Spinal Nerve Roots PDF

Chapter 8 Spinal Nerve Roots A 38-year-old man was thrown violently to the ground by an ex- plosion while he was working on a road. While in the hospital re- covering from burn injuries, he noticed mild back pain with a numb “pins and needles” sensation running down his left leg int...

Chapter 8 Spinal Nerve Roots A 38-year-old man was thrown violently to the ground by an ex- plosion while he was working on a road. While in the hospital re- covering from burn injuries, he noticed mild back pain with a numb “pins and needles” sensation running down his left leg into the sole and pinky toe of his left foot. He was unable to stand on his toes with his left foot, and his left Achilles tendon reflex was absent. This patient’s symptoms illustrate the kinds of sensory and motor deficits associated with spinal nerve root damage. In this chapter, we will learn about the anatomical course of exiting spinal nerve roots, nerve root functions, and the clinical conse- quences of injury to these structures. 320 Chapter 8 ANATOMICAL AND CLINICAL REVIEW (A) I C1 1 C1 N THE PRECEDING TWO CHAPTERS, we studied the three main motor and C2 2 C2 sensory pathways in the central nervous system (see Table 7.1). We C3 3 will now follow these somatic sensory and somatic motor pathways C4 C3 4 into the peripheral nervous system (see Figure 2.1) to explore the C4 5 anatomy and clinical disorders of the peripheral nerves. In this chap- C5 ter, we will concentrate on the anatomy of spinal nerve roots and their Cervical cord C5 6 C6 relation to the vertebral structures, regions of innervation, and common C6 7 clinical disorders. Peripheral autonomic functions were already dis- C7 C7 8 cussed in Chapter 6. In Chapter 9, we will follow the nerves further into C8 the periphery, and we will discuss the brachial and lumbosacral plexuses T1 T1 T1 and peripheral nerve branches. T2 2 T2 T3 3 Segmental Organization of the Nervous System T3 Like their invertebrate ancestors, humans retain some degree of segmen- T4 4 T4 tal organization, especially in the spinal cord. There are 8 cervical (C1–C8), 12 thoracic (T1–T12), 5 lumbar (L1–L5), 5 sacral (S1–S5), and 1 coccygeal T5 5 T5 (Co1) spinal segments (Figure 8.1). During development, the bones of T6 6 the spine continue to grow after the spinal cord has reached its full T6 size. Therefore, in adults the spinal cord normally ends with the conus T7 7 medullaris at the level of the L1 or L2 vertebral bones. The nerve roots (see T7 Thoracic cord Figure 8.1; see also Figure 6.3) travel downward to reach their exit points 8 at the appropriate level. Below the L1 or L2 vertebral bones, the spinal T8 T8 9 canal contains nerve roots with no spinal cord, forming the cauda equina, T9 T9 10 meaning “horse’s tail” (see Figures 8.1A and 8.3C). The conus medullaris tapers into the filum terminale, a thin strand of connective tissue running 11 T10 T10 in the center of the cauda equina. The roots of the cauda equina are or- 12 ganized such that the most centrally located roots are from the most cau- Lumbar cord T11 L1 dal segments of the spinal cord (see Figure 8.3C). T11 2 3 T12 4 T12 5 Sacral cord (B) L1 Dorsal (posterior) horn Conus medullaris L1 Intermediate zone Dorsal column L2 Cauda equina Ventral (anterior) Dorsal median septum L2 horn L3 White matter Central canal Gray matter L3 L4 Dorsal root (sensory) L4 L5 Ventral median fissure Spinal nerve L5 S Ventral root Ventral Lateral (motor) S1 column column S2 FIGURE 8.1 Spinal Cord and Nerve Roots in Relation to the Vertebral Spinal Canal S3 (A) Sagittal view. The cervical (C5–T1) and lumbosacral (L1–S3) spinal cord enlarge- S4 ments supply nerves to the arms and legs, respectively. At the level of the L1 or L2 ver- S5 tebral bones the spinal cord ends, and nerve roots continue as the cauda equina. (B) Mo- Co1 tor (ventral) roots and sensory (dorsal) roots join at each segment to form spinal nerves.  Spinal Nerve Roots 321 Left and right sensory and motor nerve roots arise from each segment of the spinal cord (see Figure 8.1B) except for the C1 segment, which has no sensory roots. As discussed in Chapter 6, the cervical enlargement (C5–T1) gives rise to the nerve roots for the arms, and the lumbosacral enlargement (L1–S3) gives rise to the nerve roots for the legs (see Figure 8.1A). A short distance from the spinal cord, before leaving the spinal canal, the sensory and motor nerve roots fuse to form a single mixed spinal nerve for each segment (see Figure 8.1B). The spinal nerves then further fuse and intermingle peripherally to form plexuses and nerve branches, as will be discussed in Chapter 9. In this chapter we will focus on the innervation provided by single spinal segments and the distinct clinical disorders associated with damage to individual spinal nerve roots. Nerve Roots in Relation to Vertebral Bones, Discs, and Ligaments The vertebral bones function both as the central mechanical support for the body and as protection for the spinal cord. Each vertebral bone has a sturdy cylindrical vertebral body located anteriorly (Figure 8.2A,B). The vertebral bod- ies are separated from each other by connective tissue intervertebral discs, con- sisting of a central nucleus pulposus surrounded by a capsule called the annu- lus fibrosus (Figure 8.2C). Posteriorly, the neural elements are surrounded by an arch of bone formed by the pedicles, transverse processes, laminae, and spin- ous processes (see Figure 8.2B). The superior and inferior articular processes or facet joints form additional points of mechanical contact between adjacent ver- tebrae (see Figure 8.2A,B). The spinal cord runs through the spinal canal (vertebral foramen) and is sur- rounded by pia, arachnoid, and dura mater (see Figure 8.2). As the dura exits the skull at the foramen magnum, the inner layer continues and the outer layer becomes indistinguishable from periosteum (see Figure 5.10). Unlike in the cra- nium, there is a layer of epidural fat between the dura and the periosteum in the spinal canal (see Figure 8.2C, and Figure 8.2D), which is a useful landmark on MRI scans. In addition, there is a valveless meshwork of epidural veins called Batson’s plexus that is thought to play a role in the spread of metastatic cancers and infections in the epidural space. The elastic ligamentum flavum is particularly prominent in cervical and lumbar regions and can sometimes be- come hypertrophied and contribute to spinal cord or nerve root compression. The nerve roots exit the spinal canal via the neural (intervertebral) foramina (see Figure 8.2A,D and Figure 8.3). Disc herniations (see KCC 8.3) are most com- mon at the cervical and lumbosacral levels. An understanding of the anatomy of the nerve roots and discs should make clear the following important rule of thumb: For both cervical and lumbosacral disc herniations, the nerve root involved usually corresponds to the lower of the adjacent two vertebrae. For example, a MNEMONIC C5–C6 disc herniation usually produces a C6 radiculopathy, an L5–S1 disc usu- ally produces an S1 radiculopathy, and so on. The explanation for this rule is different for cervical versus lumbosacral discs, as we will now discuss. Thoracic, lumbar, and sacral nerve roots exit below the correspondingly num- bered vertebral bone (see Figure 8.1). Cervical nerve roots, on the other hand, exit above the correspondingly numbered vertebral bone—except for C8, which has no corresponding vertebral bone and exits between C7 and T1. Cervical nerve roots have a fairly horizontal course as they emerge from the dural or the- cal sac near the intervertebral disc and exit through the intervertebral foramen (see Figure 8.3B). Cervical discs are usually constrained by the posterior longi- tudinal ligament to herniate laterally toward the nerve root, rather than centrally toward the spinal cord. Thus, in the cervical cord the nerve root involved usu- ally corresponds to the lower vertebral bone of the disc space (see Figure 8.3A). 322 Chapter 8 (A) Anterior Posterior (C) Anterior Posterior Superior Pedicle articulatory Spinal cord process Pia Vertebral Nucleus Transverse Arachnoid body pulposus process CSF in subarachnoid Inferior space Annulus articulatory fibrosus Dura Intervertebral process disc (thecal sac) Facet Posterior Epidural fat joint longitudinal ligament Periosteum Neural (intervertebral) Spinous Ligamentum foramen process flavum Plexus of veins Interspinous (Batson’s plexus) ligament and arteries (B) Posterior (D) Posterior Inferior Lamina Spinous Plexus of veins (Batson’s plexus) Periosteum articulatory process and arteries in epidural fat process Transverse process Ligamentum flavum Superior articulatory Dura process Pedicle (facet) Spinal cord Arachnoid Vertebral foramen (spinal canal) CSF in Lateral subarachnoid recess space Vertebral Spinal nerve Neural Epidural Pia body (intervertebral) fat foramen Anterior Anterior FIGURE 8.2 Vertebral Bones, Meninges, and Other Tissues (A) Lateral view of vertebral spinal Unlike cervical nerve roots, lumbar and sacral nerve roots must travel down- bones. (B) View of a vertebral bone from ward several levels before they exit the spinal canal (see Figure 8.1). In addi- above. (C) Sagittal section of spinal col- tion, the intervertebral foramina of the lumbosacral spine are such that the umn including vertebral bones, discs, ligaments, spinal cord, meninges, nerve roots exit some distance above the intervertebral discs (see Figures 8.2 epidural fat, and blood vessels. (D) Axial and 8.3B). As they are about to exit, the nerve roots move into the lateral recess section of spinal column and its contents. of the spinal canal (see Figure 8.2B), and it is at this point that they are closest to the disc (see Figure 8.3B). Thus, posterolateral disc herniations in the lum- bosacral spine usually impinge on nerve roots on their way to exit beneath the next lower vertebral bone, which corresponds to the number of the nerve root involved (see Figure 8.3B,C). Occasionally, a far lateral disc herniation will reach the nerve root exiting at that level, resulting in impingement of the next higher nerve root. For exam- ple, a far lateral L5–S1 disc herniation can cause an L5 radiculopathy (see Fig- ure 8.3C). In addition, a central disc herniation at the level of the cauda equina can impinge on nerve roots lower than the level of herniation, or it can com- press the spinal cord if it occurs above L1.  Spinal Nerve Roots 323 FIGURE 8.3 Relation of Cervical and Lum- Anterior Posterior (A) Anterior Posterior bosacral Nerve Roots to Intervertebral Discs (A) Cervical disc herniation usually compresses the nerve root exiting at that level. This corresponds to the number of the C3–C4 disc lower vertebral bone at that interspace. (B) C4 nerve Lumbosacral disc herniation usually spares C4–C5 disc the nerve root exiting at that level and com- C5 nerve presses the nerve root exiting at the next level down. However, this again corre- C6 nerve sponds to the number of the lower vertebral C5–C6 disc bone at the level of the herniation. (C) Far lateral lumbosacral disc herniation affects C7 nerve the nerve root exiting at that level, and cen- C7 vertebral tral lumbosacral disc herniation can cause C6–C7 disc body cauda equina syndrome (see KCC 8.4). (B) Anterior Posterior L2–L3 disc Dermatomes and Myotomes The sensory region of skin innervated by L3 nerve a nerve root is called a dermatome (Fig- L3–L4 disc ure 8.4). Interestingly, dermatome maps L5 nerve vary somewhat from one text to the next. L4 nerve in lateral This variation is likely due to differences recess L4–L5 disc in both the methods of testing and indi- vidual patients studied. Nevertheless, L5 nerve L5 nerve some familiarity with the usual locations exiting of dermatomes can be very helpful clin- L5–S1 disc inter- ically for localizing lesions. vertebral S1 nerve foramen Sensation for the face is provided by the trigeminal nerve (see Figure 12.7), while most of the remainder of the head is supplied by C2 (via the greater and (C) Left Right lesser occipital nerves). The nipples are usually at the T4 level, while the umbilicus is at ap- L4 vertebral proximately T10. When testing sensation on the body chest or back, remember that there is normally a L4 nerve L4–L5 central skip between C4 and T2, with C5 through T1 rep- disc herniation resented mainly on the upper extremities (see Fig- Far lateral L4–L5 disc herniation ure 8.4B). C5 is represented in the shoulder, C6 in Posterolateral the lateral arm and first two digits, C7 in the mid- L5 vertebral herniation of dle digit, and C8 in the fourth and fifth digits. The body L5–S1 disc L4 representation extends over the anteromedial L5 nerve shin, L5 extends down the anterolateral shin and dorsum of the foot to the big toe, and S1 is in the S1 nerve small toe, lateral foot, sole, and calf. S2, S3, and S4 innervate the perineal area in a saddlelike distribu- S2 nerve tion. Note that there is considerable overlap be- tween adjacent dermatomes, so lesions of a single S3 nerve nerve root ordinarily cause a decrease but not a Coccygeal S4 nerve nerve complete loss of sensation in a given dermatome. S5 nerve There may be less overlap for smaller fibers, so pin- prick is a more sensitive test for dermatomal sen- Filum terminale sory loss than touch. 324 Chapter 8 (A) (B) C2 CN V C2 C3 C4 C3 C5 C4 C4 C6 T2 T3 T2 T2 C5 T4 C5 C5 T3 C5 T5 T4 T6 T5 T7 T2 T2 T6 T8 T2 T9 T2 T7 T10 T8 T11 T1 T9 T1 T12 T10 L1 C6 C6 T1 T1 S1 T11 L2 S2 T12 L1 L3 L3 S3 S3 S3 C6 C6 C7 C8 L2 S4 L2 C8 C7 C7 C8 S4 C8 C7 L2 L2 Co1 S5 S2 S2 L3 L3 L3 L3 L4 L4 L5 L5 L4 L4 S1 L5 L5 S1 S1 S1 S1 S1 L5 FIGURE 8.4 Dermatomes The muscles innervated by a single nerve root constitute a myotome. The segmental innervation of the muscles is summarized in Table 8.1. This table also includes a summary of nerve roots supplying each peripheral nerve and the main functions of each muscle (see Chapter 9 for further details). Strength testing and reflexes for individual muscles, nerves, and nerve roots is discussed in Chapter 3 (see Tables 3.4–3.7) and is demonstrated on neuroexam.com Videos Upper extremity strength 54–60.  Spinal Nerve Roots 325 TABLE 8.1 Summary of Peripheral Nerves, Muscles, and Nerve Roots in the Upper and Lower Extremities NERVE MUSCLE(S) FUNCTION OF THE MUSCLE(S) NERVE ROOTSa Spinal accessory nerve Trapezius Elevates shoulder/arm and fixes scapula CN XI, C3, C4 Phrenic nerve Diaphragm Inspiration C3, C4, C5 Dorsal scapular nerve Rhomboids Draw scapula up and in C4, C5 Levator scapulae Elevates scapula C3, C4, C5 Long (Bell’s) thoracic nerve Serratus anterior Fixes scapula on arm raise C5, C6, C7 Lateral pectoral nerve Pectoralis major Pulls shoulder forward C5, C6 (clavicular head) Medial pectoral nerve Pectoralis major Adducts and medially rotates arm C6, C7, C8, T1 (sternal head) Pectoralis minor Depresses scapula and pulls C6, C7, C8 shoulder forward Suprascapular nerve Supraspinatus Abducts humerus from 0° to 15° C5, C6 Infraspinatus Externally rotates humerus C5, C6 Subscapular nerve Subscapularis Internally rotates humerus C5, C6 Teres major Adducts and internally rotates humerus C5, C6, C7 Thoracodorsal nerve Latissimus dorsi Adducts and internally rotates humerus C6, C7, C8 Axillary nerve Teres minor Adducts and externally rotates humerus C5, C6 Deltoid Abducts humerus beyond 15° C5, C6 Musculocutaneous nerve Biceps brachii Flexes and supinates arm and forearm C5, C6 Brachialis Flexes forearm C5, C6 Coracobrachialis Flexes and adducts arm C6, C7 Radial nerve Triceps Extends forearm C6, C7, C8 Brachioradialis Flexes forearm C5, C6 Extensor carpi radialis Extend wrist, abduct hand C5, C6 (longus and brevis) Posterior interosseus nerve Supinator Supinates forearm C6, C7 (branch of radial nerve) Extensor carpi ulnaris Extends wrist, adducts hand C7, C8 Extensor digitorum Extends fingers (test at C7, C8 communis metacarpophalangeal joints) Extensor digiti quinti Extends little finger C7, C8 Abductor pollicis longus Abducts thumb in plane of palm C7, C8 Extensor pollicis Extends thumb C7, C8 (longus and brevis) Extensor indicis proprius Extends index finger C7, C8 Median nerve Pronator teres Pronates and flexes forearm C6, C7 Flexor carpi radialis Flexes wrist, abducts hand C6, C7 Palmaris longus Flexes wrist C7, C8, T1 Flexor digitorum Flexes metacarpophalangeal and C7, C8, T1 superficialis proximal interphalangeal joints Lumbricals (I, II) For second and third digits, C8, T1 flexes metacarpophalangeal joints, extends other joints Muscles affected Opponens pollicis Flexes, opposes thumb C8, T1 in carpal tunnel Abductor pollicis brevis Abducts thumb perpendicular C8, T1 syndrome to plane of palm Flexor pollicis brevis Flexes first phalanx of thumb C8, T1 (superficial head) (continued on p. 326) 326 Chapter 8 TABLE 8.1 (continued) NERVE MUSCLE(S) FUNCTION OF THE MUSCLE(S) NERVE ROOTSa Anterior interosseous nerve Flexor digitorum Flexes second and third fingers C7, C8 (branch of median nerve) profundus (digits 2, 3) (best tested in distal phalanges) Flexor pollicis longus Flexes distal phalanx of thumb C7, C8 Pronator quadratus Pronates forearm C7, C8, T1 Ulnar nerve Flexor carpi ulnaris Flexes wrist, adducts hand C7, C8, T1 Flexor digitorum Flexes fourth and fifth fingers C7, C8 profundus (digits 4, 5) (best tested in distal phalanges) Lumbricals (III, IV) For fourth and fifth digits, flex metacarpo- C8, T1 phalangeal joints, extend other joints Palmar interossei Adduct fingers, flex metacarpo- C8, T1 phalangeal joints, extend other joints Dorsal interossei Abduct fingers, flex metacarpo- C8, T1 phalangeal joints, extend other joints Flexor pollicis brevis Flexes and adducts thumb C8, T1 (deep head) Adductor pollicis Adducts thumb C8, T1 Muscles of Opponens digiti minimi Internally rotates fifth finger C8, T1 hypothenar Abductor digiti minimi Abducts fifth finger C8, T1 eminence Flexor digiti minimi Flexes fifth finger at metacarpo- C8, T1 phalangeal joint Obturator nerve Obturator externus Adducts and outwardly rotates leg L2, L3, L4 Adductor longus Adducts thigh L2, L3, L4 Adductor magnus Adducts thigh L2, L3, L4 Adductor brevis Adducts thigh L2, L3, L4 Gracilis Adducts thigh L2, L3, L4 Femoral nerve Iliopsoas Iliacus Flexes leg at hip L1, L2, L3 muscle Psoas Flexes leg at hip L2, L3, L4 Rectus femoris Extends leg at knee, flexes hip L2, L3, L4 Quadriceps Vastus lateralis Extends leg at knee L2, L3, L4 femoris Vastus intermedius Extends leg at knee L2, L3, L4 Vastus medialis Extends leg at knee L2, L3, L4 Pectineus Adducts thigh L2, L3, L4 Sartorius Inwardly rotates leg, flexes hip and knee L2, L3, L4 Sciatic nerve Adductor magnus Adducts thigh L4, L5, S1 Semitendinosus Flexes knee, medially rotates thigh, L5, S1, S2 extends hip Hamstring muscles Semimembranosus Flexes knee, medially rotates thigh, L5, S1, S2 extends hip Biceps femoris Flexes knee, extends hip L5, S1, S2 Tibial nerve Triceps Gastrocnemius Plantar flexes foot S1, S2 (branch of surae sciatic nerve) muscles Soleus Plantar flexes foot S1, S2 Popliteus Plantar flexes foot L4, L5, S1 Tibialis posterior Plantar flexes and inverts foot L4, L5 Plantaris Spreads, brings together, flexes L4, L5, S1 proximal phalanges Flexor digitorum longus Flexes distal phalanges, aids L5, S1, S2 plantar flexion  Spinal Nerve Roots 327 TABLE 8.1 (continued) NERVE MUSCLE(S) FUNCTION OF THE MUSCLE(S) NERVE ROOTSa Flexor hallucis longus Flexes great toes, aids plantar flexion L5, S1, S2 Small foot muscles Cup sole S1, S2 Superficial peroneal nerve Peroneus longus Plantar flexes and everts foot L5, S1 (branch of sciatic nerve) Peroneus brevis Plantar flexes and everts foot L5, S1 Deep peroneal nerve Tibialis anterior Dorsiflexes and inverts foot L4, L5 (branch of sciatic nerve) Extensor digitorum longus Extends phalanges, dorsiflexes foot L5, S1 Extensor hallucis longus Extends great toe, aids dorsiflexion L5, S1 Peroneus tertius Plantar flexes foot in pronation L4, L5, S1 Extensor digitorum brevis Extends toes L5, S1 Superior gluteal nerve Gluteus medius Abducts and medially rotates thigh L4, L5, S1 Gluteus minimus Abducts and medially rotates thigh L4, L5, S1 Tensor fasciae latae Abducts and medially rotates thigh L4, L5, S1 Inferior gluteal nerve Gluteus maximus Extends, abducts, and laterally L5, S1, S2 rotates thigh, extends lower trunk Source: Modified and reproduced with permission from Devinsky O and Feldmann E. 1988. Examination of the Cranial and Peripheral Nerves. Churchill Livingstone, New York. a Bold text indicates the most important nerve roots, where applicable. KEY CLINICAL CONCEPT 8.1 DISORDERS OF NERVE, NEUROMUSCULAR JUNCTION, AND MUSCLE A variety of disorders can affect the peripheral nervous system at multiple levels. This text focuses on neuroanatomical localization, so in this chapter and in Chapter 9 we concentrate mainly on localized disorders of the spinal nerve roots, nerve plexuses, or individual nerve branches. Here, we will place these disorders in the wider context of peripheral nervous system dis- ease so that a more complete differential diagnosis can be formulated. Disorders of the peripheral nervous system can often be distinguished from central nervous system dysfunction by the anatomical pattern of sensory or motor deficits (see KCC 6.3 and KCC 7.3; Table 8.1). In addition, presence of lower motor neuron signs (see KCC 6.1) such as atrophy, fasciculations, de- creased tone, or hyporeflexia suggests peripheral nervous system dysfunction, as do paresthesias in a peripheral nerve distribution (see KCC 7.1). When the location of lesions in the central versus peripheral nervous system remains un- certain based on the history and physical examination, diagnostic tests such as neuroimaging studies (see Chapter 4), blood tests, CSF analysis, and electrodi- agnostic studies (see KCC 9.2) can be helpful. Disorders of the peripheral nervous system can be produced by a large number of mechanical, toxic, metabolic, infectious, autoimmune, inflammatory, degenerative, and congeni- tal causes that are beyond the scope of this text (see the References at the end of this chapter for additional details). We will now briefly discuss several com- mon disorders of nerve, neuromuscular junction, and muscle. Common Neuropathies Neuropathy is a general term meaning nerve disorder. The site of pathology can be in the axons, myelin, or both and can affect large-diameter fibers, small-diameter fibers, or both. Usually, neuropathies affect both sensory and 328 Chapter 8 motor fibers in the nerve, although one or the other may be preferentially in- volved. Damage can be reversible or permanent. The location of neuropathy can be focal (mononeuropathy), multifocal (mononeuropathy multiplex), or generalized (polyneuropathy). Neuropathy affecting the spinal nerve roots is called radiculopathy, which we will discuss in greater detail in KCC 8.3. Like neuropathies, motor neuron disorders (see KCC 6.7) can also cause lower motor neuron–type weakness but motor neuron disorders do not cause sen- sory involvement. Important causes of neuropathy include diabetes; mechanical causes; in- fectious disorders such as Lyme disease, HIV, CMV, varicella-zoster virus, or hepatitis B (see KCC 5.9); toxins; malnutrition; immune disorders such as Guillain–Barré syndrome; and hereditary neuropathies such as Charcot- Marie-Tooth disease, among others. We will only discuss a few of the more common causes of neuropathy here. Diabetic neuropathy is produced by a number of mechanisms, including compromise of the microvascular blood supply of the peripheral nerves (other possible mechanisms include oxidative stress, autoimmunity, and neurotrophic and biochemical disturbances). The most common pattern of diabetic neuropathy is distal symmetrical polyneuropathy, which results in a characteristic glove and stocking pattern of sensory loss (see Figure 7.9D). Mononeuropathies are also relatively common in diabetes. Acute diabetic mononeuropathy can affect any cranial or spinal nerve but is most common in CN III and the femoral and sciatic nerves. Onset is usually fairly sudden, and sensorimotor deficits in the nerve distribution may be accompanied by painful paresthesias. There is often partial or complete recovery over the course of weeks to months after onset. Mechanical causes of nerve injury include extrinsic compression, traction, laceration, or entrapment by intrinsic structures such as bone or connective tissue. Mild mechanical disruption of a nerve causes neurapraxia, temporary impairment of nerve conduction that usually resolves within hours to weeks. More severe injury can interrupt the axons, leading to Wallerian de- generation (degeneration of axons and myelin) distal to the site of injury. As long as the structural elements of the nerve are intact, axonal regeneration may occur at a rate of about 1 mm/day (a little more than 1 in./month). Oc- casional long-term complications include incomplete or aberrant reinnerva- tion and the complex regional pain syndrome. Complex regional pain syn- drome Type 1, also called reflex sympathetic dystrophy, is more common and follows an injury without specific nerve damage, while Type 2, also called causalgia, follows damage to a specific nerve. Both types are characterized by intense local burning pain accompanied by edema, sweating, and changes in skin blood supply. In some situations, when peripheral nerves are severed or otherwise disrupted they can be reanastomosed surgically. In addition, some entrapment syndromes may be amenable to surgical decom- pression. Painful paresthesias associated with neuropathies of all causes are often treated with medications such as anticonvulsants, serotonin-norepi- nephrine reuptake inhibitors, or tricyclic antidepressants. Common mechan- ical neuropathies are discussed further in KCC 8.3 and KCC 9.1. Guillain–Barré syndrome, also known as acute inflammatory demyelinating polyneuropathy (AIDP), is an important form of neuropathy caused by im- mune-mediated demyelination of peripheral nerves. Onset typically occurs 1 to 2 weeks following a viral illness, Campylobacter jejuni enteritis, HIV in- fection, or other infections. Presentation is with progressive weakness, are- flexia, and tingling paresthesias of the hands and feet, with motor involve- ment typically much more severe than sensory involvement. Symptoms usually reach their worst point 1 to 3 weeks after onset; recovery occurs over  Spinal Nerve Roots 329 many months. Diagnosis is based on typical clinical presentation, cere- brospinal fluid (CSF) demonstrating elevated protein without a significantly elevated white blood cell count, and EMG/nerve conduction studies com- patible with demyelination (see KCC 9.2). Recovery occurs more quickly when patients are treated with plasmapheresis or intravenous immunoglobu- lin therapy. In severe cases, patients require intubation and mechanical ven- tilation. Autonomic dysfunction can be prominent in some cases, requiring careful monitoring. With good supportive care and immune therapy, the ma- jority of patients enjoy complete or near-complete recovery, although about 20% of patients have some residual weakness 1 year after onset. Common Disorders of the Neuromuscular Junction Impaired neuromuscular transmission can lead to motor weakness without sensory deficits. Causes include myasthenia gravis, neuromuscular blocking agents and other drugs, Lambert–Eaton myasthenic syndrome (usually paraneoplastic), and botulism. Myasthenia gravis is an immune-mediated disorder in which there are circulating antibodies against the postsynaptic nicotinic acetylcholine re- ceptors at the neuromuscular junction of skeletal muscle cells. The disorder can sometimes be accompanied by other autoimmune phenomena such as hypothyroidism, lupus, rheumatoid arthritis, and vitiligo. Myasthenia gravis has a bimodal age-related onset, with onset in the second or third decades more common in women and onset in the sixth or seventh decades more common in men. The prevalence is 50 to 125 cases per million. Clini- cal features include generalized symmetrical weakness, especially of proxi- mal limb muscles, neck muscles, the diaphragm, and eye muscles. Involve- ment of bulbar muscles can cause facial weakness, a nasal-sounding voice, and dysphagia (see KCC 12.8). Reflexes and sensory exam are normal. Characteristically, weakness becomes more severe with repeated use of a mus- cle or during the course of the day. About 15% of cases have weakness in- volving only the extraocular muscles and eyelids, a condition called ocular myasthenia. Diagnosis of myasthenia gravis is based on clinical features, and several diagnostic tests, including the ice pack test, repetitive nerve stimulation, measurement of anti-acetylcholine antibodies or muscle specific receptor ty- rosine kinase (MuSK) antibodies, single fiber EMG, and chest CT or MRI. The ice pack test, which can be performed in patients with ptosis, is administered by placing a bag of ice on the closed eyelids for 2 minutes, and reevaluating for improvement in the ptosis (possibly due to reduced cholinesterase func- tion at lower temperatures). Formerly, the Tensilon test, using a short-acting acetylcholinesterase inhibitor (edrophonium) was administered at bedside while observing clinical effects on involved muscles. Commercial manufac- ture of this agent was discontinued in 2008, however, making future avail- ability of this test uncertain. Clinical response to intermediate-acting acetyl- cholinesterase inhibitors, such as neostigmine, may also be diagnostically helpful in some cases. Compound motor action potential measurement (see KCC 9.2) with repetitive nerve stimulation at a rate of 3 per second often pro- duces a characteristic decrement in amplitude in myasthenia and is consid- ered positive if there is a decrement greater than 10%. Single-fiber EMG is more sensitive (about 90%) but is not specific for myasthenia. Anti-acetylcholine receptor antibodies (AchR-Ab) are positive in about 85% of cases of generalized myasthenia but in only about 50% of cases of ocular myasthenia. About half of the patients with generalized myasthenia who are AchR-Ab negative have positive serology for muscle specific receptor tyrosine kinase antibodies (MuSK-Ab). 330 Chapter 8 About 12% of patients with myasthenia have a thymoma, a tumor of the thymus gland, and many others have thymic hyperplasia, so CT or MRI of the chest should be performed. In addition, testing for associated conditions such as thyroid disease and other immune disorders is appropriate. Myasthenia gravis is treated by immune therapy. Anticholinesterase med- ications are also helpful to relieve symptoms. Pyridostigmine (Mestinon) is a long-acting cholinesterase inhibitor, with onset of action beginning about 30 minutes after oral administration and duration of about 2 hours. Patients’ doses are individually titrated but should not ordinarily exceed about 120 mg every 3 hours, since excess anticholinesterase can actually worsen weak- ness. Most patients in the age range of adolescence to 60 years are treated surgically with thymectomy (whether a thymoma is present or not), as this usually leads to improvement by unclear mechanisms, likely involving a re- duced autoimmune response. Use of thymectomy outside this age range or in patients with ocular myasthenia is more controversial but has been used in some cases. Thymectomy should be performed at a time when patients are relatively clinically stable in order to minimize complications in the peri- operative period. Short-term immunotherapy with plasmapheresis or intra- venous immune globulin (IVIg) can be helpful, particularly when patients are in myasthenic crisis requiring intubation, experiencing other severe worsen- ing in symptoms, or in preparation for elective surgery. Longer-term im- munosuppressive agents, including steroids, azathioprine, mycophenolate, and cyclosporine, are also typically prescribed. Common Muscle Disorders Muscle disorders, or myopathies, produce weakness that is typically more severe proximally than distally, without loss of sensation or reflexes. Com- mon causes of myopathy include thyroid disease, malnutrition, toxins, viral infections, dermatomyositis, polymyositis, and muscular dystrophy. Der- matomyositis and polymyositis are immune-mediated inflammatory my- opathies. The blood creatinine phosphokinase (CPK) is typically elevated, and electromyography (EMG) studies (see KCC 9.2) are compatible with myopathy. In dermatomyositis there is a characteristic violet-colored skin rash, typically involving the extensor surface of the knuckles and other joints. Although numerous other forms exist, Duchenne muscular dystrophy is the most common form of muscular dystrophy. Transmitted by X-linked inheritance, it affects male children and causes progressive proximal weak- ness. The abnormal protein (dystrophin) has been identified, providing hope for a cure in the near future.! KEY CLINICAL CONCEPT 8.2 BACK PAIN Back pain is one of the most common reasons that people seek medical atten- tion. In this chapter we focus on back pain caused by nerve root disorders; however, it is important to briefly review causes of back pain in general. Table 8.2 is a partial list intended to emphasize the diverse nature of condi- tions that can be associated with back pain. Many of the diagnoses listed can be elucidated on the basis of a careful history and physical exam. Muscu- loskeletal causes are most common in all age groups. However, in individu- als with onset of back pain over age 50, a neoplasm should be suspected. Back pain in a younger person that worsens with exertion and improves with rest is usually caused by a musculoskeletal problem, including disc hernia- tion in some cases (see KCC 8.3). Symptoms and signs of a radiculopathy (see  Spinal Nerve Roots 331 TABLE 8.2 Differential Diagnosis of Back Pain TRAUMA/MECHANICAL Disc herniation; spondylolysis; vertebral fracture; arthritis; muscle strain/ligament sprain; soft tissue injury VASCULAR Spinal arteriovenous malformation; spinal cord infarct; subarachnoid hemorrhage; spinal epidural hematoma INFECTIOUS/ Osteomyelitis; arachnoiditis; spinal epidural abscess; INFLAMMATORY/ myositis; cytomegalovirus radiculitis; muscle aches NEOPLASTIC in viral illness; Guillain–Barré syndrome; primary or metastatic neoplasms (extradural, extramedullary, or intramedullary) DEGENERATIVE/ Scoliosis; degenerative joint disease; amyotrophic DEVELOPMENTAL lateral sclerosis REFERRED/OTHER Normal pregnancy; ectopic pregnancy; menses; (NON-NEUROLOGIC) urinary tract infection; pyelonephritis; renal stone; retroperitoneal abscess; retroperitoneal hematoma; retroperitoneal tumor; pancreatitis; aortic aneurysm; aortic dissection; angina; myocardial infarction; pulmonary embolism KCC 8.3) should be sought. Back pain in any age group that progressively worsens or does not improve over time should be evaluated with appropri- ate imaging studies (usually an MRI of the spine). In addition, one should never neglect to evaluate bowel, bladder, and sexual function in patients with back pain, so that irreversible loss of function can be prevented (see KCC 7.2 and KCC 8.4). Several clarifying definitions for degenerative disorder of the spine are given in Table 8.3.! KEY CLINICAL CONCEPT 8.3 RADICULOPATHY Sensory or motor dysfunction caused by pathology of a nerve root is called radiculopathy. (Neuropathies in general are discussed in KCC 8.1; radiculopa- thy is a specific subtype involving nerve roots.) Radiculopathy is often asso- TABLE 8.3 Clarifying Definitions for Degenerative Disorders of the Spine DISORDER DEFINITION SPONDYLOLYSIS A general term for degenerative disorders of the spine. From the Greek spondylos, meaning “vertebra.” SPONDYLOLYSIS Fractures that appear in the interarticular portion of the vertebral bone, between the facet joints (see Figure 8.2A). Lysis means “loosening” in Greek. SPONDYLOLISTHESIS Displacement of a vertebral body relative to the vertebral body beneath it. Includes anterolisthesis or retrolisthesis, meaning “anterior” or “posterior” displacement of the upper vertebral bone, respectively. Anterolisthesis often coexists with spondylolysis. In Greek, olisthesis means “slipping and falling.” OSTEOPHYTES Bony spurs that form on regions of apposition between adjacent vertebrae because of chronic degeneration. From the Greek osteo, meaning “bone,” plus phyton, meaning “plant” or “outgrowth.” SPINAL STENOSIS Congenital or acquired narrowing of the spinal canal. 332 Chapter 8 ciated with a burning, tingling pain that radiates or shoots down a limb in TABLE 8.4 Common Causes of the dermatome of the affected nerve root (see Figure 8.4). There may be loss Radiculopathy of reflexes and motor strength in a radicular distribution (see Tables 3.4–3.7 Disc herniation and 8.1). Chronic radiculopathy can result in atrophy and fasciculations (see Osteophytes KCC 6.1). Sensation may be diminished if a single dermatome is involved, Spinal stenosis but because of overlap from adjacent dermatomes, sensation is usually not Trauma absent. Testing with pinprick is more sensitive than touch for detecting Diabetes radicular sensory loss. Relatively mild or recent-onset radiculopathy can Epidural abscess cause sensory changes without motor deficits. T1 radiculopathy can inter- Epidural metastases rupt the sympathetic pathway to the cervical sympathetic ganglia (see Fig- Meningeal carcinomatosis ure 6.13), resulting in Horner’s syndrome (see KCC 13.5). Involvement of Nerve sheath tumors (schwannomas multiple nerve roots below L1 can result in a cauda equina syndrome (see and neurofibromas) KCC 8.4). Guillain–Barré syndrome Common causes of radiculopathy are listed in Table 8.4. The most com- Herpes zoster (shingles) mon cause by far is intervertebral disc herniation, which occurs when part or Lyme disease all of the nucleus pulposus extrudes through a tear in the annulus fibrosus, Cytomegalovirus often causing root compression (see Figures 8.2C and 8.3). It usually occurs Idiopathic neuritis without any recent history of traumatic injury, but it can occasionally be caused or exacerbated by trauma. Disc herniation as a cause of radiculopa- thy is common for the C6, C7, L5, and S1 nerve roots and less common at other levels. Lumbosacral radiculopathies are about two to three times as common as cervical radiculopathies. Thoracic disc herniations are less com- mon, since this region of the spinal column is less mobile and fixed by the rib cage. Patients with intervertebral disc herniation typically present with back or neck pain, as well as sensorimotor symptoms in a radicular distribu- tion. As the spine degenerates over time, bony osteophytes form (see Table 8.3). Osteophytes, together with disc material, may contribute to narrowing of the intervertebral foramina or may protrude more centrally into the canal, causing spinal stenosis and chronic injury to the spinal cord (myelopathy). The straight-leg raising test can be helpful in the diagnosis of mechanical nerve root compression in the lumbosacral region (Figure 8.5A). In this test the patient lies supine and the examiner slowly elevates the patient’s leg at an increasing angle to the table while keeping the leg straight at the knee (B) FIGURE 8.5 Straight Leg Raising and Spine Percussion Tests (A) Straight leg raising or crossed straight leg raising may reproduce typical radicular symptoms. (B) Pain on per- cussion of the spine may indicate metastatic, infectious, or other disorders of the vertebral bones. (A) Pain on straight leg raising Pain on percussion of spine  Spinal Nerve Roots 333 joint. This provides traction on the nerve roots, and the test is considered positive if it reproduces the patient’s typical radicular pain and paresthesias. A response to less than 10° or more than 60°F of straight-leg raising is proba- bly not caused by root compression. In the crossed straight-leg raising test, el- evating the asymptomatic leg causes typical symptoms in the symptomatic leg. The crossed straight-leg raising test has a specificity of over 90% for lumbosacral nerve root compression. Radicular symptoms may also be in- creased by the Valsalva maneuver (e.g., coughing, sneezing, straining). In cervical radiculopathy, radicular symptoms may be increased by flexing or turning of the head toward the affected side, likely because of increased nar- rowing of the intervertebral foramina by these movements. Pain on percus- sion of the spine (Figure 8.5B) may indicate metastatic disease, epidural ab- scess, osteomyelitis, or other disorders of the vertebral bones, although this sign can be absent in these conditions. Back pain that is persistent, progressively worsens, or occurs in an older individual, in a patient with prior history of neoplastic disease, or where there is a possibility of epidural abscess, should always be evaluated with a neuroimaging study. An MRI of the spine is usually the test of choice (see Chapter 4). It is important, however, to carefully interpret the MRI in the context of the history and physical examination, since incidental disc bulges and other degenerative changes of the spine are common findings even in individuals without symptoms. In some cases, CT-myelography (see Chap- ter 4) can help define abnormalities that are not well visualized on MRI. When diagnostic uncertainty remains, EMG and nerve conduction studies (see KCC 9.2) may be helpful. Other causes of radiculopathy are listed in Table 8.4. Spinal stenosis, meaning “narrowing of the spinal canal,” can arise congenitally; gradually, as the result of degenerative processes; or by a combination of both factors. Lumbar stenosis may result in neurogenic claudication, in which bilateral leg pains and weakness occur with ambulation. Cervical stenosis can cause a mixture of radicular and long tract signs. Trauma produces radiculopathy through root compression, traction, or avulsion of nerve roots off the spinal cord. Diabetic neuropathy can occasionally involve nerve roots, particularly at thoracic levels, producing abdominal pain. Epidural metastases most com- monly occur in the vertebral bodies, but they can extend laterally to com- press nerve roots. Spread of cancer cells such as adenocarcinoma, lym- phoma, medulloblastoma, and glioblastoma within the cerebrospinal fluid can involve the nerve roots. Many causes of radiculopathy are similar to those causing neuropathy in general (see KCC 8.1) but may have an increased tendency to involve the nerve roots. For example, some autoimmune disorders, such as Guillain–Barré syndrome, have a predilection for nerve roots. Reactivation of latent varicella-zoster virus (chickenpox virus) in dorsal root ganglia pro- duces the painful blistering lesions of herpes zoster, or shingles. These occur in a dermatomal distribution, associated with sensory and, less commonly, motor loss in the affected nerve roots. Herpes zoster is most common in tho- racic dermatomes but can occur anywhere. Treatment with oral antiviral agents such as valacyclovir, famciclovir, or acyclovir can shorten the dura- tion of blistering lesions. Severe pain, referred to as postherpetic neuralgia, can persist after the blistering eruption and is shortened by treatment with antiviral treatment. When herpes zoster occurs in the ophthalmic division of the trigeminal nerve, it can threaten vision, so prompt treatment is critical. Lyme disease, a tick-borne illness caused by the spirochete Borrelia burgdor- feri, can cause radiculopathies. Cytomegalovirus polyradiculopathy can be seen in patients with HIV infection, most commonly in the lumbosacral 334 Chapter 8 roots. A milder form of radiculopathy can also be caused by HIV itself. C5 Dumbbell-shaped nerve sheath tumors, such as schwannomas and neurofi- C6 bromas (in neurofibromatosis), can occasionally occur in a neural foramen, producing radiculopathy.! C5–C6 Simplification: Three Nerve Roots to Remember in the Arm For practical purposes, the most clinically important nerve roots in the arm C7 are C5, C6, and C7. It is important to be familiar with the reflexes and the FIGURE 8.6 Three Roots to Remem- motor and sensory functions associated with these nerve roots, as summa- ber in the Arm C5 mediates arm ab- rized in Figure 8.6 and Table 8.5. When examining patients, it is helpful to duction at the shoulder; C5 and C6 me- have memorized at least one muscle that gets its major innervation from diate flexion at the elbow and the biceps each of these three nerve roots. In addition to the nerve roots listed in Table reflex; C6 mediates wrist extension; C7 8.5, it is also worth knowing that C8 radiculopathy accounts for about 6% of mediates elbow extension and the tri- cervical radiculopathies, is usually caused by C7–T1 disc herniation, and is ceps reflex (see also Table 8.5). associated with weakness of the intrinsic hand muscles and decreased sensa- tion in the fourth and fifth digits and the medial forearm. About 20% of all cervical radiculopathies involve two or more cervical levels. Simplification: Three Nerve Roots to Remember in the Leg The most clinically important nerve roots in the leg are L4, L5, and S1. Re- L4 flexes and the motor and sensory functions associated with L4, L5, and S1 are summarized in Figure 8.7 and Table 8.6. As with the cervical nerve roots, it is helpful, when examining patients, to have memorized at least one mus- cle that gets its major innervation from each of these nerve roots. L5 S1 KEY CLINICAL CONCEPT 8.4 CAUDA EQUINA SYNDROME Impaired function of multiple nerve roots below L1 or L2 is called cauda FIGURE 8.7 Three Roots to Remem- equina syndrome. If the deficits begin at the S2 roots and below, there may be ber in the Leg L4 mediates leg exten- no obvious leg weakness. Sensory loss in an S2 to S5 distribution (see Figure sion at the knee and the patellar tendon reflex; L5 mediates dorsiflexion at the 8.4) is sometimes called saddle anesthesia. Involvement of the S2, S3, and S4 ankle; S1 mediates plantar flexion at the nerve roots can produce a distended atonic bladder with urinary retention ankle and the Achilles tendon reflex (see or overflow incontinence (see KCC 7.5), constipation, decreased rectal tone, also Table 8.6). TABLE 8.5 Three Important Nerve Roots in the Arm APPROXIMATE REGION OF PERCENTAGE NERVE MAIN REFLEX SENSORY USUAL DISC OF CERVICAL ROOT WEAKNESSa DECREASEDa ABNORMALITYb INVOLVED RADICULOPATHIES C5 Deltoid, Biceps, Shoulder, upper C4–C5 7% infraspinatus, pectoralis lateral arm biceps C6 Wrist extensors, Biceps, First and second C5–C6 18% biceps brachioradialis fingers, lateral forearm C7 Triceps Triceps Third finger C6–C7 46% a See Figure 8.6. b See Figure 8.4.  Spinal Nerve Roots 335 TABLE 8.6 Three Important Nerve Roots in the Leg APPROXIMATE REGION OF PERCENTAGE NERVE MAIN REFLEX SENSORY USUAL DISC OF LUMBOSACRAL ROOT WEAKNESSa DECREASEDa ABNORMALITYb INVOLVED RADICULOPATHIES L4 Iliopsoas, Patellar tendon Knee, medial L3–L4 3%–10% quadriceps (knee jerk) lower leg L5 Foot dorsiflexion, None Dorsum of foot, L4–L5 40%–45% big toe extension, big toe foot eversion, inversion S1 Foot plantar Achilles tendon Lateral foot, L5–S1 45%–50% flexion (ankle jerk) small toe, sole a See Figure 8.7. b See Figure 8.4. fecal incontinence, and loss of erections. It is essential to detect and treat cauda equina syndrome promptly to avoid irreversible deficits. Cauda equina syndrome can sometimes be difficult to differentiate from conus medullaris syndrome, in which similar deficits occur as the result of a lesion in the sacral segments of the spinal cord (see Figure 8.1). Causes of cauda equina syndrome include compression by a central disc herniation (see Fig- ure 8.3C), epidural metastases, schwannoma, meningioma, neoplastic meningitis, trauma, epidural abscess, arachnoiditis, and cytomegalovirus polyradiculitis.! KEY CLINICAL CONCEPT 8.5 COMMON SURGICAL APPROACHES TO THE SPINE Most patients with radiculopathy caused by disc herniation recover within a few months without surgery. Indications for urgent surgery include the rare instances in which cord compression or cauda equina syndrome occurs. Semiurgent surgery is indicated in patients with progressive or severe motor deficits or in the occasional patient with intolerable, medically intractable pain. Elective surgery is contemplated when a clear radiculopathy is present and conservative measures such as rest, physical therapy, and traction have been tried for 1 to 3 months but were ineffective. In the cervical spine, surgical options include a posterior approach with laminectomy, meaning removal of the lamina over affected levels (see Figure 8.2B), combined with discectomy to remove herniated disc material, and foraminotomy to widen the lateral recess through which the nerve root passes just before it exits the intervertebral foramen. An anterior approach can also be used in the cervical spine. In this procedure, an incision is made in the anterior neck and the dissection is carried down to the vertebral bod- ies. The anterior approach provides direct access to the discs without tra- versing the spinal canal and also allows mechanical fusion of adjacent verte- bral bodies, usually using a bone graft. An anterior approach is also often favored in cases of thoracic disc herniation, which is rare. In the lumbar spine, a posterior approach is generally used. Sometimes a variety of hard- ware is implanted to increase mechanical stability.!