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Editorials: Editorials

Ultrasound-Guided Nerve Blocks

The Real Position of the Needle Should Be Defined

Choquet, Olivier MD; Capdevila, Xavier MD, PhD

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doi: 10.1213/ANE.0b013e31823207b9
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In the 1914 edition of Regional Anesthesia, Victor Pauchet,1 the mentor of Gaston Labat, claimed that with perineural or endoneural injection of novocaine “Anesthesia of a large nerve trunk is governed by definite principles. … The length of time one must wait after having made the injection depends upon the manner in which the nerve has been reached. If the needle has been introduced into the nerve root, abolition of sensation is instantaneous. If the anesthetic has merely been injected around the nerve trunk, 5 to 20 minutes elapse before complete insensibility is established. After a direct endoneural injection, a fusiform swelling of the nerve is provoked, which besides disappears quickly.”

Passionate and unresolved controversies on peripheral nerve blocks (PNBs) have ensued in the century since Regional Anesthesia: long- or short-bevel needle, paresthesia or nerve stimulation (NS), PNB or not in anesthetized patients. The paradigm of peripheral nerve location has been transformed by ultrasound (US) guidance. US-guided PNB allows us to visualize needle-nerve interaction in real time. The local anesthetic (LA) spread around the nerve and intraneural (or intravascular) injection are identified. We have learned that a needle can penetrate a nerve without paresthesia or without any twitch at 0.5 mA. We know that many injections at low current intensity are intraneural without nerve damage and that an intraneural injection results in rapid onset of sensory blockade. Because the paresthesia/NS debate is out of date, the epistolary battle still revolves around the target nerve. Some authors promote deliberate US-guided intraneural injection as a powerful tool for achieving successful blockade.2 Most others are conservative because intraneural injection may cause nerve damage.3 The main issue is that we should be consistent with our anatomical terminology and do not “overread” the US.

In this issue of Anesthesia & Analgesia, Sala-Blanch et al.4 report on an appealing study comparing a traditional NS-guided popliteal sciatic nerve block (PSNB) above the division of the sciatic nerve with a US-guided block with a single injection of LA between the tibial nerve (TN) and common peroneal nerve (CPN) just below the division. The location and spread of LA was evaluated by US in both groups. The increase in nerve area and proximal and distal diffusion of LA were more often present in patients in the US group than in the NS group. Patients in both groups completed successful block for surgery 30 minutes after injection. Sensory and motor blockades were achieved faster in the US group than in the NS group. More surprisingly, there was no difference in block onset time in patients in the NS group who had an intraepineural injection (19 patients) and those who had not (7 patients). These results are not actually original but confirm earlier studies. Buys et al.5 and Prasad et al.6 reported that US-guided separate injections around the TN and CPN resulted in a faster onset of block than a US-guided block above the division of the sciatic nerve. Morau et al.,7 using single-injection NS-guided PSNB, reported that the highest success rate was obtained when the sciatic nerve was divided into its 2 components, each surrounded by the LA solution, i.e., the double doughnut sign. Sala-Blanch et al.4 achieved a circumferential spread of LA around the TN and CPN for all patients without needle redirection. As in the study by Morau et al.,7 only the changes in the longitudinal spread and diameter of the nerve were associated with faster-onset block. We assume that the LA injection was probably extrafascicular and extraepineural beyond the external layer of the sciatic nerve, with or without nerve swelling.

We think that specific needle positions are able to produce a double doughnut sign without nerve swelling. We do not think that all nerve-area increases and longitudinal spreads correspond to intraneural injections. Anesthesiologists should be cautious in interpreting US images. On the one hand, with the resolution of current US machines, only one-third of sciatic nerve fascicles can be seen ultrasonically compared with light microscopy.8 Even with a high-frequency transducer, the different layers of the nerve cannot be differentiated from each other. On the other hand, the names of the different layers of connective tissue between nerve fibers and bundles of nerves differ greatly in the literature. Most authors consider from the inside outward: the endoneurium, perineurium, epineurium, and paraneurium (Fig. 1A).911 Individual nerve fibers and their Schwann cells are surrounded by the endoneurium. Groups of endoneurium-sheathed fibers, called fascicles, are surrounded by a thin, dense, multilayered connective tissue sheath, known as the perineurium. Connective tissue surrounding each fascicle is termed interfascicular or inner epineurium, surrounded by the epifascicular or outer epineurium (Fig. 1B). This outermost layer defines the nerve trunk and protects the peripheral nerve against mechanical stress. A loose connective tissue fills the space between the nerve and the surrounding tissue in connection with the epineurium. This tissue is called the paraneurium, mesoneurium, adventitia, or gliding apparatus.911 It suspends the nerve trunk within the soft tissue, fuses with similar loose connective tissue around vessels in neurovascular bundles, and brings the segmental blood supply of the nerve. The paraneurium is usually not described in textbooks.11 It may be overlooked in cadaver dissection. The paraneurium allows longitudinal nerve motion and caliber changes if adjacent joints are extended or flexed. It protects the nerve and allows it to glide against the surrounding tissue. Any attempt to touch the paraneurium leads to fibrosis, covering the nerve in a longitudinal direction. The next level is fusion with the fascicular epineurium and the 2 sheaths cannot be differentiated. This explains why the paraneurium has escaped the attention of surgeons and is thought to be the epineurium by anesthesiologists. This common nerve sheath was described after careful dissection of the sciatic nerve12 and the brachial plexus.13 Sala-Blanch et al.4 have to be commended for reporting that the most important parameter for rapid-onset PSNB was the spread of LA around both nerve components rather than the intraepineural injection itself. That result highlights the virtual space between the paraneurium and the epifascicular epineurium. We are convinced that the paraneurium can confine the LA solution around and along the epineurium and give a doughnut sign on US without nerve swelling (Fig. 2). We believe that in the case of swelling, when nerve fascicles are separated from each other, the needle should be withdrawn. We do not claim that the paraneural injection is the highway that leads to 100% success and no complications. We assume that the paraneural space is a target for the needle that can lead us to an adequate risk/benefit ratio.

Figure 1
Figure 1:
Cross-section of a peripheral nerve. A, Drawing of peripheral nerve showing a characteristically round bundle of nerve processes surrounded by connective tissue sheaths. B, Human sciatic nerve cross-section histologic view/×20. En = endoneurium; Pe = perineurium; IE = inner epineurium; OE = outer epineurium; Pa = paraneurium (or mesoneurium).
Figure 2
Figure 2:
Ultrasound-guided popliteal sciatic nerve block. A, The 2 components (tibial and common peroneal) in cross-section before local anesthetic (LA) solution injection. B, The sciatic nerve is divided into its 2 components, each surrounded by the LA solution, i.e., the “double doughnut sign” after injection. Note the absence of swelling into each nerve component.


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