Inadvertent Femoral Nerve Impalement and Intraneural Injection Visualized by Ultrasound

To the Editor: We present a case of femoral nerve impalement and subtotal intra-neural injection during femoral nerve block (Fig.1) that was recognized in retrospect after review of the recorded ultrasound imaging (see videos available at www.anesthesia-analgesia.org). A total of 35 mL mixture of 0.5% mepivacaine and 0.25% levobupivacaine was administered via a 22-gauge blunt Quincke tip needle. On follow-up 24 h later, quadriceps function was intact, but sensory block remained, resolving the following day.Figure 1.: Short Axis (transverse cross-sectional) ultrasound scan of the inguinal region (left) and the corresponding labeled image (right). (A) The femoral nerve (Femoral Nerve) is shown impaled by a 22-gauge blunt Quincke tip needle (solid line, Needle Path) with the needle tip (Tip) appearing at the underside of the nerve. Local anesthetic spread around the nerve is shown between the large arrowheads. (B) Intraneural injection is shown between small arrowheads. The femoral nerve lies on top of the psoas muscle (Psoas Muscle). Tick marks are spaced 10 mm. Supplemental material available at www.anesthesia-analgesia.org.With nerve impalement, crucial barriers to local anesthetic diffusion are disrupted, which probably renders the impaled nerve more susceptible to conduction blockade, independently of intraneural injection (1). Nerve injuries following intentional impalement with microelectrodes (5–300 μm diameter) generally repair without sequelae (2–4). Similarly, no substantial injury resulted from femoral nerve impalement in the present case with a 22-gauge needle (700 μm diameter) (5). The sonographically measured cross-sectional area of the intraneural injection was 7.3 mm2 (only 22% of the femoral nerve cross-sectional area). Therefore, the increase in intraneural pressure due to injection likely was within the tolerable pressure range (6,7). Moreover, the femoral nerve increases in caliber along its course in the inguinal region (8), indicating dispersion of nerve fibers within the femoral nerve before separating into its cutaneous and muscular branches. This polyfascicular architecture may have prevented a significant increase in intraneural pressure in our patient. The present case demonstrates that femoral nerve impalement and intraneural injection of local anesthetic may occur without major adverse sequelae. We hope this report will serve to improve the understanding of these potentially harmful events. Ingeborg Schafhalter-Zoppoth, MD Ivan D. Zeitz, MD Andrew T. Gray, MD, PhD Department of Anesthesia and Perioperative Care, University of California, San Francisco, San Francisco General Hospital, San Francisco, CA