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Department of Anesthesia and Perioperative Care; University of California, San Francisco; San Francisco General Hospital; San Francisco, CA; graya{at}anesthesia.ucsf.edu
In Response:
We thank Dr. Chan for his interest in our report and his insightful commentary. Although intraneural injection has been the focus of recent investigations (1), sonographic evidence has been limited. One case of ultrasound imaging obtained in follow-up after presumed intraneural injection has been published (2). Since acceptance of our manuscript a case series of ultrasound guided injections into neuromas has been described (3). In contrast, our letter reports injection into the normal fasciculated tissue of the femoral nerve (4).
We emphasize that most of the 35-mL injection is extraneural, with only approximately 0.1 mL being considered intraneural (assuming limited longitudinal distribution with equal spread in all directions within the nerve). We agree with Dr. Chan and other authors on the basic principle that intraneural injection acutely expands nerve structure (5–8). Our transverse sonograms show fasciculated femoral nerve architecture (9) on both sides of the block needle. Even without a surgical pathology specimen, our ultrasound scans and clinical course make a highly compelling case for intraneural injection. In clinical practice we have appreciated anatomic divisions of the femoral nerve only rarely during these procedures (approximately 2 in 73 cases of femoral nerve blocks with ultrasound guidance) and do not include our case as one such example.
Dr. Chan presents new experimental data demonstrating high-volume (10 mL) injections into porcine brachial plexus nerves. However, one potentially important issue is that the consequences of intraneural injection in a dividing nerve may be quite different than injection in a discrete nerve without local branching. These new data raise many exciting questions regarding characteristic internal signs of nerve injection injury that can be answered with real time high-resolution ultrasound imaging. More importantly, a major safety goal with imaging is to develop techniques that improve needle (10) and nerve visibility to reduce block-related complications.
References
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