and the resultant instantaneous Na channel block, this phenomenon is most likely the result of a physical displacement of the tissues, along with the nerve, away from the needle. As such, this phenomenon is observed at any distance from the nerve.[3]

The authors began nerve localization using the current intensity of 2 to 3 mA and decreased the current to less than 1.0 mA before injecting the local anesthetic. Although the manufacturer of the Stimuplex DIG suggests that the initial current should be “approx. 1.0 mA and reduced until visible muscle contractions occur at lower current levels (approx. 0.2 mA)”, Benzon et al considered nerve localization successful when a response was obtained using the current intensity that was almost five times greater than that recommended by the manufacturer.

While Benzon’s method of nerve stimulation was based on the report by Singelyn et al. [4], the nerve stimulator used by Singelyn et al. featured a pulse width of 40 microseconds, while the Stimuplex DIG used by Benzon et al. has a pulse width of 100 microseconds. This presents a subs-tantial difference, since the unit of electrical stimulation (Coulomb) can be defined as a function of current and time (electric charge = current (mA) x time (sec)).[3] Since the 1.0 mA stimulus intensity used by Benzon et al. can be estimated as being 2.5 times stronger than the 1.0 mA stimulus in the report by Singelyn, this could not indicate adequate nerve localization. Additionally, the use of a stimulating current of an intensity >1.0 mA is frequently associated with a burning sensation during needle advancement due to the high current density at the tip of the insulated needle.[3]

In our practice we use a nerve stimulator identical to the one used in the study by Benzon et al. However, we inject a local anesthetic only after a response is obtained using a stimulating current of 0.4 mA or less in healthy patients. Using this endpoint, we have a nearly 100% success rate in achieving surgical anesthesia after a sciatic nerve block, regardless of the type of motor response obtained.[5,6,A] For instance, in our prospective study comparing two different approaches to sciatic nerve block at the popliteal fossa [7], the response to nerve stimulation significantly varied between the two techniques, with stimulation of the common peroneal nerve (dorsiflexion) being the most common first response (72%) in the lateral approach group, and the tibial nerve response (plantar flexion) being the most common response (76%) in the posterior approach group. Regardless of the type of motor response, using a stimulating current of 0.4 mA or less, we obtained blocks of all branches of the sciatic nerve in 49 (98%) of 50 patients undergoing ankle and foot surgery. Importantly, in addition to the sensory and motor evaluations, the quality of the blocks in our study were also confirmed by the ultimate test – lower extremity surgery with bone incision. The differences in the success rates between the Benzon study and our reports [5,A] is likely due to the use of lower intensity stimulating current and the consequent closer needle-nerve distance at the time of local anesthetic injection in our series. In return, this could result in injection of local anesthetic within the common epineural sheath [8] and resultant block in the distribution of both divisions of the sciatic nerve, regardless of the level of sciatic nerve division. [9]

In conclusion, we suggest that the results of Benzon et al. should be interpreted within the limits of the methods that they used. For those who use a low intensity stimulating current (more than half of US anesthesiologists who utilize nerve blocks in their practice use current intensity

A) Hadzic A, Vloka JD. A Comparison of the Posterior versus Lateral Approaches to the Block of the Sciatic Nerve in the Popliteal Fossa. Anesthesiology; 1988:88 (6):1480-1486.

B) Vloka JD, Hadzic A. Lower extremity nerve blocks. Video material featured at the Scientific Exhibitions at the 1996 and 1997 ASA Annual meeting in New Orleans and San Diego.

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