Faruk Dilberović

Combination of intraneural injection and high injection pressure leads to fascicular injury and neurologic deficits in dogs.

Background Unintentional intraneural injection of local anesthetics may cause mechanical injury and pressure ischemia of the nerve fascicles. One study in small animals showed that intraneural injection may be associated with higher injection pressures. However, the pressure heralding an intraneural injection and the clinical consequences of such injections remain controversial. Our hypothesis is that an intraneural injection is associated with higher pressures and an increase in the risk of neurologic injury as compared with perineural injection. Methods Seven dogs of mixed breed (15-18 kg) were studied. After general endotracheal anesthesia, the sciatic nerves were exposed bilaterally. Under direct microscopic guidance, a 25-gauge needle was placed either perineurally (into the epineurium) or intraneurally (within the perineurium), and 4 mL of lidocaine 2% (1:250,000 epinephrine) was injected by using an automated infusion pump (4 mL/min). Injection pressure data were acquired by using an in-line manometer coupled to a computer via an analog digital conversion board. After injection, the animals were awakened and subjected to serial neurologic examinations. On the 7th day, the dogs were killed, the sciatic nerves were excised, and histologic examination was performed by pathologists blinded to the purpose of the study. Results Whereas all perineural injections resulted in pressures ≤4 psi, the majority of intraneural injections were associated with high pressures (25-45 psi) at the beginning of the injection. Normal motor function returned 3 hours after all injections associated with low injection pressures (≤11 psi), whereas persistent motor deficits were observed in all 4 animals having high injection pressures (≥25 psi). Histologic examination showed destruction of neural architecture and degeneration of axons in all 4 sciatic nerves receiving high-pressure injections. Conclusions High injection pressures at the onset of injection may indicate an intraneural needle placement and lead to severe fascicular injury and persistent neurologic deficits. If these results are applicable to clinical practice, avoiding excessive injection pressure during nerve block administration may help to reduce the risk of neurologic injury.

Intensity of the Stimulating Current May Not Be a Reliable Indicator of Intraneural Needle Placement

Background and Objectives: The current intensity at which a motor response is elicited with an intraneural needle placement has been inadequately studied. We hypothesized that electrical current delivered through an intraneurally placed needle does not always result in an evoked motor response. Our secondary objective was to determine the relationship between electrical current intensity and needle‐to‐nerve distance. Methods: Twenty pigs were given general anesthesia and the sciatic nerves (SN) were exposed bilaterally. Electrical nerve stimulation was applied 2 cm, 1 cm, 0.5 cm, 0.2 cm, and 0.1 cm away from the SN, transepineurally, and intraneurally (in the subepineurium). Stimulation was started at 2.0 mA and decreased to the minimal current at which visible motor response was obtained. Two blinded observers agreed on the intensity and type of motor response. Specific response of SN was defined as a distal motor response (hoof twitch); nonspecific response was defined as a local muscle twitch (no hoof response). Results: At a distance of 0.5 cm to 2 cm away from the SN, only nonspecific muscle responses were observed. Specific SN responses were obtained starting at 0.1 cm away from the nerve and transepineurally with currents of 0.92 ± 0.33 mA (median 1.00 mA; range 0.24‐1.48 mA) and 0.39 ± 0.33 mA (median 0.3 mA; range 0.15‐1.4 mA), respectively. With the needle tip positioned intraneurally, specific motor response could be obtained at 0.56 ± 0.54 mA (median 0.3 mA; range 0.08‐1.80 mA). Five (12.5%) intraneurally positioned needles only elicited a specific motor response at 0.8‐1.8 mA. Conclusions: Specific response to nerve stimulation with currents <0.2 mA occurred only when the needle tip was positioned intraneurally. However, motor response could be absent with intraneural needle placement at a current intensity of up to 1.7 mA.