To the Editor:-We read with interest the study by Benzon et al., [1]which was published in the September 1997 issue of Anesthesiology. Using the peripheral nerve stimulator technique, 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. Benzon et al. 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 < 1.0 mA. The proximity of the needle to the nerve was confirmed when an injection of 1 or 2 ml of local anesthetic abolished the elicited motor response. The authors concluded that elicitation of foot inversion was associated with the most complete sciatic nerve blockade.

Because of several inherent methodologic flaws in this study, we are compelled to comment on their methods and offer an alternative explanation for the obtained results.

1. 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 four 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.

2. 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] 

Because an immediate cessation of this response cannot be the result of local anesthetic uptake by the nerve and the resultant instantaneous Na channel blockade, 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] 

3. The authors began nerve localization using the current intensity of 2–3 mA and decreased the current to < 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. Although 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 micro s, whereas the Stimuplex DIG used by Benzon et al. has a pulse width of 100 micro s. This presents a substantial difference because the unit of electrical stimulation (Coulomb) can be defined as a function of current and time (electric charge = current [mA] x time [s]). [3]Because 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 as a result of 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]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 <or= to 0.4 mA, 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]* is likely a result of 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 blockade 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 use nerve blocks in their practice use current intensity < 0.6 mA), [10]their findings may not have significant practical implications. Based on our clinical experience and experimental data, the intensity of the current at which the nerve stimulation is achieved is the most important factor determining the quality and extent of the block, rather than the type of motor response obtained using higher stimulating currents. In addition to higher success rates, the use of a lower stimulating current is associated with greater patient comfort during the block placement. It is a fact to which we can attest having performed a series of sciatic nerve blocks on each other.**

Jerry D. Vloka, M.D.

Admir Hadzic, M.D.

Assistant Professors of Clinical Anesthesiology; St. Luke's-Roosevelt Hospital Center; College of Physicians and Surgeons; Columbia University; New York, New York

(Accepted for publication January 8, 1998.)

*Hadzic A, Vloka JD: The lateral approach to popliteal nerve block. A comparison with the posterior approach. Manuscript submitted to Anesthesiology, October 3, 1997.

**Vloka JD, Hadzic A: Lower extremity nerve blocks[copyright sign]. 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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