Chapter 69

### Basic Considerations: Introduction

Although there are relatively few published reports of anesthesia-related nerve injury associated with peripheral nerve blocks (PNBs), it is likely that the commonly cited incidence (0.4%) of neurologic injury is underestimated owing to underreporting.1–3 Most complications of PNBs were reported with upper extremity blocks. The less frequent clinical application of lower extremity nerve blocks may be the main reason why there are even fewer reports of anesthesia-related nerve injury associated with lower extremity PNBs compared with upper extremity PNBs.4 Although neurologic complications after PNBs can be related to factors associated with the block technique (eg, needle trauma, intraneuronal injection, neuronal ischemia, and toxicity of local anesthetics), a search for other common causes should include positional and surgical factors (eg, positioning, stretching, retractor injury, ischemia, and hematoma formation). In some instances, the neurologic injury may be a result of a combination of these factors.

In all four sections of this chapter, mechanisms and consequences of acute neurologic injury related to the nerve block procedure are discussed and, where appropriate, methods and techniques to reduce the risk of complications are suggested. Specific nerve injuries with upper and lower nerve block techniques, neuraxial anesthesia, and local anesthetic toxicity are discussed elsewhere in this volume.

### Functional Histology of the Peripheral Nerves

Knowledge of the functional histology of the peripheral nerve is important to understand the mechanisms of peripheral nerve injury; the reader is referred to Chapters 3 and 4 for more in-depth discussion on this subject. Here we briefly review salient features of the organization of the peripheral nerves. A peripheral nerve is a complex structure consisting of fascicles held together by the epineurium, an enveloping, external connective sheath (Figure 69–1). Each fascicle contains many nerve fibers and capillary blood vessels embedded in a loose connective tissue, the endoneurium.5 The perineurium is a multilayered epithelial sheath that surrounds individual fascicles and consists of several layers of perineural cells. Therefore, in essence, a fascicle is a group of nerve fibers or a bundle of nerves surrounded by perineurium. Of note, fascicles can be organized in one of three common arrangements: monofascicular (single, large fascicle); oligofascicular (few fascicles of various sizes); and polyfascicular (many fascicles of various sizes).6

###### Fig. 69-1

Histology of the peripheral nerve. Shown is a large fascicle of the peripheral nerve with its axons, surrounded by perineurium, epineurium, and nourishing blood vessels.

Nerve fibers can be myelinated or unmyelinated; sensory and motor nerves contain both in a ratio of 4:1, respectively. Unmyelinated fibers are composed of several axons, wrapped by a single Schwann cell. The axons of myelinated nerve fibers are enveloped individually by a single Schwann cell. A thin layer of collagen fibers, the endoneurium, surrounds the individually myelinated or groups of unmyelinated fibers.

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