Electrical nerve stimulation and subepineurial staining: not only mechanical factors count.

To the Editor:—We read with interest the article published by Rigaud et al. The authors addressed the very important and insufficiently explored relationship between the parameters of electrical nerve stimulation and the precise position of the stimulating needle tip. Unfortunately, little data exist defining this relationship in special clinical situations like neuromuscular and metabolic diseases, and we applaud the authors for their effort. While acknowledging the factual results of this study, we disagree with the authors’ interpretation of the findings regarding hyperglycemic dogs. Specifically, ink streaks under the epineurium were interpreted as needle penetration and intraneural injection. In our opinion, review of the photomicrographs (fig. 3 in the publication) does not necessarily support this conclusion. In these figures the amount of ink lodged under the epineurium appears negligible in comparison with that located outside the nerve, and the internal neural architecture remains intact. It is possible that such marginal staining could have a biochemical rather than a mechanical explanation. In diabetes, an impairment in energy balance and tissue edema could result in a sufficient increase in epineurial permeability to allow some ink already in close contact to the nerve (as in fig. 2 in the publication) to penetrate the epineurium in the absence of any direct trauma. An alternative explanation could be migration of ink via dilated vasa nervorum. Eventually, performing the same experiment with ultrasound-guided injection would be very interesting. In the absence of clinical data suggesting frequent nerve damage from performing electrical guided nerve blocks on diabetic patients, one of two possible conclusions of this study should be considered: penetration of local anesthetic inside the epineurium (with or without needle penetration) does not result in nerve damage, or that the results of this study are pertinent only to this specific experimental condition and do not warrant clinical extrapolation. The answer to this question has particular importance in the context of the ongoing debate about the relative risk of electrical stimulationguided blocks in comparison with ultrasound guidance.