To the Editor:—
We read with interest Dr. Franco's recently published article concerning his new approach to sciatic nerve block. 1We believe there are important omissions in the methodology and interpretation of the results in both stages of the study that substantially limit the applicability of the findings.
First Stage, Anatomy Laboratory:
The author stated that his anatomical data resulted from dissection of 24 sciatic nerves. However, this information came from only 12 cadavers. Because there is no indication that individuals lack bilateral symmetry of their sciatic nerves, in reality, only 12 specimens were evaluated. No presentation of the inter- or intraindividual variability of these values was provided. The variation reported in the linear distance from the intergluteal sulcus and its variability could be substantially underestimated by combining both inter- and intraindividual data. This error may in part account for the substantial number of subjects (60%) who required multiple attempts to locate the nerve, as well as the significant block failure rate (10%).
The author stated that “variation in the hip width reflects soft tissue, not bony, differences between the sexes [p 724],” implying an inherent inconsistency of surface landmarks. Further, he stated that gender was not a factor in determining his calculation of the surface anatomical relationship. However, he never indicated how many of his sciatic dissections were from male and female cadavers. The lack of association between gender and surface anatomical characteristics cannot be assumed from the data he presented. Although we concur with his observations with respect to the consistent relationship between the sciatic nerve and bony structures, it is difficult to accept the extrapolation of this spatial information to the use of a surface anatomical approach to the nerve block that relies on a predetermined distance (10.1 ± 0.2 cm), primarily because variations in the contour of the buttocks are likely to alter these surface characteristics.
Second Stage, Clinical Stage (doesn't the author mean Clinical Study?): The clinical study has substantial methodologic limitations. The limited clinical evaluation (20 cases) fell far short of convincing us of the validity of the author's stage 1 hypotheses.
The endpoint stated by the author as appropriate for local anesthetic injection was “sciatic nerve response” at 0.6 mA. To ensure the highest degree of success with peripheral nerve blocks using neurostimulation technology, the endpoint for injection is an appropriate evoked motor response at < 0.5 mA. 2,3In addition, the latency and success of a complete sciatic nerve block depends on the type of motor response obtained via neurostimulation. 3,4The author used the very vague, nonspecific term, “sciatic nerve response.” No attempt was made to categorize whether inversion, plantar flexion, eversion, or dorsiflexion were elicited, nor how the elicited response influenced his latency and success rate.
The success of a peripheral nerve block, in terms of sensory anesthesia, is represented by both analgesia and anesthesia , and with regard to motor block, by paresis and paralysis . The latency of block onset is judged by the time taken from the injection of local anesthetic until these parameters are reached. We do not believe that the “time to incision” (patient's response to a surgeon's knife) is an appropriate measure of block latency and success. Obviously, according to this definition, a surgeon who scrubs and drapes the surgical field quickly will have a different “latency of onset” for a given patient than one who scrubs and drapes more slowly. The use of this parameter to judge the efficacy of a block in response to surgical incision is not only questionable ethically, but it also lacks scientific validity because it offers little indication as to whether both components of the sciatic nerve (tibial and common peroneal) were actually blocked.
The author stated, “Most of the patients were given 1 mg of midazolam plus 50 μg of fentanyl in the OR; some patients received less [p 724].” There is no validity to this vague statement, but it implies that the amount of sedation administered to individual patients was not carefully monitored.
The author states, “… a handheld timer (Casio, Japan) was … left running continuously. The time periods were later calculated by subtracting the previous elapsed time from the actual reading [pp 724-5].” It is difficult to imagine that five parameters (time to skin mark; time to sciatic nerve response; time to injection; time to incision; total accumulated time) could have been accurately calculated from a continuously running timing device with the described precision. It would appear that this approach might have involved a great deal of “estimating.” It is not clear if these values represent the total time of all attempts or only the time of the successful attempt. This is not a minor consideration, because in 60% of the cases more than one attempt was required. In addition, no mention was made of what adjustments were made to locate the nerve when the first attempt was not successful. Also, the actual distance from the intergluteal sulcus to the needle entry point at the successful attempt was not reported.
The author presented durations of anesthesia and analgesia as 2– 4 h after injection in all patients, “but a strong analgesia persisted in some patients up to 24 h later [p 726].” How was this anesthesia and analgesia monitored? How is “strong” analgesia defined? How many patients required rescue analgesics, and in what quantities, during this 24 h? Furthermore, analgesic requirements will vary widely depending on the type of surgery (bunions vs. below-the-knee amputations). Was “strong” analgesia observed in all patients across the board?
The author stated,“… the technique was successfully used in adult patients of different gender, height, body habits [habitus?], and ethnic backgrounds, including a 147-cm tall Hispanic woman and a 196-cm tall Caucasian male …[pp 726].” We are not convinced from the present work (or any other) that ethnicity affects sciatic nerve anatomy, particularly with respect to the significant diversity encompassing the descriptors Hispanic and Caucasian. Furthermore, “We could not reach one Hispanic male patient …” (for follow-up evaluation). We fail to see why the patient's ethnicity needed to be stated with regard to follow-up data collection.
We believe the author's speculation as to the applicability of his work could have easily been resolved by addressing the issues raised above.
