Page 386 - Atlas of Small Animal CT and MRI

3.6

Brachial and lumbosacral plexus

Normal anatomy of the brachial Muscle denervation
and lumbosacral plexus Although muscle denervation pathology is not limited

Brachial plexus to plexus disorders, clinical and imaging manifestations
Spinal segment dorsal and ventral nerve roots arise of muscle denervation can be striking in these patients.
separately from the spinal cord before merging to form Acute muscle denervation is rarely recognized in
a spinal nerve. The nerve exits the intervertebral fora­ dogs and cats, although it is described in people. Acute
men then separates into a dorsal and ventral branch. denervation results in muscle injury with increased
The brachial plexus is formed by a complex convergence extracellular fluid volume. CT imaging features in
of the ventral branches of the sixth, seventh, and eighth people include mild increase in muscle volume and
cervical nerves and the first and second thoracic nerves, variable, mild contrast enhancement. MR features
although contributions are variable, and the fifth include mild increase in muscle mass, no change in T1
cervical nerve is sometimes included (Figure 3.6.1). The intensity, increased T2 and STIR intensity, and mild
ventral spinal nerve branches divide and reorganize contrast enhancement. Chronic muscle denervation
deep within the axilla to form the peripheral nerves that results in marked reduction in muscle mass and fatty
supply the forelimb and parts of the cranial thoracic replacement. CT features include hypoattenuation of
wall. Because of their location relative to their respective remaining muscle volume due to increased fat content.
ribs, the ventral branches of the first and second tho­ MR findings include heterogeneous increased T1 and
2
racic nerves course along the internal aspect of the cra­ T2 intensity (Figure 3.6.2).
nial thoracic wall for a short distance before exiting at
the thoracic inlet. 1 Trauma

Lumbosacral plexus Brachial plexus traction injury
The lumbosacral plexus is normally formed by ventral Traction injuries of the plexus involve abnormal tensile
branches of the third through seventh lumbar nerves forces applied to the limb, which cause avulsion of spinal
and the first through third sacral nerves (Figure 3.6.1), nerve roots or injury to the spinal nerves. In people, a
although variations do occur. Similar to the brachial distinction is made between preganglionic brachial
plexus, the lumbosacral plexus results from the plexus injuries, those occurring at or near the spinal
divergence of the spinal nerves with reorganization roots and proximal to the dorsal root ganglion, and
proximally along the caudal paraspinal region and within postganglionic injuries, since this can determine the
the pelvis to form peripheral nerves that innervate the applicable surgical approaches for repair or nerve trans­
pelvic limb and parts of the pelvis and pelvic viscera. 1 fer. Although brachial plexus injury has been described
3

Atlas of Small Animal CT and MRI, First Edition. Erik R. Wisner and Allison L. Zwingenberger.
© 2015 John Wiley & Sons, Inc. Published 2015 by John Wiley & Sons, Inc.
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