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Basic Considerations: Introduction
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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.
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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.
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Functional Histology of the Peripheral Nerves
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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
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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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