We thank Drs. Yanovski et al.  for their interest in our paper.1They raise the point that there are alternative explanations for our observations of traces of ink within the sciatic nerves of hyperglycemic dogs other than our conclusion that the needle tip was positioned there during the ink injection. They argue that the accumulation of ink within the nerve is less than what was found outside the nerve, and ink penetration through the intact epineurium is a more likely explanation.

Publication necessarily degrades the images and limits their number, so not all of the relevant sections can be shown. The original high-resolution images (see figure 1, Supplemental Digital Content 1, http://links.lww.com/A1219; figure 2, Supplemental Digital Content 2, http://links.lww.com/A1220; figure 3, Supplemental Digital Content 3, http://links.lww.com/A1212) show clear dissection of ink among the fascicles of the nerves and travel of this ink as rivulets within the nerve to areas distant from the external accumulation in patterns not expected for direct diffusion. In other images, there is substantial destruction of the normal nerve anatomy at the injection site.

Finding the majority of the ink outside the nerve is compatible with our interpretation that injection was originally into the nerve, since the nerve is not capacious and the short path for retrograde flow along the outside of the needle shaft is not likely to be occluded by adjacent tissue pressure. Passage of the ink through membranous barriers is unlikely. The tissue was harvested immediately and frozen within 10 min, so limited time was available for such a process. Also, particulate ink such as was used for this study does not transit through membranes or vascular walls, and for this reason is routinely employed for vascular labeling.2 

Bleeding seen within the nerves is clearly visible in the original photographs only in specimens showing intraneural ink (please see the supplemental digital content), which independent of ink distribution patterns conclusively indicates an intraneural needle placement. Overall, we believe the most likely explanation for these various observations is that needle insertion guided by electrical stimulation resulted in intraneural placement in the hyperglycemic dogs.

*Medical College of Wisconsin, Milwaukee, Wisconsin. qhogan@mcw.edu

Rigaud M, Filip P, Lirk P, Fuchs A, Gemes G, Hogan Q: Guidance of block needle insertion by electrical nerve stimulation: A pilot study of the resulting distribution of injected solution in dogs. Anesthesiology 2008; 109:473–8
Kachlik D, Baca V, Stingl J, Sosna B, Lametschwandtner A, Minnich B, Setina M: Architectonic arrangement of the vasa vasorum of the human great saphenous vein. J Vasc Res 2007; 44:157–66