We appreciate the interest of Dr. Blumenthal et al. in our case report on intraneural administration of local anesthetic during an anterior approach of the sciatic nerve.1We agree completely with these authors but would like to clarify a few details.
Our patients had been included in a study intended to evaluate the accessibility of the sciatic nerve using the anterior approach. We aimed at determining the reliability and stability of the catheter for postoperative analgesia in this approach. Despite this explanation of why we conducted these blocks in the computerized tomographic radiologic room, we believe that the near future will see an increasing participation of neuroimaging techniques in helping with anesthetic block procedures.2Moreover, the sciatic nerve is hidden behind the minor trochanter in a nonnegligible proportion of patients (> 10%) when we effect an anterior approach, independently of the rotation of the hip, and the risk of puncture of the femoral vessels exists.3,4The computerized tomography–guided approach should help to avoid risks.
We agree with Blumenthal et al. in that, with the current state of our knowledge, the best praxis would have been to withdraw the catheter. This was what we did in the second patient reported. In the first patient, however, we were not aware of the intraneural puncture and anesthetic administration because the images were analyzed off-line.
The structure of the nerve trunk is complex (fig. 1). Basically, the nerves run along different body compartments surrounded by tissues with their own fascia. These fascias, which are not part of the nerve, are the “recommended points” for injection to avoid nerve damage—or, that is what we thought. Inside the nerve trunk, the axons, enwrapped by the endoneurium, are grouped in fascicles surrounded by the perineurium, a relatively resistant membrane that is difficult to puncture because of its elasticity, mobility, and adaptability to external forces. The fascicles are embedded within the epineurium, which is surrounded by a thicker membrane, the epineurium sheath. The epineurium is a slack collagen and fatty tissue, which contains the vasa and nervi nervorum.5–8Most of the nerve section (between 30–75%) is occupied by nonneural structures.5,9The sciatic nerve produces abundant epineurium, which covers between 72 and 78% of the nervous section.10
Intraepineural injection of anesthetic would not necessarily lead to nerve damage.11On the contrary, intrafascicular injection is more likely to induce nerve lesions.12However, because of their elastic properties, the fascicles probably separate from each other and get out of the way if a needle penetrates the nerve trunk. Although direct neural tissue lesion is unlikely, it should be taken into account that intraepineural injection of a substance can cause an increase in neural pressure and secondary damage because of compression or a vascular lesion.
Clinical experience supports neurostimulation as a safe and effective technique with minimal incidence of nerve lesions. However, a large number of unresolved questions stemming out from our observations remain: How often does intraepineural injection occur in routine practice? Does it occur preferentially in certain nerves? What are the clinical and radiologic signs suggesting intraneural puncture? What is the safe threshold for electrical near-nerve stimulation?
* Hospital Clínic, Barcelona, Spain. firstname.lastname@example.org