Differences in quantitative architecture of sciatic nerve may explain differences in potential vulnerability to nerve injury, onset time, and minimum effective anesthetic volume.

BACKGROUND: In sciatic nerve (SN) blocks, differences are seen in risk of nerve damage, minimum effective anesthetic volume, and onset time. This might be related to differences in the ratio neural:nonneural tissue within the nerve. For the brachial plexus, a higher proximal ratio may explain the higher risk for neural injury in proximal nerve blocks. A similar trend in risk is reported for SN; however, equivalent quantitative data are lacking. The authors aimed to determine the ratio neural:nonneural tissue within SN in situ in the upper leg.
METHODS: From five consecutive cadavers, the region between the sacrum and distal femur condyle was harvested and frozen. Using a cryomicrotome, consecutive transversal sections (interval, 78 mum) were obtained and photographed. Reconstructions of SN were made strictly perpendicular to its long axis in the midgluteal, subgluteal, midfemoral, and popliteal regions. The epineurial area and all neural fascicles were delineated and measured. The nonneural tissue compartment inside and outside SN was also delineated and measured.
RESULTS: The amount of neural tissue inside the epineurium decreased significantly toward distal (midfemoral/popliteal region) (P < 0.001). The relative percentage of neural tissue decreased from midgluteal (67 +/- 7%), to subgluteal (57 +/- 9%), to midfemoral (46 +/- 10%), to popliteal (46 +/- 11%). Outside the SN, the adipose compartment increased significantly toward distal (P < 0.007).
CONCLUSION: In SN, the ratio neural:nonneural tissue changes significantly from 2:1 (midgluteal and subgluteal) to 1:1 (midfemoral and popliteal). This suggests a higher vulnerability for neurologic sequelae in proximal SN, and may explain differences observed in minimum effective anesthetic volume and onset time between proximal and distal SN blocks.