The Maximum Effective Needle-to-Nerve Distance for Ultrasound-Guided Interscalene Block

One of the most fundamental, 1 yet controversial, tenets of regional anesthesia practice has been the adage “no paresthesia, no anesthesia.” Implicit to this concept is the requirement for direct needle-nerve contact to achieve a successful block. The advent of ultrasound (US) guidance for peripheral nerve blockade (PNB) has enabled providers to position the needle tip purposefully as close as possible to, and even inside, the target nerve. Consequently, much of the contemporary regional anesthesia literature has focused on the question “How close is too close?” while investigators challenge the safety limits of US-guided PNB. Regrettably, the risk of nerve injury persists despite US guidance and is underscored by reports of new functional deficits after interscalene brachial plexus block (ISB) performed under US guidance by experienced providers. Given that mechanical needle-nerve trauma is an important mechanism of peripheral nerve injury, providers are cautioned to avoid intentional intraneural injection or needle-nerve contact during US-guided PNB.8,14,15 Potentially hazardous needleto-nerve proximity may be especially relevant during US-guided ISB, where inadvertent injection beneath the epineurium may be as high as 50%. Subepineural, and particularly intrafascicular, injection of local anesthetic may increase the risk of nerve injury.17 Neural elements of the interscalene brachial plexus are predominantly comprised of axonal tissue and may be especially