The paraneural sheath of the sciatic nerve, formerly called the "gliding apparatus,"22 continues to surround the tibial and common peroneal nerves below the bifurcation.15,19 In essence, local anesthetic injected extraneurally may spread within the subparaneural space (space between the paraneural sheath and the epineurium of the sciatic nerve), the subepimyseal space (space between the epimysium and the paraneural sheath), or both. Recent studies11,12,14,15 comparing subepimyseal to subparaneural local anesthetic injection show that subparaneural injection is associated with faster block onset,12,14,15 higher block success rate,11,14,15 and prolonged block duration,14,15 suggesting that the subparaneural compartment is a desirable site for local anesthetic injection.14,19 Subparaneural local aesthetic spread can be both circumferential18 and noncircumferential14,19 with extensive spread proximally and distally along the nerve,14,15,19 thus exposing a larger surface area of the nerve to the local anesthetic.
Although the paraneural and epimyseal sheaths are anatomically distinct, they are not easily visualized under current US technology unless the subparaneural and subepimyseal spaces are distended with local anesthetic after injection.14,19 Using high-definition US, the report of Karmakar et al18 describes the sonographic appearance of the paraneural sheath separating the subepimyseal and subparaneural compartments around the sciatic nerve in 4 subjects. It also describes local anesthetic spread patterns within the subparaneural compartment, thus confirming the findings of earlier cadaveric19,20 and clinical studies.14 The accompanying sonographic images elegantly illustrate the outer epimyseal connective tissue sheath surrounding muscles. Karmakar reiterates earlier calls14,19 to standardize terminology and eliminate misnomers regarding the 3 sheaths of connective tissue surrounding the sciatic nerve: the outermost epimysium, the innermost epineurium, and the paraneural sheath in between. It is recommended that earlier terminology (eg, epineurial sheath, paramysium, and paraneurium that refer to the extraneural connective tissue layer) be replaced by a simpler and easily understandable term, the paraneural sheath. Finally, Karmakar underscores the need to develop a safe future technique for subparaneural injection, possibly using high-definition ultrasonography.
By definition, a subparaneural injection is extraneural in contradistinction to an intraneural (subepineurial) injection. In a recent cadaveric study, Moayeri et al23 showed early and sensitive US detection of intraneural injection with as little as 0.5 mL into the supraclavicular brachial plexus and infragluteal sciatic nerve. The best US indicator is an increase in the cross-sectional area of the nerve (∼9% in this study) with concomitant change in nerve echogenicity. However, the degree of nerve swelling depends on the amount of local anesthetic injected, and some recommend an increase of greater than 15% as clinical evidence of an intraneural injection.24 Although the subparaneural space is deemed the desirable site of local anesthetic injection for sciatic nerve block,15 the technique of placing a needle accurately into this narrow compartment without trespassing the epineurium (ie, intraneural puncture) can be challenging. Interestingly, even with high-definition US, Karmakar relied on electrical stimulation to guide needle placement and to elicit a foot motor response at 0.3 to 0.4 mA before local anesthetic injection.
The site of bifurcation of the sciatic nerve represents a unique opportunity for subparaneural injection as US often permits identification of a plane of cleavage between the tibial and peroneal nerves that is large enough for relatively easy needle access into the subparaneural compartment using either an in-plane or out-of-plane approach. Ultrasound recognition of the paraneural sheath and epimysium above and below the sciatic nerve bifurcation can be difficult, even with high-resolution US.18 For this reason, we believe it is not practical or safe to rely on US imaging alone to accurately identify the subparaneural space for needle insertion. Rather, it is technically prudent to first advance the needle toward the small hypoechoic perineural space that is often visualized at the 2 corners of the nerve (3 and 9 o'clock) as shown in Karmakar's report. We believe this is the most suitable space for initial needle entry with less risk of contacting the nerve. With the hydrodissection technique, a small fluid bolus of local anesthetic or D5W (if electrical stimulation is intended) can peel the hyperechoic paraneural sheath off the sciatic nerve and further injection can confirm needle tip position within the hypoechoic subparaneural space, now distended with fluid. It is also important to recognize a suboptimal subepimyseal injection indicated by a failure to visualize the distinct hyperechoic paraneural sheath and its separation from the sciatic nerve as well as the absence of circumferential local anesthetic spread.
Although effective, the practice of subparaneural injection also raises some safety concerns as transient paresthesias have been reported in 20%17 to 29.4%19 of patients, inadvertent nerve swelling in 2.6%18 to 9%,19 and prolonged neurologic sequelae in 3% after PSB.19 Closeness of the paraneural sheath to the sciatic nerve poses some potential risk of needle trauma during needle attempt to enter the narrow subparaneural space. To avoid unintentional intraneural injection, we believe it is worthwhile to further evaluate the clinical use of the periplexus injection technique, which was found to be as effective as perineural injection during interscalene brachial plexus block in 1 preliminary study.25 Unlike Karmakar, we do not believe that high-definition US is the answer for safe subparaneural injection. Rather a slightly more distant injection technique to produce a compartment block (ie, needle injection close enough to surround the nerve with local anesthetic but far enough to avoid needle to nerve contact) may be the ultimate anesthetic goal in the future.
The Paraneural Compartment: A New Destination?
Abdallah, Faraj W. MD*; Chan, Vincent W. MD, FRCPC
