This feature plays a “safety” role in nerve block by allowing the nerves to be “pushed” rather than pierced by the advancing needle, as often seen on ultrasound. For instance, the axons are somewhat “wavy,” and when stretched, the connective tissue around them is also stretched giving it some protection. The connective tissue of a nerve is tougher, compared to the nerve fibers themselves, and allows a certain amount of “stretch” without damage to the nerve fibers.
This arrangement of the peripheral nerve helps explain why intraneural injections may result in disastrous consequences. If a short segment of the nerve is missing, however, the fascicles in the various quadrants of the stump may no longer correspond with one another, good axial alignment may not be possible, and functional recovery is greatly compromised or improbable. In such cases, good functional recovery is more likely. In such a scenario, there is a greater chance that the distal segment of nerves synapsing with the muscles will be sutured to the central stump of motor or sensory axons. If the cut is clean, it may be possible to suture individual fascicular bundles together. This arrangement is a practical concern to the surgeons trying to repair a severed nerve.
However, the axons within a small set of adjacent bundles redistribute themselves so that the axons remain in approximately the same quadrant of the nerve for several centimeters. They divide and anastomose with one another as frequently as every few millimeters.
Of note, the fascicular bundles are not continuous throughout the peripheral nerve. In contrast, a more dense collagenous tissue forms the epineurium that surrounds the entire nerve and holds it loosely to the connective tissue through which it travels.įIGURE 2. The fascicular bundles in turn are embedded in loose connective tissue called the interfascicular epineurium, which contains adipose tissue, fibroblasts, mastocytes, blood vessels (with small nerve fibers innervating these vessels), and lymphatics. The perineurium surrounds each fasciculus and splits with it at each branching point. This barrier helps to preserve the ionic milieu of the axon and functions as a blood–nerve barrier. In addition to its mechanical strength, the perineurium functions as a diffusion barrier to the fascicle, isolating the endoneural space around the axon from the surrounding tissue. In surgical procedures, the perineurium holds sutures without tearing. Groups of axons are closely associated within a bundle called a nerve fascicle that is surrounded by the perineurium, which imparts mechanical strength to the peripheral nerve. Individual nerve fibers bind together, somewhat like individual wires in an electric cable ( Figure 2). In the peripheral nerve, individual axons are eveloped by the endoneurium, which is a delicate layer of loose connective tissue around each axon. The reader is referred to Figure 1 for an easier orientation of the body planes discussed throughout the book. The goal of this chapter is to provide a generalized and rather concise overview of anatomy relevant to the practice of regional anesthesia more specific anatomic discussions pertaining to individual regional anesthesia techniques are detailed in their respective chapters. Moreover, the introduction of ultrasound in the practice of regional anesthesia has further clarified the relationship of the needle and the nerve and the dynamics of local anesthetic spread. Jerry Vloka in the 1990s, has contributed to better understanding of the anatomy of regional nerve block. Much research on functional regional anesthesia, a term introduced by Dr. This is because accurate placement of the needle and the spread of the local anesthetic after an injection depend on the interplay between neurologic structures and the neighboring tissues where local anesthetic pools and accumulates, rather than on the mere anatomic organization of the nerves and plexuses. However, once the anatomic layers and tissue sheets are dissected, the anatomy of nerve structures without the tissue sheaths around them is of little relevance to the clinical practice of regional anesthesia. In the past, many nerve block techniques and approaches were devised by academicians merely relying on idealized anatomic diagrams and schematics, rather than on functional anatomy. Just as surgical technique relies on surgical anatomy or pathology leans on pathologic anatomy, the anatomic information necessary for the practice of regional anesthesia must be specific to this application.
The practice of regional anesthesia is inconceivable without sound knowledge of the functional regional anesthesia anatomy. Lopez, Xavier Sala-Blanch, Eldan Kapur, Ilvana Hasanbegovic, and Admir Hadzic INTRODUCTION Table of Contents Functional Regional Anesthesia AnatomyĪnna Carrera, Ana M.
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