Ultrasound-guided Supraclavicular Block: What Is Intraneural?

We read with interest the article by Bigeleisen et al. 1in the June issue of the journal. This study brings two important questions to the fore: first, can a minimum stimulating current detect intraneural needle placement; and second, can this minimum current predict whether needle placement and local anesthetic injection will cause neurologic injury? The research of Bigeleisen et al. is designed to deal with the former question, but in a wider context, it is concerned with the fundamental issue of avoiding nerve injury.Intraneural needle placement does not inevitably lead to nerve injury,2,3in the etiology of which the perineurium may be a more critical barrier than the epineurium.4,5Nerve fascicles may escape direct injury from subepineural needles because of the tough perineurium that surrounds them and the considerable amount of compliant connective tissue within the epineurium.6,7Nevertheless, the possibility of causing nerve injury by direct trauma with a needle, or by toxic or ischemic effects of injection of local anesthetic, has made the avoidance of intraneural injection a basic rule of regional anesthesia.1,4However, the anatomical site and the methodology chosen by Bigeleisen et al. have led us to question whether the authors achieved their primary objective of comparing intraneural and extraneural minimum stimulating currents and to address our own technique of ultrasound-guided supraclavicular block.Our question rests on the definition of intraneural needle placement at the level of the supraclavicular brachial plexus. Bigeleisen et al. describe the outer border of the entire plexus as the epineurium, breach of which defines intraneural. The brachial plexus is a network of nerves—if each of these is considered to have its own epineurium, then the definitions in this study do not hold true. However, the area of enquiry is a compact segment of a plexus, ultrasound images of which rarely resolve into distinct trunks or divisions (fig. 1) and where separate trunks and divisions may not be visually distinct on cadaver cross sections.1,7In our practice of ultrasound-guided supraclavicular block, we intentionally breach the layer that Bigeleisen et al. describe as epineurium.8,9This is observed in real time and often felt by the operator as a loss of resistance or “pop.” We attempt to avoid injecting into what we believe to be nerves (seen as predominantly hypoechoic circular structures lateral to the subclavian artery; fig. 1) by observing both the needle tip as it advances and the spread of local anesthetic (fig. 2), which will likely cause the nerves to move but should not cause them to distend. If parasthesia is encountered, we redirect the needle, and we do not inject in the presence of pain or parasthesia, or against unusually high resistance (although this last feature is subjective, not measured).If the definition of Bigeleisen et al. is correct, we are performing intraneural injections with a mean volume of 33 ml of local anesthetic9on a daily basis, although we believe that we are depositing local anesthetic outside the nerves of the plexus, after breaching extraneural fascial layers. In a recent case series from our institution, evidence of possible neurologic injury was sought from 510 consecutive supraclavicular blocks. Two instances of numbness in the fingers of the operative hand were found in retrospect. Both of these had resolved spontaneously after several weeks and were not commented on at surgical follow-up.9Given our question about their definition of intraneural at the level of the supraclavicular brachial plexus block, we would reserve judgment on the generalizability of the results of Bigeleisen et al. to nerves in other anatomical sites. An examination of the question of stimulating thresholds and nerve injury, particularly in relation to the perineurium, would be of great interest, although we would be wary of conducting such a study on human subjects.Whichever term is used for the outer border of the brachial plexus, the technique of supraclavicular block that we describe seems to be safe and reliable. We firmly believe that neurologic complications of regional anesthesia must be the subject of continued investigation, both in terms of quantifying the incidence and understanding the means of avoidance, and we congratulate Bigeleisen et al. for their contribution to our understanding of the subject.*University of Toronto, Toronto Western Hospital, Toronto, Ontario, Canada. dmorfey@aol.com

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