Fascial Plane Blocks: More Questions Than Answers?

Abstract

A well-educated mind will always have more questions than answers. —Helen Keller Ultrasound-guided fascial plane blocks have been embraced enthusiastically as an alternative to epidural, paravertebral, and perineural injections. They represent a paradigm shift as there is no need to visualize nerves or deposit local anesthetic close to nerves. However, despite growing popularity, these novel blocks have their share of controversies, namely inconsistent effect, efficacy, evidence, indication, and technique. This has not stopped their popularity nor their acceptance into guidelines, protocols, and clinical practice. While they certainly have a role in modern anesthesia, a better understanding of fascia is a prerequisite to greater success. Fascia is a complicated collagenous, fibrous connective tissue that encloses muscles and separates them from other structures (eg, bones, nerves, vessels, and viscera). It provides a framework for different parts of the body to function together.1 There are 2 types of fascia: superficial fascia and deep fascia.2 Superficial fascia is a layer of loose connective tissue that provides skin integrity and supports subcutaneous structures.2 Deep fascia is the fibrous structure that sheathes muscles and has functions such as maintaining posture, force absorption and transmission, load transfer, movement stability, and proprioceptive communication.3 It blends with other structures such as muscles, ligaments, and joints.4 The deep fascia is the target for fascial plane blocks.4 Many characteristics of the deep fascia may influence the extent of injectate spread and clinical effectiveness of fascial plane blocks (Figure 1), but exactly how they affect an individual block is largely unknown.Figure 1.: A summary of the features of fascia to consider when performing fascial plane blocks.In this article, we examine how the structure and function of fascia influences fascial plane block effectiveness and consider the definition of block success and failure. Also, we challenge the reader to scrutinize the cadaveric and radiological evidence applies and deliberate on how high-quality patient-centered research may be accomplished. FASCIAL STRUCTURE AS A POTENTIAL CAUSE OF BLOCK INCONSISTENCY Local anesthetic injected into a facial plane must travel to reach the nerve targets for the block to be effective; however, fascia is complex and exhibits regional variation in structure which influences spread. Thickness of a Fascial Plane Deep fascia has 2 subtypes: epimysial and aponeurotic fascia (Figure 2).2 Epimysial fascia, as found in the pectoral region, is thinner and adherent muscles and is the site of the PECS II block (Table).5 Aponeurotic fascia is thicker, easily separable from muscles and is seen in the thoracolumbar fascia (TLF) and the rectus sheath.2 The TLF is the target for quadratus lumborum (QL) and erector spinae plane (ESP) blocks. The TLF has been described as both a 2- and 3-layered model; however, the 3-layered model is more commonly used.7,8 Table 1. - Summary of the Types of Deep Fascia and Relevant Fascial Plane Blocks Epimysial Aponeurotic Thickness Thinner (150–200 μm) Thicker (600–1400 μm) Grouping Specific to each muscle May envelop several muscles Action Localized Transmits muscular forces over greater distance Adherence Usually adherent to muscles via fibrous septa Easily separable from muscle Anatomical location Found in deep fascia of trunk muscles (eg, pectoralis major and latissimus dorsi) and the epimysium of limbs Found in the thoracolumbar fascia, rectus sheath, and deep fascia of limbs (eg, fascia lata) Block examples PECS II, SAP, and TAP Adductor canal, ESP, fascia iliaca, QL, and rectus sheath Data were derived from Stecco.6Abbreviations: ESP, erector spinae plane; QL, quadratus lumborum; SAP, serratus anterior plane; TAP, transversus abdominus plane. Figure 2.: Types of deep fascia. A, Epimysial fascia enveloping the pectoralis major muscle. B, Aponeurotic fascia of the thigh (fascia lata). The epimysial fascia is thin and totally adherent to the muscle, while the aponeurotic fascia is a thick fibrous layer separated from the underlying muscle by adipose tissue.The thickness of a fascial plane may influence the ability to accurately deposit local anesthetic within a plane and its spread. For example, a thicker aponeurotic fascial plane may represent a greater physical barrier to local anesthetic diffusion and a thin epimysial fascial layer might make this diffusion easier. Alternatively, a thicker layer (eg, the middle layer of the TLF) may provide a more distinct sonographic and tactile end point for needle puncture and keep the local anesthetic within the interfascial plane, allowing for a greater spread to reach target nerves. It may be more difficult to inject local anesthetic with precision within a fascial plane that is bordered by thin epimysial fasciae, and it may be less likely that the drug stays within the fascial plane because it can easily penetrate the perimysium to enter the muscle, especially with high injection pressure. However, this is speculation; ultimately how fascial thickness, structure, and adherence to surrounding tissue impacts the performance and efficacy of fascial plane blocks is uncertain. Additionally, fascial planes are perforated by blood vessels and nerves which allow injectate to spill out of the target plane and may hamper spread. These perforations are subject to significant variability. New technologies may be useful in assessing these unknowns, and use of newer devices (eg, higher resolution ultrasound, injection pressure monitors, ultrasound contrast agents, and optical coherence tomography) may all prove useful. Furthermore, changes in fascial architecture (eg, thickening, thinning, and scarring) due to aging, trauma, disease states (eg, diabetes mellitus), diet, and exercise may all affect injectate spread and block effectiveness.9 Lines of Fusion Lines of fusion (LOF) are sites where fasciae fuse, creating a place where muscular forces can converge.6 LOF exist throughout the body and permit coordination between groups of muscles; the linea alba is a well-known example.6 Another example is the LOF between the layers of the TLF where the muscles and fasciae of 1 layer blend with the muscles and fasciae of adjacent layers.6 These LOF can limit local anesthetic spread by creating a compartment. Consider the rectus sheath block where local anesthetic is injected between the rectus abdominus muscle and the posterior rectus sheath.10 The aim of this block is to anesthetize the anterior cutaneous branches of the thoracoabdominal nerves as they travel through this plane.10 This block must be performed bilaterally to provide coverage for midline surgery as the linea alba will prevent spread to the contralateral side. Likewise, the linea semilunaris is likely to impede spread laterally to anesthetize the lateral cutaneous nerves. However, injecting into this defined compartment might be useful as the injectate will, in theory, be able to concentrate the effect on the target nerves without spread beyond the target plane. Understanding where these fascial convergences are is important in trying to predict the spread of local anesthetic.Figure 3.: Adhesions between 2 fascial layers. The blue arrows highlight the points of adhesions. A, Physiological adhesions in the erector spinae compartment. B, Postsurgical adhesion between the internal oblique fascia and transversus abdominus. These adhesions may block the diffusion of injectate into these spaces.New pathological LOF from adhesion formation may develop after surgery or trauma (Figure 3). This can act as a roadblock in a fascial space, restricting local anesthetic diffusion, and possibly contributing to an inconsistent block. Excluding scars from previous surgery, it may be impossible to predict where, or if, these new LOF exist. This might provide an explanation to an unexpectedly limited block extent in some patients. Fascial Interconnectivity In addition to LOF, the concept of fascial interconnectivity is fundamental in understanding the coverage of fascial plane blocks. Many fascial planes are continuous and communicate with each other without a clear anatomical boundary.6 For example, the TLF is continuous with the endothoracic fascia in the thorax through the medial and lateral arcuate ligaments and the aortic hiatus of the diaphragm, and with the gluteal fascia in the lower limb.6,8 It is important to differentiate the concepts of LOF and fascial interconnectivity. A LOF is a distinct point at which fasciae fuse. However, some fascial planes are continuous for large distances without any interrupting boundary and will allow injectate to spread far and wide. For example, it has been suggested that a paraspinal block could reach the plane deep to the pectoralis major muscle.4 Fascial interconnectivity may improve or impair plane block effectiveness. For example, local anesthetic spread from the anterior surface of the QL muscle (anterior TLF) to reach the thoracic paravertebral space is a proposed mechanism of the transmuscular QL block.11 Injectate may also spread within the fascial plane to the nearby ventral rami.11 However, because of fascial interconnectivity, a local anesthetic injection can spread in several directions (medially, laterally, cephalad, and caudad) along the path of least resistance. This can result in an unpredictable extent and direction of local anesthetic spread beyond the operator’s control, thus leading to inconsistent analgesic effects. FASCIAL FUNCTION AS A POTENTIAL CAUSE OF BLOCK INCONSISTENCY Fascia has complex physiology and allows the gliding of one tissue plane over another. Alterations in the ability of fascial planes to glide freely will have different implications for each individual and can cause pain, inflammation, and potentially loss of function.6 Fascial Gliding Fascia permits gliding between fascial sublayers and between fascia and muscles, bones, and joints.3 The purpose of gliding is to reduce the friction associated with movement and ease of gliding is influenced by hyaluronan (HA), a lubricating glycosaminoglycan secreted by fasciacytes.3 Fasciacytes are a recently described fibroblast-like cell that can be found on the surface of fascial sublayers.3 Fasciacyte function and location and quantity of HA influences the ease of fascial gliding and the fascial plane’s resistance to flow. Also, the viscosity of HA is altered by changes in body temperature, pH, and physical strain, which could alter local anesthetic spread within a fascial plane.9 There is currently no way to visualize this with ultrasound. What impact these issues may have on the extent and direction of injectate spread and success of plane blocks is unknown, but it is an interesting opportunity for future research. Some potential research questions include: what influence will additives (eg, hyaluronidase) have on injectate spread? Do disordered fasciacyte function and myofascial pain reduce the efficacy of fascial plane injections? Can this information be used to predict which patient may benefit more or less from these techniques? The difference in the regional degree and ease of fascial gliding and fascial plane resistance to flow may also influence the extent of injectate spread. For example, gliding between the erector spinae muscles and the underlying transverse processes and muscles is probably greater than the transversus abdominus plane (TAP) between the internal oblique and transversus abdominus muscles, thus the spread of local anesthetic following an ESP block is potentially more widespread than a TAP block. New methodology and new thinking are required to reliably predict the spread of local anesthetic and subsequent block value. OTHER POSSIBLE FACTORS THAT MAY INFLUENCE SPREAD It is unclear how local anesthetic spread or block dynamics are influenced by patient position, ventilation, and/or muscular contraction. Additionally, we do not know how technical factors such as needle size, needle orientation, and injection pressure influence spread. The key question is can local anesthetic spread be predictable, and if not, will the success of plane blocks ever be predictable? NERVE VARIATIONS AS A POTENTIAL CAUSE OF BLOCK INCONSISTENCY There are 3 main types of nerves to consider when performing fascial plane blocks: 1. Somatic nerves traveling through the fascial plane 2. Sympathetic nerves traveling through the fascial plane 3. Nerves to the fascia (which are from said somatic and sympathetic nerves) Somatic and sympathetic nerves travel in fascial planes and cross through them as they travel to reach their targets.6 The intrafascial journey of nerves is highly variable. How far they travel within the plane and where they enter and exit the plane is inconsistent. This is another possible contributor to inconsistent block results since the local anesthetic injection into the fascial plane may not reach the nerve targets. Fascia is a sensitive organ richly innervated with sympathetic and nociceptive fibers and is active in proprioception and nociception.12 The innervation pattern of the deep fascia has been described as the “fasciatome.”12 The fasciatome is believed to be more closely related to the underlying muscles (myotome) than the overlying skin (dermatome). Fascia contributes to both acute and chronic myofascial pain syndromes.9 Radiating pain generating from the fasciatome follows the organization of the fascial anatomy and is distinctly different from localized pain derived from the dermatome.12 A fasciatome “map,” like those commonly seen for dermatomes has not yet been described. Direct anesthesia of the fascia is one possible explanation for why it is commonly not possible to detect sensory block by conventional cold and pinprick testing in a seemingly efficacious block. It may transpire that some of the analgesia benefits of fascial plane blocks is due to relief of myofascial pain rather than somatic pain. A sympathetic blockade has also been proposed as a potential mechanism of pain relief for various plane blocks which may provide clinically useful analgesia, particularly for visceral pain.11 This may arise from blockade of sympathetic nerves traveling in the fascial plane, directly innervating the fascia or by spread outside the fascia plane. The 3 types of sensory nerves in the fascial plane are probably all involved in fascial plane injections. It would be tidy to assume that each block has a definable and separately measurable effect on each type of nerve, but this is unlikely. Given that these nerves are not consistently located, and probably not consistently affected in the same way by all fascial plane blocks, will fascial plane blocks ever be truly consistent? CLINICAL EVIDENCE BASED ON INJECTATE SPREAD MAY BE FLAWED The extent of injectate spread is used by cadaveric and radiological investigations as a surrogate marker of clinical efficacy with the assumption that greater spread causes wider anesthetic coverage and also is more clinically effective. However, this may not be true. For example, considering the ESP block, cadaveric and clinical studies have demonstrated variable extent of cephalocaudal injectate spread, with a 20–25 mL volume injected at T5 spreading between 6 and 14 vertebral levels.13–15 These results are likely true and represent that spread will be variable in clinical practice. However, the pattern of spread is uncontrollable. The most consistent and intense anesthesia is probably located at the level of injection. There is a limited correlation between dye spread in cadaveric studies or volunteer imaging studies and clinical analgesia because clinical efficacy does not only depend on the extent of coverage, but also the intensity of coverage. Dye studies have shown spread to the epidural space following ESP blocks; however, epidural analgesia and common epidural-related complications are not commonly reported.13 Nerve staining in cadaveric studies or dye spread in volunteer imaging studies provides little information on injectate concentration, and local anesthetic must reach a target with effective concentration to provide analgesia. Consider the iPACK (interspace between the popliteal artery and posterior capsule of the knee)injection for total knee arthroplasty. Cadaveric evidence would suggest that the common fibular nerve is frequently stained, yet in clinical practice a foot drop is very uncommon.16 Caution is advised in interpreting cadaveric staining and radiological spread as evidence of clinical efficacy. Furthermore, there is significant variability within and between studies, which is representative of clinical practice. DEFINING BLOCK SUCCESS AND FAILURE Sensory anesthesia after fascial plane block injection is inconsistent, making it challenging to assess block success and failure. Many plane block clinical studies report a statistically significant reduction in pain scores and opioid consumption in the absence of recorded cutaneous loss of sensation. A pooled review of ESP block cases noted that only 34.7% of cases in the literature recorded assessment of sensory or motor block.17 This is contrary to more traditional perineural techniques where sensory loss and motor block are considered reliable indicators of block success. If a fascial plane block is not associated with any discernible loss in cutaneous sensation, has it failed? Is this sensory loss necessary for block success? If these techniques are performed and the patient derives benefit, does the distribution of block matter? There are no easy answers to these questions. One possible explanation might be a differential block, that is, a low concentration of local anesthetic that is sufficient to block small unmyelinated C-fibers without affecting larger A-delta fibers, thus providing analgesia without a discernible sensory blockade.18 If this is true, the block duration may be short-lived as a small quantity of local anesthetic would reach the nerves. This would be an elegant answer but would need innovative research to prove. At present, there is no consensus on the definition of a successful or failed fascial plane block. While the use of fascial plane blocks is rapidly expanding, and the majority of publications extol their use in ever-increasing clinical scenarios, caution needs to be exercised to ensure scientific rigor. We encourage the regional anesthesia community to discuss, document, and publish the rates and reasons for fascial plane block failure and not just success. Furthermore, practitioners must critically examine the reasons for a failed block, for example, suboptimal technique, and be nonjudgmental when discussing this issue. FUTURE RESEARCH We believe the future of ultrasound-guided plane blocks is exciting, but significant improvement in the understanding of fascia and high-quality clinical trials are needed to improve block technique and analgesic consistency. For these blocks to become more consistent, studies are needed to determine the optimal site, plane, and direction of needle access, as well as the speed and pressure of injection and the desired sonographic end points.19 Ideally, these trials should include a relevant and consistent definition of success and failure. Many studies report encouraging opioid-sparing effects in favor of plane blocks when compared to placebo in the absence of multimodal analgesia, but well-designed comparative studies using active comparators are encouraged to evaluate clinically meaningful outcomes for these blocks. The rate of absorption of local anesthetic varies by site with the highly vascular paravertebral and intercostal regions being rapid. Variation is also seen with fascial plane blocks and there is a longer time to peak plasma local anesthetic concentration following a rectus sheath block when compared to a TAP block.20 This may be because the TAP plane is bordered by thin epimysial fascial and the rectus sheath by thicker aponeurotic fascia. However, many questions remain. Is duration of block related to uptake of local anesthetic? Is uptake predictable based on fascial subtype or anatomical location? Is there an ideal local anesthetic drug and/or additive(s)? How does the volume and concentration of local anesthetic matter for each specific block? Patient-reported outcomes (PRO) are increasingly important in perioperative research and one such PRO measurement is the minimum clinically important difference (MCID) in the different Quality of Recovery scores.21 These are objective and structured measurements that assess the quality of life, patient satisfaction, and postoperative well-being.21 Using the MCID will allow studies to be powered to detect a difference that will matter to patients. Hopefully, studies using a standardized plane block compared to current best practice, in conjunction with appropriate multimodal analgesia, with a defined MCID in PRO will become commonplace and provide real-world evidence for novel techniques in clinical practice. CONCLUSIONS Current understanding of the anatomy, physiology, and pharmacology of injected drugs pertinent to fascial plane blocks is incomplete and imperfect. We do not know which patient, block, and anatomical factors influence each technique. Although this is a rapidly expanding and exciting area of regional anesthesia research and clinical practice, a healthy cynicism is necessary pending objective scientific findings. DISCLOSURES Name: Nick D. Black, MB BCh BAO, FRCA. Contribution: This author helped with the conception, manuscript writing and editing, and final approval of the manuscript. Name: Carla Stecco, MD. Contribution: This author helped with manuscript writing and editing and final approval of the manuscript. Name: Vincent W. S. Chan, MD, FRCPC. Contribution: This author helped with the conception, manuscript writing and editing, and final approval of the manuscript. This manuscript was handled by: Honorio T. Benzon, MD.