The Peripheral Nervous System and Introduction to Electromyography/Nerve Conduction Studies

INTRODUCTION TO ANATOMY AND DISEASES OF THE PERIPHERAL NERVOUS SYSTEM

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The peripheral nervous system includes the nerve roots, dorsal root ganglia, brachial and lumbosacral plexuses, and peripheral nerves. Nerve roots join to form plexuses (cervical plexus, brachial plexus, lumbosacral plexus), which give rise to peripheral nerves.

Individual peripheral nerves may be sensory, motor, autonomic, or mixed. Mixed nerves are two-way conduits: Efferent motor information travels from the spinal cord to the muscles and afferent sensory information travels in from the periphery to the spinal cord.

Efferent motor signals travel from the anterior horn cells (alpha motor neurons) into peripheral nerves by way of ventral roots. These are the lower motor neurons that are under the control of the corticospinal tracts (see Ch. 4).

Afferent sensory information travels from the peripheral nerves to dorsal root ganglia, and from dorsal root ganglia into the spinal cord by way of dorsal roots to enter the ascending sensory pathways (see Ch. 4).

Efferent peripheral nervous system sympathetic autonomic signals originate in the intermediolateral columns of the thoracic spinal cord, synapse in paraspinal ganglia, and then travel in postganglionic neurons to end organs. The parasympathetic control of the organs of the thorax and most of the organs of the abdomen comes from the vagus nerve (cranial nerve 10), with the exception of the final third of the gastrointestinal tract, the bladder, and the reproductive organs, which receive parasympathetic input from nerves arising from nerve roots from sacral levels 2 through 4. The central control of the autonomic peripheral nervous system comes from hypothalamic-brainstem pathways.

Peripheral nervous system pathology can affect the roots (radiculopathy), dorsal root ganglia (ganglionopathy; also known as sensory neuronopathy), alpha motor neurons (motor neuron disease), brachial or lumbosacral plexus (plexopathy), or one or more peripheral nerves (peripheral neuropathy).

Localization of focal or multifocal peripheral nervous system findings requires determining whether a pattern of weakness, sensory disturbance, and/or reflex diminution/absence can be explained by a problem with:

A single nerve (mononeuropathy)
A single root (radiculopathy)
Multiple individual nerves (mononeuropathy multiplex)
Multiple roots (polyradiculopathy)
The brachial or lumbosacral plexus (plexopathy)

If a peripheral nervous system process is generalized rather than focal, clinical symptoms and signs can help to determine which level(s) of the peripheral nervous system is/are affected (i.e., polyneuropathy, polyradiculopathy, polyradiculoneuropathy, or ganglionopathy). The main symptoms of each category of peripheral nervous system disorder are listed in Table 15–1 and discussed in more detail in their respective sections in this chapter. Localization guides differential diagnosis since different types of pathologic processes can selectively affect different levels of the peripheral nervous system. Common causes of pathology at each level of the peripheral nervous system are listed in Table 15–2 and discussed in more detail below. Note that diabetes and HIV can cause a variety of different types of peripheral nervous system dysfunction.

TABLE 15–1Clinical Features of Peripheral Nervous System Disorders.

	Key Clinical Feature	Focal Versus Diffuse	Characteristics of Pain, if Present
Radiculopathy	Pain	Focal symptoms limited to root distribution, often asymmetric	Radiating in root distribution
Ganglionopathy (Sensory neuronopathy)	Sensory ataxia	Diffuse, usually symmetric	If present, often burning in quality
Plexopathy	Weakness > sensory loss	Focal symptoms limited to involved limb	Depends on etiology
Mononeuropathy	Weakness and/or sensory loss	Focal symptoms limited to single nerve distribution	If present, in distribution of involved nerve
Mononeuropathy multiplex	Weakness and/or sensory loss	Multifocal, usually asymmetric	Present when vasculitis is etiology
Polyneuropathy	Weakness and/or sensory loss Paresthesias and/or pain	Diffuse, usually symmetric	If present, symmetric and usually begins distally

TABLE 15–2Etiologies of Peripheral Nervous System Disorders.

	Structural	Inflammatory	Infectious	Neoplastic	Toxic	Systemic Diseases	Hereditary
Radiculopathy	Disc disease Spondylosis Trauma	AIDP CIDP	Lyme disease CMV HSV2	Leptomeningeal Compressive		Ankylosing spondylitis
Ganglionopathy		Idiopathic inflammatory	HIV	Paraneoplastic	Vitamin B6 toxicity Platins	Sjögren’s syndrome
Plexopathy	Trauma Operative positioning	Parsonage-Turner Nondiabetic lumbosacral radiculoplexus neuropathy		Infiltration Radiation-induced		Diabetic lumbosacral radiculoplexus neuropathy (Bruns-Garland syndrome)	Hereditary neuralgic amyotrophy
Mononeuropathy	Entrapment Trauma		Leprosy	Amyloidosis	Lead poisoning (radial nerve most common)	Diabetes Hypothyroidism	Hereditary neuropathy with liability to pressure palsies
Mononeuropathy multiplex		Primary nonsystemic vasculitic neuropathy Multifocal motor neuropathy	Leprosy HIV Hepatitis C	Neurolymphomatosis		Diabetes Systemic vasculitis Amyloidosis	Hereditary neuropathy with liability to pressure palsies
Generalized symmetric polyneuropathy		AIDP CIDP	HIV	Multiple myeloma Paraneoplastic	Chemotherapy Heavy metals Medications	Diabetes Vitamin B12 deficiency Uremia	Charcot-Marie-Tooth Hereditary sensory and autonomic neuropathy

ANATOMY AND DISEASES OF NERVE ROOTS: RADICULOPATHY

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Anatomy of Nerve Roots

In the cervical spine, each nerve root is numbered according to the cervical vertebra above which it exits: The C1 root exits above the C1 vertebra, the C2 root exits above the C2 vertebra (between C1 and C2), the C7 root exits above the C7 root vertebra (between C6 and C7) (Fig. 15–1). The root exiting between C7 and T1 is labeled as the C8 root (although there is no C8 vertebra). This “resets” the numbering from T1 forward, so at the thoracic, lumbar, and sacral levels, roots are numbered by the level below which they exit: the T1 root exits below the T1 vertebra (between T1 and T2), the L1 root exits below the L1 vertebra (between L1 and L2), the S1 root exits below the S1 vertebra (between S1 and S2). The spinal cord ends at L1, and the lumbar and sacral roots (the cauda equina) must descend inferiorly from the L1 level to their corresponding neural foramina to exit.

FIGURE 15–1

Schematic of the relationship of nerve roots to the spinal column. A: Sagittal view of the spine, showing cervical roots exiting above their corresponding vertebrae, whereas thoracic, lumbar, and sacral roots exit below their corresponding vertebrae. B: Axial view of disc herniation causing nerve root compression. C: Posterolateral lumbar disc herniation affecting the root exiting at the next level down (here, disc between L4 and L5 compresses L5 root). D: Central lumbar disc herniation affecting multiple roots of the cauda equina. Reproduced with permission from Aminoff M, Greenberg D, Simon R: Clinical Neurology, 9th ed. New York: McGraw-Hill Education; 2015.

One of the most common causes of radiculopathy is intervertebral disc prolapse, which occurs most commonly at the cervical and lumbar levels where the spine is most mobile. At both cervical and lumbar levels, disc prolapse most commonly impinges on the nerve root whose number corresponds to the inferior vertebra of the pair of vertebrae surrounding the disc. For example, the disc between C5-C6 most commonly impinges on the C6 root, and the L4-L5 disc most commonly impinges on the L5 root. Note that because of the numbering scheme just described, this means that a herniated cervical disc compresses the root at the level of exit, whereas a herniated lumbar or sacral disc most commonly compresses the root that is going to exit at the next level down. For example, a herniated disc at C5-C6 typically compresses the C6 root, which exits between the C5 and C6 vertebral bodies. Most commonly, lumbar disc herniation impinges upon the root on the way down to the next level, called posterolateral disc herniation. Therefore, a herniated disc at L4-L5 most commonly compresses the L5 root, which is the root that is going to exit between the L5 and S1 vertebral bodies (the root that exits between L4 and L5 is the L4 root, which exits below its corresponding L4 vertebra). Less commonly, far lateral disc herniation can affect the root exiting at the level of the disc (e.g., L4-L5 disc affecting the L4 root) (see Fig. 17–3).

A few reference points are important in remembering which sensory dermatomes correspond to which nerve roots (Fig. 15–2):

On the thorax and abdomen, the nipples are at the T4 level, the umbilicus at T10, and the waist line at L1.
On the back, the most prominent cervical vertebra is C7, and the iliac crests correspond to the L3-L4 level.
The upper extremity is supplied by C5-T2.
The lower extremity is supplied by L1-S2.
S3-S4-S5 supply the area around the anus, and S3-S4 supply the genitalia.

FIGURE 15–2

Schematic of dermatomes. Reproduced with permission from Martin J: Neuroanatomy Text and Atlas, 4th ed. New York: McGraw-Hill Education; 2012.

The root supply of the upper and lower extremities is discussed in detail in Chapters 16 and 17.

Diseases of Nerve Roots (Radiculopathy)

Nerve roots can be affected by:

Compression by local structures: disc, osteophyte
Infection (radiculitis): Lyme disease, cytomegalovirus (CMV), herpes simplex virus 2 (HSV2), syphilis (along with the dorsal columns in tabes dorsalis)
Inflammation: Guillain-Barré syndrome and chronic inflammatory demyelinating polyradiculoneuropathy (CIDP), both of which are polyradiculoneuropathies (i.e., affect both roots and nerves)
Malignancy: compression due to spine metastases or leptomeningeal metastases