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Anatomy of Nerve Injury

   

Jeff Gadsden, MD. Published by B. Braun Medical Inc.

Knowledge of the functional histology of nerves is essential to understanding the consequences of intraneural injection. Nerves are made up of fascicles supported and enveloped by perineurium and a loose collection of collagen fibers termed the epineurium. The epineurium is easily permeable and carries the nutritive vessels of larger nerves. Each fascicle is made up of bundles of nerve fibers (axons) and their associated Schwann cells held together by a tough squamous epithelial sheath called the perineurium, which acts as a semipermeable barrier to local anesthetics. The nerve fibers are supported within the perineurium by a delicate connective tissue matrix called the endoneurium, which contains capillaries that arise from the larger epineurial vessels.

The etiology of perioperative nerve injury is complex and multifactorial.7 Postulated surgical mechanisms include:

  • Compression due to pneumatic tourniquets or casts, stretch injuries from positioning
  • Surgical injuries


Postulated anesthesia related mechanisms include:

  • Chemical neurotoxicity from either the local anesthetic solution itself or from vasoconstrictive additives such as epinephrine
  • Mechanical injury from needle-nerve contact and/or puncture.8
  • Rupturing of the fascicle as a result of a high injection pressure. 29


Strategies aimed at preventing nerve injury include using the lowest effective concentration of local anesthetic, taking care with limb positioning, and cast application, and the use of meticulous surgical technique. However, since mechanical injury from the needle and the affects of high injection pressure remains the most likely causative mechanism following a peripheral nerve block, much effort has been directed at preventing harmful needle-nerve contact & detecting high pressures through the use of multiple monitors.

Intrafascicular Injection

It is well established that injection of even very small amounts of local anesthetic within the fascicle can lead to widespread axonal degeneration and permanent neural damage in animals, whereas extra-fascicular injection does not disrupt the normal nerve architecture. 9,10

Fortunately, the incidence of intrafasicular injection is rare, probably because the tough perineurium prevents needle advancement.

The risk of an intrafascicular injection differs from site to site in the peripheral nervous system, and it correlates with the cross‐sectional fascicle‐connective tissue ratio. For example, the sciatic nerve at the popliteal fossa contains a relatively high proportion of loose connective tissue compared to fascicles, which corresponds with its low incidence of post-PNB neuropathy.11 By contrast, the brachial plexus at the level of the trunks is nearly all neural tissue, a needle entering the nerve here is more likely to encounter a fascicle on its trajectory that may contribute to the disproportionately higher rate of postoperative neuropathy following PNB with interscalene blocks.12

Intraneural Injection

In 2006, Bigeleisen published a series of axillary brachial plexus blocks performed on 22 patients undergoing thumb surgery.13 Using ultrasound guidance, the authors deliberately placed the needle intraneurally and injected 2 to 3 mL of local anesthetic, which resulted in 72 intraneural injections as evidenced by nerve swelling. Despite the common occurrence of paresthesia or dysesthesia (66 times), none of the patients developed an overt neurologic deficit up to 6 months postoperatively.

Similarly, Robards et al studied 24 patients receiving sciatic nerve blocks in the popliteal fossa using both nerve stimulation and ultrasound guidance.14 The end point for needle advancement was a motor response using a current intensity of 0.2 to 0.5 mA, or an apparent intraneural needle tip location, whichever came first. There was no postoperative neurologic dysfunction.

While interesting, these results are far from reassuring to the provider given the small number of subjects and relatively low incidence of PONS, and these data should not be interpreted as evidence that intraneural injections are safe. Intraneural injection observed with ultrasound has been reported to lead to long-term injury.15 Moreover, there are fine inter-fascicular neural connections and blood vessels within the epineurium that are likely easily damaged with intraneural needling, even if the fascicles themselves are not penetrated.15

Injection Against the Epineurium

Injecting local anesthetic while the needle tip is apposed to the epineurium may present a real danger of intraneural deposition of the injectate, since part or all of the needle opening may be embedded within this layer. Even if injection is not performed, simply contacting the epineurium with the needle and displacing the nerve has been shown to cause injury.17

Perineural Injection

Most regional anesthesiologists agree that injection of local anesthetic into the nerve is a risk factor for injury, and that extra-neural deposition minimizes the potential for an intrafascicular injection.18

 

Intramuscular Injection


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