Intraneural Injections

Safety issues for ultrasound in regional anesthesia practice have been raised by several authors.^1,2 Tremendous controversy and debate surround this field, particularly in regard to the subject of intraneural injection.

A large number of seemingly innocuous intraneural injections imaged with ultrasound have been reported.^1,3,4 In these cases, although nerve expansion from injection was reported, no loss of nerve border integrity was detected. All these intraneural injections were believed to be low pressure. Most of these harmless intraneural injections have been detected by retrospective review of ultrasound recordings of blocks and were not recognized online in real time. All these intraneural injections were in nerves with high collagen content and polyfascicular architecture. There have been no reported cases of intraneural injections in monofascicular nerves, which would more likely result in functional damage.

Peripheral nerves normally have substantial regenerative capacity. However, in a number of clinical conditions, this capacity is limited (e.g., neuropathy from diabetes, alcoholism).

We have yet to establish the sonographic signs of intraneural injections that produce functional damage. To date, there has been only one published report of sonographic nerve imaging following nerve injury resulting from needle injection.⁵ Sonography cannot distinguish between subepineurial and subperineurial needle tip placement. Subperineurial injection is thought to be more harmful.⁶

Intraneural injections have been dichotomized into high-pressure and low-pressure types.^7,8 The low-pressure type has not been associated with injury and may be common in clinical practice. Collateral damage may be the mechanism of adjacent fiber injury after high-pressure injection (nerve blast injury from explosion with surrounding nerve fascicles damaged).

Ultrasound may be able to identify some factors that influence the chance of nerve injury during regional anesthetics. Nerve echotexture may play an important role in perioperative nerve injuries. Polyfascicular nerves contain more protective connective tissue than monofascicular nerves. The outcomes after monofascicular nerve injuries are probably worse than after polyfascicular injuries.⁹

Nerves are usually mobile and distensible structures. However, if nerve fascicles become trapped on both sides of the advancing needle, paresthesia often occurs as the needle crosses into or even through the nerve. The complex architecture of peripheral nerves in regard to somatotopic orientation of motor and sensory fibers may account for the variability in mechanically and electrically evoked responses. Ultrasound also can identify proximity to underlying structures that are an unyielding surface. Nerve blocks performed near bone have been associated with a high incidence of nerve dysfunction.¹⁰

Nerve shape also may play a role in nerve injury. Round nerves are less likely to be impaled because the needle moves to the side during needle advancement. Injection within a nerve can be similar to blowing up a deflated balloon; the nerve becomes more round with injection. Any change in nerve shape or increase in cross-sectional area during injection should be considered serious. The injection should be halted because there is no benefit from further injection inside the nerve, and there is potential harm. Nerve fascicles themselves are always round, and therefore monofascicular nerves are normally round.

By identifying ultrasound characteristics of needle-induced nerve injury, we can potentially reduce the incidence of neurologic complications after regional block. New strategies for ultrasound block techniques can be developed that minimize future risks.

FIGURE 64-1 Short-axis view of an intraneural injection during musculocutaneous nerve block in the axilla. Before injection (A) and after injection (B). This event was recognized afterward by retrospective review of the block recording.

FIGURE 64-2 Simulated intraneural injection in an excised ex vivo specimen before (A) and after (B) injection. Long-axis view of a peripheral nerve demonstrating fusiform enlargement after intraneural injection with in-plane approach of the needle. Change in the nerve caliber occurs in the region of the injection.