The purpose of the EDX is to assess the motor and sensory function related to the spinal nerves. Each spinal nerve contains both motor and sensory fibers and contributes to the formation of the peripheral nerve. The cell bodies for the motor axons are situated within the anterior horn of the spinal cord. The cell bodies for the sensory axons are located within the dorsal root ganglion near its junction with the ventral root. There it forms the mixed spinal nerve in the region of the intervertebral foramina. After exiting the neural foramen, the spinal nerve root divides into anterior and posterior rami. The anterior rami supply the anterior trunk muscles and, after entering the brachial or lumbosacral plexus, the muscles of the extremities. The posterior rami supply the paraspinal muscles and skin over the neck and trunk (Fig. 17-1).
Lesions can be classified as either preganglionic (localized to spinal cord or nerve root) or postganglionic (localized to plexus or distal mixed peripheral nerve). Lesions within the spinal canal (myelopathy, radiculopathy) compromise sensory fibers proximal to their cell bodies in the dorsal root ganglion. Such lesions do not affect the sensory NCS studies because the injured sensory fibers degenerate centrally between the cell body in the dorsal root ganglion and the nerve root. Cells in the dorsal root ganglion continue to supply nutrition to the peripheral sensory fibers, thereby preserving sensory nerve conduction in this region. With more peripheral lesions (e.g. within and distal to the plexuses), sensory fibers degenerate distally, resulting in abnormal sensory NCS. In contrast, nerve root compression distal to the motor cell bodies in the anterior horn cell results in distal degeneration of motor fibers that can be detected on motor NCS and EMG studies.
It is possible for the dorsal root ganglion to be situated slightly more proximal in the foramina and be affected by direct compression or indirectly by vascular insult and edema formation. The dorsal root ganglion can also be damaged in diseases such as diabetes mellitus, herpes zoster, and malignancy. In these conditions, the sensory NCS may be abnormal. However, abnormal sensory NCS rarely occur with discogenic radiculopathies.
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