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. 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. Chapter 9 Major Plexuses and Peripheral Nerves A 3-week-old infant was not moving her left arm normally. She was a large baby and had endured significant traction on her left shoulder during delivery. Her left arm had decreased tone and appeared internally rotated. She was able to extend her left arm at the elbow and could open and close her hand, but she could not abduct her left arm at the shoulder or flex it at the elbow. The left biceps reflex was absent. In this chapter, we will learn the sensory and motor functions of the major nerves in the arms and legs. As we shall see, this patient’s symptoms are characteristic of injury to specific nerves. 360 Chapter 9 FIGURE 9.4 Lumbosacral Plexus: Sim- plified Schematic IlHyp = iliohypogas- tric nerve; IlIng = ilioinguinal nerve; GF = genitofemoral nerve; LFC = lateral femoral cutaneous nerve; F = femoral nerve; Obt = obturator nerve; Saph = saphenous nerve; SG = superior gluteal nerve; IG = inferior gluteal nerve; Sc = sciatic nerve; CP = common peroneal nerve; SP = superficial peroneal nerve; DP = deep peroneal nerve; T = tibial nerve; Sur = sural nerve; PFC = posterior femoral cutaneous nerve; Pud = puden- dal nerve. TABLE 9.1 Five Important Nerves in the Arm REGION OF SENSORY NERVE MOTOR FUNCTIONS LOSS WITH NEUROPATHY REVIEW EXERCISE Radial Extension at all arm, 1. Practice drawing the simplified wrist, and proximal finger Posterior cutaneous joints below the shoulder; schematic of the brachial plexus nerve of arm forearm supination; shown in Figure 9.2. thumb abduction in Posterior cutaneous plane of palm nerve of forearm 2. Practice drawing the simplified schematic of the lumbosacral Dorsal digital nerves (radial) plexus shown in Figure 9.4. Figure 9.5. The actual area of sensory loss following a nerve injury is some- Median Thumb flexion and Median nerve what smaller than the territories opposition, flexion of digits 2 and 3, wrist shown because of overlap from adja- flexion and abduction, cent nerves. Compare this figure to forearm pronation the dermatomal sensory distribution of nerve roots shown in Figure 8.4. Ulnar Finger adduction and abduction other than thumb; thumb adduction; Simplification: Five Nerves flexion of digits 4 and 5; to Remember in the Arm wrist flexion and adduction Ulnar nerve It is most clinically important to be fa- miliar with the functions of the radial, Axillary Abduction of arm at Axillary nerve median, ulnar, axillary, and musculocu- shoulder beyond first 15° taneous nerves in the arm. Table 9.1 summarizes the motor and sensory functions of these nerves, and they are demonstrated on neuroexam.com Musculo- Flexion of arm at Back Front Videos 54 and 55. Additional details cutaneous elbow, supination are found in Table 8.1. In general, the of forearm Lateral Lateral radial nerve is important for extension cutaneous cutaneous nerve of of all joints in the arm and proximal nerve of forearm forearm fingers, the median nerve is important for the thumb side of the hand and wrist, and the ulnar nerve is impor- tant for the pinky side of the hand and wrist. Note that (1) the sensory terri-  Major Plexuses and Peripheral Nerves 361 FIGURE 9.5 Regions of Sensory Innervation Supplied by Cutaneous Nerve Branches (After Devinsky O, Feldmann E. 1988. Examination of the Trigeminal nerve Cranial and Peripheral Nerves. Churchill Livingstone, New York.) Lateral cutaneous rami of thoracic Supraclavicular nerves nerves Axillary nerve Anterior cutaneous rami Medial cutaneous of thoracic nerves nerve of arm Lower lateral cutaneous nerve Iliohypogastric nerve of arm Medial cutaneous nerve of forearm Ilioinguinal nerve Lateral cutaneous nerve of forearm Radial nerve Genitofemoral nerve (femoral branch) Median nerve Genitofemoral nerve Ulnar nerve (genital branch) Lateral cutaneous Dorsal nerve nerve of thigh of penis Obturator nerve Perineal nerve (scrotal branch) Intermediate and medial cutaneous nerves of thigh (femoral) Saphenous nerve Lateral cutaneous nerves of calf (common peroneal) Superficial peroneal nerve Sural nerve Deep peroneal nerve Medial Medial and lateral calcaneal plantar nerves nerve tories shown in Table 9.1 are smaller than in Figure 9.5 because adjacent nerves overlap somewhat, and (2) Table 9.1 shows regions of sensory loss with nerve injury rather than the whole region innervated by the nerve. Finger flexion is best tested at the distal interphalangeal joints (see neuroexam.com Video 55), where the flexor digitorum profundus (median nerve for digits 2 and 3; ulnar nerve for digits 4 and 5) acts without significant contributions from other mus- cles (Table 9.2). 362 Chapter 9 TABLE 9.2 Muscles Contributing to Flexion and Extension at Finger Joints Other than the Thumb FLEXIONa EXTENSIONa MUSCLE NERVE MCP PIP DIP MCP PIP DIP Flexor digitorum profundus Median (2nd, 3rd digits), X X X ulnar (4th, 5th digits) Flexor digitorum superficialis Median X X Flexor digiti minimi (fifth digit) Ulnar X Lumbricals Median (2nd, 3rd digits), X X X ulnar (4th, 5th digits) Palmar and dorsal interossei Ulnar X X X Extensor digitorum Radial X X X Extensor indicis (second digit) Radial X X X Extensor digiti minimi (fifth digit) Radial X X X Note: Bold text indicates the most important muscles. a MCP, metacarpophalangeal joint; PIP, proximal interphalangeal joint; DIP, distal interphalangeal joint. Simplification: Three Nerves Acting on the Thumb Different thumb muscles are innervated by the radial, ulnar, and median nerves. It is easiest to remember these by the mnemonic RUM (Radial, Ulnar, Median), as shown in Figure 9.6. Thumb abduction in the plane of the palm (ab- ductor pollicis longus) is mediated by the Radial nerve, adduction (adductor pollicis) by the Ulnar nerve, and opposition (opponens pollicis) and flexion MNEMONIC (flexor pollicis longus and superficial head of the flexor pollicis brevis) by the Median nerve. It should also be recalled that thumb abduction perpendicular to the palm (see Table 3.4; neuroexam.com Video 55) is mediated by the abduc- tor pollicis brevis, which is innervated by the median nerve after it passes through the carpal tunnel. Intrinsic and Extrinsic Hand Muscles The intrinsic hand muscles include the muscles of the thenar eminence at the base of the thumb (opponens pollicis, abductor pollicis brevis, flexor pollicis bre- U vis, adductor pollicis), the muscles of the hypothenar eminence at the base of the pinky finger (opponens digiti minimi, abductor digiti minimi, flexor digiti R minimi), the lumbricals, and the interossei. Intrinsic hand muscles are inner- M vated by the ulnar nerve, except for the LOAF (Lumbricals I and II, Opponens MNEMONIC pollicis, Abductor pollicis brevis, Flexor pollicis brevis—superficial head) mus- cles, which are innervated by the median nerve after it passes through the carpal tunnel. All intrinsic hand muscles are supplied by C8 and T1 (see Table 8.1). In addition to the intrinsic hand muscles, extrinsic muscles in the forearm are important for finger movements (see Table 8.1). Intrinsic and extrinsic muscles contributing to flexion and extension at finger joints other than the thumb are FIGURE 9.6 Three Nerves Acting on summarized in Table 9.2. As we have already mentioned, it should be clear from the Thumb The radial nerve abducts this table that the flexor digitorum profundus (median nerve for digits 2 and 3; the thumb in the plane of the palm. The ulnar nerve adducts the thumb in the ulnar nerve for digits 4 and 5) is best tested at the distal interphalangeal joints, plane of the palm. The median nerve op- since other muscles participate in flexion at the other joints. Similarly, the ex- poses the thumb. Note also that the ab- tensor digitorum (radial nerve and C7) is best tested at the metacarpophalangeal ductor pollicis brevis (median nerve) joints (see Table 9.2). This is because other muscles, most notably the lumbricals, abducts the thumb perpendicular to the are predominantly responsible for finger extension at the proximal and distal in- plane of the palm (not shown). terphalangeal joints (median nerve for 2nd and 3rd digits; ulnar nerve for 4th  Major Plexuses and Peripheral Nerves 363 and 5th digits). See Tables 8.1 and 9.1 TABLE 9.3 Important Nerves in the Leg for muscles contributing to finger ad- duction, abduction, and opposition. MOTOR REGION OF SENSORY NERVE Note, for example, that the palmar in- NERVE FUNCTIONS LOSS WITH NEUROPATHY terossei adduct the fingers, while the Femoral Leg flexion dorsal interossei abduct them. at the hip, Femoral leg extension nerve at the knee Simplification: Five Nerves to Saphenous Remember in the Leg nerve It is most clinically important to be familiar with the functions of the femoral, obturator, sciatic, tibial, and peroneal nerves in the leg. Table 9.3 summarizes the motor and sensory functions of these nerves, and they Obturator Adduction of the thigh Obturator are demonstrated on neuroexam.com nerve Videos 56 and 57. Table 8.1 provides ad- ditional details. Note again that the sen- sory territories shown in Table 9.3 are smaller than in Figure 9.5, since here we are interested in regions of sensory loss. The tibial and common peroneal nerves are the two most important branches of the sciatic nerve. The ham- string muscles (semitendinosus, semi- Sciatic Leg flexion at membranosus, and biceps femoris) the knee (see are innervated by the sciatic nerve it- also tibial and self before it divides into the tibial and peroneal nerves, in column at left) Common common peroneal nerves. The com- peroneal Posterior mon peroneal nerve divides further to nerve tibial give rise to the superficial and deep Posterior tibial nerve peroneal nerves (see Figures 9.3 and Sural nerve nerve 9.4; Table 9.3). Tibial Foot plantar flexion and inversion, REVIEW EXERCISE toe flexion Posterior tibial nerve 1. Turn back to Tables 3.4–3.6 in Chapter 3, where we discussed strength and reflex testing (see Superficial Foot eversion also neuroexam.com Videos peroneal 54–58). In these tables, cover all columns except for the left-most Superficial column. For each action or reflex, peroneal list the appropriate muscle, nerve nerves, and nerve roots being tested (refer to Table 8.1). Deep Foot 2. In Tables 9.1 and 9.3, cover the peroneal dorsiflexion, Deep columns showing the regions of toe extension peroneal nerve sensory loss and sketch the region of sensory loss for each of the five nerves in the arm and the leg. 364 Chapter 9 KEY CLINICAL CONCEPT 9.1 COMMON PLEXUS AND NERVE SYNDROMES In Chapter 8, we introduced the general causes of neuropathy (see KCC 8.1). Here we focus on clinical localization of common mononeuropathies and plexus syndromes caused mainly by mechanical factors or diabetes. Note that in some cases trauma to a limb produces nerve injury, with the exact mechanism of such injury remaining unclear. Upper-Extremity Nerve Injuries BRACHIAL PLEXUS, UPPER TRUNK INJURY (ERB–DUCHENNE PALSY) Common causes include traction on an infant’s shoulder during a difficult delivery as well as motorcycle accidents. Damage to the upper trunk of the brachial plexus (see Figures 9.1 and 9.2) causes loss of function in C5- and C6-inner- vated muscles, resulting in prominent weakness of the deltoid, biceps, infra- spinatus, and wrist extensors (see Table 8.1). The arm assumes a characteris- tic “bellman’s tip” or “waiter’s tip” pose, held at the side, internally rotated, and with the wrist flexed (Figure 9.7). Finger and hand movements are rela- tively spared. Most infants recover fully, but prognosis depends on the sever- ity of the injury. Surgical repair of the plexus is occasionally pursued. Differ- ential diagnosis includes traumatic avulsion of the C5 and C6 nerve roots, or other causes of a C5 and C6 radiculopathy (see KCC 8.3). BRACHIAL PLEXUS, LOWER TRUNK INJURY (KLUMPKE’S PALSY) Common causes include upward traction produced by grabbing a branch during a fall from a tree, thoracic outlet syndrome, and Pancoast’s syndrome. Damage to the lower trunk of the brachial plexus (see Figures 9.1 and 9.2) causes weakness of C8- and T1-innervated muscles, resulting in hand and finger weakness, and atrophy of the hypothenar muscles, together with sensory loss on the ulnar aspect of the hand and forearm. If the T1 nerve root is damaged proxi- mal to the sympathetic trunk (see Figure 6.13), there may also be an associ- ated Horner’s syndrome (see KCC 13.5; see also Figure 13.10). Differential diagnosis includes ulnar neuropathy or C8–T1 radiculopathies. In thoracic outlet syndrome, the lower brachial plexus is compressed as it passes between the clavicle and the first rib (see Figure 9.1). Symptoms may be increased by raising and external rotation of the arm, which may also de- crease brachial arterial pulses. EMG and X-rays (looking for a cervical rib or other bony abnormalities) are important diagnostically. Treatments include exercises to strengthen shoulder muscles and surgical decompression for well-documented refractory cases. Treatment for equivocal cases has been a source of controversy. In Pancoast’s syndrome, an apical lung tumor (usually non-small cell car- cinoma) extends into the lower brachial plexus. In addition to lower-plexus signs (sometimes including Horner’s syndrome), the recurrent laryngeal nerve is occasionally involved as it loops downward into the thorax, produc- ing hoarseness (see KCC 12.8). Ultimately, the entire brachial plexus may be invaded, producing a flail, insensate upper extremity. AXILLARY NEUROPATHY Dislocation or fracture of the proximal humerus can compress the axillary nerve, causing deltoid weakness and numbness in the shoulder (see Table 9.1). Differential diagnosis includes C5 radiculopathy (although axillary neuropathy does not involve the biceps, while C5 radicu- lopathy does). FIGURE 9.7 “Bellman’s,” or “Waiter’s BRACHIAL PLEXITIS (BRACHIAL NEURITIS, PARSONAGE-TURNER SYNDROME) This Tip,” Pose Assumed in Upper-Plexus is a disorder of unknown, possibly inflammatory cause with onset in adult- Lesions  Major Plexuses and Peripheral Nerves 365 hood and slightly more commonly in males, of burning shoulder or lateral (A) (B) neck pain followed by weakness of muscles innervated by the brachial plexus. Most patients recover fully within 6 to 12 weeks of onset. RADIAL NEUROPATHY Common causes include sleeping with the arm slung over a park bench (“Saturday night palsy”), compression in the axilla by im- proper crutch use (“crutch palsy”), or fracture of the humerus damaging the nerve as it travels in the spiral groove. There is weakness of all extensors of Wrist drop Preacher’s hand the arm, hand, and fingers below the shoulder, weakness of forearm supina- (radial palsy) (median palsy) tion, loss of the triceps reflex, and sensory loss in a radial nerve distribution (see Table 9.1). A wrist drop is often present (Figure 9.8A). The triceps may be (C) (D) spared, depending on how distal the lesion is in the arm. The posterior in- terosseous nerve is a purely motor branch of the radial nerve. Trauma or en- trapment of the posterior interosseous nerve results in weakness of radial nerve-innervated muscles, sparing the triceps (see Table 8.1) and with no sensory loss. Tight wrist bands or handcuffs can sometimes compress the su- perficial branch of the radial nerve, causing isolated sensory loss in the dor- sal lateral hand called cheiralgia paresthetica, or handcuff neuropathy. Ulnar claw Simian hand (ulnar palsy) (median and ulnar palsy) MEDIAN NEUROPATHY Causes include sleeping with a lover’s head resting on the upper arm (“honeymooner’s palsy”). Fractures of the humerus or dis- Sensory loss Atrophy tal radius can occasionally injure the median nerve. In addition, entrapment (see KCC 8.1) can occur as the nerve passes through the pronator teres mus- FIGURE 9.8 Classic Hand Poses in Le- cle in the forearm. There is weakness of wrist flexion and abduction, opposi- sions of the Radial, Median, and Ulnar Nerves (A) “Wrist drop” in radial nerve tion of the thumb, and flexion of the second and third digits, together with lesions, resulting from weakness of wrist sensory loss in a median nerve distribution (see Table 9.1). In an attempt to extensors. (B) “Preacher’s hand,” or “or- make a fist, the hand may assume a “preacher’s hand” or “orator’s hand” ator’s hand,” in proximal median nerve pose (Figure 9.8B). lesions, resulting from weakness of the CARPAL TUNNEL SYNDROME This common entrapment syndrome is caused hypothenar and first two flexor digito- by compression of the median nerve as it passes together with the tendons rum profundus muscles when the pa- of the hand under the flexor retinaculum on the flexor surface of the wrist. It tient is asked to make a fist (flex fingers). (C) “Ulnar claw,” or “benediction pos- is seen most often in women over age 30 and can be associated with activi- ture,” in chronic ulnar nerve lesions, re- ties such as typing or housepainting that can cause repetitive stress injury, sulting from weakness of the lumbricals edema, and inflammation in the wrist. Other causes include pregnancy, oral (see Table 9.2) for digits 4 and 5 when contraceptives, hypothyroidism, arthritis, wrist fracture, acromegaly, ure- the patient is asked to extend the fingers. mia, diabetes, and amyloidosis. Recall that after the median nerve passes (D) “Simian hand” in chronic median through the carpal tunnel, it innervates the LOAF (Lumbricals I and II, Op- plus ulnar nerve lesions, resulting from ponens pollicis, Abductor pollicis brevis, Flexor pollicis brevis—superficial thenar and hypothenar atrophy and loss head) muscles. The best muscle to test in suspected carpal tunnel syndrome of thumb opposition. is the abductor pollicis brevis, which abducts the thumb perpendicular to the plane of the palm (see neuroexam.com Video 55), although thumb flexion and opposition (see Figure 9.6) may also be weak. Symptoms often include MNEMONIC sensory loss in the first, second, and third digits, as well as prominent pares- thesias (see KCC 7.1) that are most bothersome at night and can sometimes radiate into the upper arm. Patients often report shaking the hand to try and relieve symptoms (flick sign). In advanced cases, thenar atrophy may be present. Wrist flexion, flexion of the second and third digits, and sensation over the thenar eminence are typically spared, since nerves providing these functions branch off proximal to the carpal tunnel. M >r> ah en m tereu cn d o < so n Ii Differential diagnosis of carpal tunnel syndrome includes radiculopathy of C6 and C7 or compression of the median nerve proximal to the carpal tunnel, although these conditions usually include more proximal symptoms. Tests to provoke paresthesias in a median nerve distribution may be helpful, but they are not very sensitive or specific. These include Tinel’s sign, in which the me- dian nerve is percussed in the carpal tunnel, and Phalen’s sign, in which the dorsal surfaces of the hands are pressed together, flexing the wrists for about 366 Chapter 9 1 minute. Treatments include immobilization by a removable wrist splint, steroid injections, and surgical decompression of the carpal tunnel. ULNAR NEUROPATHY The medial epicondyle of the elbow carries the name “funny bone” because mild trauma to the ulnar nerve as it passes over the elbow in the ulnar groove (between the olecranon and medial epicondyle) produces transient paresthesias (see KCC 7.1) in an ulnar distribution. A com- mon cause of ulnar neuropathy is entrapment (see KCC 8.1) at the elbow in the cubital canal, which lies in the region of the ulnar groove. This condition is sometimes called tardy ulnar palsy, a delayed result of a posttraumatic, de- generative, or congenital increased carrying angle at the elbow. The ulnar nerve can also be damaged acutely by fractures of the medial epicondyle, or it may be compressed by a habit of resting the elbows on a hard table. Findings include weakness of wrist flexion and adduction, finger adduction and abduction, and flexion of the fourth and fifth digits, together with sensory loss and paresthesias in an ulnar distribution (see Table 9.1). As with most neu- ropathies, motor findings may be absent in mild cases. Severe cases may in- clude atrophy and fasciculations in the hypothenar eminence. Because of weak lumbricals at the fourth and fifth digits, these fingers may assume a character- istic “ulnar claw” or “benediction posture” (Figure 9.8C). Differential diagnosis includes C8 and T1 radiculopathy, Pancoast’s syndrome, thoracic outlet syn- drome, or other lesions of the brachial plexus inferior trunk or medial cord (see Figures 9.1 and 9.2). Unlike ulnar neuropathy, these conditions sometimes pro- duce a Horner’s syndrome, sensory changes in the T1 dermatome of the upper medial arm (see Figure 8.4) or involvement of hand muscles innervated by the median nerve. Entrapment in the cubital canal at the elbow can be treated sur- gically by translocation of the ulnar nerve to the flexor side of the elbow. Compression of the ulnar nerve in the hand as it passes over the hamate bone in Guyon’s canal can occur from prolonged leaning forward while cy- cling. The result is weakness of finger adduction and abduction without sen- sory loss because the cutaneous branches of the ulnar nerve are given off more proximally. Combination of chronic median and ulnar nerve lesions leads to thenar and hypothenar atrophy with lack of thumb opposition, resulting in a “simian hand,” or “monkey’s paw” (Figure 9.8D). Lower-Extremity Nerve Injuries FEMORAL NEUROPATHY The femoral nerve can occasionally be injured dur- ing pelvic surgery or compressed by a retroperitoneal hematoma or pelvic mass. Abnormalities include weakness of thigh flexion and knee extension, loss of the patellar reflex, and sensory loss in the anterior thigh (see Table 9.3). Differential diagnosis includes L3 or L4 radiculopathy. L3 or L4 radicu- lopathy, however, may include weakness of thigh adduction (obturator nerve), a feature not associated with femoral neuropathy (see Table 8.1). SCIATIC NEUROPATHY Causes of sciatic neuropathy include posterior hip dis- location, acetabular fracture, and intramuscular injection placed too medially and inferiorly in the buttocks. There is weakness of all foot and ankle muscles and of knee flexion, loss of the Achilles tendon reflex, and sensory loss in the foot and lateral leg below the knee (see Table 9.3). Differential diagnosis in- cludes lesions in the foot area of the motor cortex (see KCC 6.3; Figure 6.14F). The term “sciatica” is vague and refers to all disorders causing painful paresthesias in a sciatic distribution. The most common cause is compres- sion of lumbosacral roots by disc material and osteophytes (see KCC 8.3). Rarely, the sciatic nerve may be compressed more distally by muscular or skeletal elements.  Major Plexuses and Peripheral Nerves 367 PERONEAL NERVE PALSY As the common peroneal nerve passes around the fibular head near the skin surface, it is vulnerable to laceration, stretch injury by forcible foot inversion, or compression by tight stockings, a cast, crossed legs, or trauma. In peroneal nerve palsy, there is foot drop, with weakness of foot dorsiflexion and eversion, and sensory loss over the dorsolateral foot and shin. Most patients recover spontaneously when the mechanical cause is re- moved. A foot brace may improve function if foot drop is significant. Differen- tial diagnosis includes L5 radiculopathy. However, L5 radiculopathy includes weakness of foot dorsiflexion, eversion, and inversion, while in peroneal palsy, foot inversion is normally spared because this function can be carried out by the tibialis posterior (innervated by the tibial nerve) (see Table 9.3; see also Table 8.1). OBTURATOR NERVE PALSY The obturator nerve (originating from L2–L4; see Figure 9.3 and Table 9.3) can be compressed in women during complicated delivery or occasionally during pelvic trauma or surgery. Deficits include gait instability due to weakness of the leg adductor muscles and pain and numbness in the medial thigh. MERALGIA PARESTHETICA The lateral femoral cutaneous nerve (which origi- nates in L2 and L3; see Figures 9.3 and 9.4) can be entrapped as it passes under the inguinal ligament or fascia lata, producing paresthesias and loss of sensation in the lateral thigh (see Figure 9.5). This entrapment syndrome includes no motor involvement or reflex changes. Common causes include obesity, pregnancy, weight loss, or heavy equipment belts, and symptoms may be worse after prolonged walking, standing, or sitting. Differential di- agnosis includes L2 or L3 radiculopathy, although, unlike meralgia pares- thetica, these conditions usually include motor changes or decreased patel- lar reflex. Symptoms most often resolve spontaneously or by avoidance of mechanical precipitants; however, surgical decompression is occasionally at- tempted in refractory cases. MORTON’S METATARSALGIA Tight-fitting shoes can compress the digital nerves, especially of the third and fourth toes, producing patches of numb- ness and paresthesias. In conclusion, familiarity with the patterns of sensory and motor loss in the common plexus and nerve syndromes discussed in this section, as well as those seen in radiculopathies (see KCC 8.3) and other disorders of nerves, muscles, and the neuromuscular junction (see KCC 8.1), can greatly aid in localizing neurological deficits and in distinguishing disorders of the periph- eral and central divisions of the nervous system. When the diagnosis re- mains uncertain, electrodiagnostic tests can often be helpful, as discussed in the next section.! KEY CLINICAL CONCEPT 9.2 ELECTROMYOGRAPHY (EMG) AND NERVE CONDUCTION STUDIES Electromyography (EMG) and nerve conduction studies are valuable diagnos- tic tools that can help localize and determine the causes of nerve and muscle disorders. In nerve conduction studies, stimulating electrodes are placed on the skin overlying a nerve, and recording electrodes are placed either at a different point along the same nerve or overlying a muscle innervated by the nerve (Figure 9.9A–C). When a stimulus is given to the nerve, a compound motor ac- tion potential (CMAP) can be recorded over the belly of a muscle innervated by that nerve, resulting from the summated electrical activity of muscle cells (Fig- ure 9.9D). If a distal nerve branch with purely sensory function is used for 368 Chapter 9 (A) (B) (C) (D) Stimulation site: 2.9 a ms Wrist 7 cm 5.6 ms 67 m/s Below elbow b 25 cm 7.3 ms 65 m/s Above elbow c 36 cm 8.7 ms 71 m/s Axilla d 46 cm Erb’s point e 11.6 ms 72.5 m/s (brachial plexus) 67 cm a bc d e 10 mV 2 ms FIGURE 9.9 Nerve Conduction Study The compound motor action potential (CMAP) of the ulnar nerve was recorded recording or stimulation and a second set of stimulating or recording elec- in this example from a normal subject at trodes is placed somewhere along the nerve, a compound sensory nerve action several stimulus locations. (A) Stimula- potential (SNAP) can be recorded over the nerve, resulting from summated tion of the ulnar nerve at the wrist and electrical activity in the sensory neuron axons of the nerve. recording of CMAP from the abductor Lesions proximal to the dorsal root ganglia will leave the cell bodies and digiti quinti in the hypothenar eminence. distal axons of sensory neurons intact (see Figure 8.1B). Therefore, SNAPs will (B) Ulnar nerve stimulation just distal to the ulnar groove. (C) Ulnar nerve stimu- be preserved. In contrast, proximal lesions of motor nerve roots will cause de- lation just proximal to the ulnar groove. generation of the distal motor neuron axons, reducing or abolishing CMAPs. (D) CMAP recordings for different stim- There are standard values for SNAP and CMAP latencies or conduction veloc- ulation sites. For each trace, the distance ities for each major nerve when stimulated at various points along its course. from the recording electrode (in centime- These values allow nerve conduction studies to be used to determine if there ters) and the latency to CMAP onset (in is evidence for slowed nerve conduction—for example, in the case of demyeli- milliseconds) are indicated to the left. nation (see KCC 8.1). In addition, there are standard values for SNAP ampli- Conduction velocities between adjacent tudes. Decreased SNAP amplitudes suggest that conduction in some axons of stimulation sites (in meters per second) the nerve has been interrupted, as is the case in axonal damage. were calculated by dividing the distance CMAP studies can be used to evaluate the function of the neuromuscular between the two stimulation sites by the junction by the use of repetitive stimulation. Slow, repetitive stimulation (2–3 difference in motor latencies for the two sites. (After Rajesh KS, Thompson LL. Hz) depletes presynaptic stores of acetylcholine; faster repetitive stimulation 1989. The Electromyographer’s Handbook. (>5 Hz) increases presynaptic calcium, facilitating neurotransmitter release. 2nd Ed. Little, Brown, Boston.) Under normal conditions, repetitive stimulation does not significantly affect  Major Plexuses and Peripheral Nerves 369 CMAP amplitude because there is a “safety factor,” meaning that every presynaptic action potential results in a postsynaptic potential well above the threshold needed to produce a muscle cell action potential. Under pathologic conditions, however, failures in neuromuscular transmission occur. There- fore, for example in myasthenia gravis (see KCC 8.1), in which there is a de- crease in postsynaptic acetylcholine receptors on muscle cells, slow repetitive stimulation results in a gradual decrement in CMAP amplitude. Decrement of >10% is considered abnormal. In Lambert–Eaton myasthenic syndrome and in botulism, in which there is decreased presynaptic neurotransmitter release, fast repetitive stimulation (or active volitional muscle contraction) causes CMAPs to increment in amplitude from an abnormally low starting point. In electromyography (EMG), an electrode is inserted directly into a muscle, and motor unit action potentials (MUPs) are recorded from muscle cells. The EMG pattern provides information useful in distinguishing weakness caused by neuropathic disorders (nerve or motor disease) from that caused by myo- pathic disorders (muscle disease). In neuropathic disorders, increased sponta- neous activity (fibrillation potentials and positive sharp waves) often is recorded on EMG and is sometimes also visible on physical examination as fas- ciculations (see KCC 6.1). Fasciculations and other forms of spontaneous activ- ity can occur due to chronic deinnervation of muscle cells. Deinnervation also causes adjacent motor axons to sprout and reinnervate a larger region, result- ing in abnormally large motor units (a motor unit consists of all the muscle cells innervated by a single motor neuron axon). Therefore, with neuropathic disorders, MUPs are of abnormally large amplitude and duration. Reduced MUP amplitude and duration suggests a myopathic disorder is present. When a muscle is voluntarily contracted, the EMG normally shows a pat- tern of continuous firing of motor units referred to as a normal recruitment pattern. In neuropathic disorders, the recruitment pattern has normal ampli- tude but shows interrupted firing, since some motor units are not success- fully activated. This phenomenon is referred to as decreased, reduced, or in- complete recruitment. In myopathic disorders, the recruitment pattern is continuous or even increased (since more motor units need to be activated for a given force), but the amplitude is often decreased.! CLINICAL CASES CASE 9.1 COMPLETE PARALYSIS AND LOSS OF SENSATION IN ONE ARM CHIEF COMPLAINT PHYSICAL EXAMINATION A 60-year-old man with a history of lung cancer gradually de- Vital signs: T = 99.4°F, P = 110, BP = 130/80. veloped severe pain, weakness, and numbness in his right arm. Neck: Supple; no tenderness. HISTORY Lungs: Clear. The patient smoked for 34 years. Two years ago he was diag- Heart: Regular rate with no murmurs, gallops, or rubs. nosed with lung cancer and underwent a right upper-lobe Abdomen: Normal bowel sounds; soft, no masses. lung resection followed by radiation and chemotherapy. Six Extremities: Right arm swollen, with two firm, 5 cm discolored months ago he developed shooting pain and swelling of the masses—one in the right axilla and one in the upper right right arm. He gradually lost all strength and sensation in chest wall. Also marked clubbing of the fingers bilaterally. the entire right arm up to the shoulder but continued to have severe burning pain. Past medical history was notable for right eye surgery following an assault with a baseball bat 20 years ago. (continued on p. 370) 370 Chapter 9 CASE 9.1 (continued) Neurologic exam: LOCALIZATION AND DIFFERENTIAL DIAGNOSIS MENTAL STATUS: Alert and oriented × 3. 1. On the basis of the symptoms and signs shown in bold CRANIAL NERVES: Intact, except for the right eye, which had above, where is the lesion? an irregular pupil and diminished vision in both the lat- 2. What is the most likely diagnosis? eral and medial fields. 3. This patient had an abnormal right eye due to prior MOTOR: Normal tone except for the right arm, which was trauma. If his right eye had been normal previously, flaccid. 5/5 power throughout, except for 0/5 power in what additional finding might be present on exam that the right shoulder, arm, and hand. would help with the localization? REFLEXES: 0 2+ 0 2+ FIGURE 9.10 Region of 0 2+ Sensory Loss Compare to Figures 8.4 and 9.5. 2+ 2+ 2+ 2+ COORDINATION: Normal on finger-to-nose (except unable to test right arm) and heel-to-shin testing. GAIT: Normal. SENSORY : Absent light touch, pinprick, and vibration sense in the entire right arm up to the deltoid (Figure 9.10). Sensation otherwise normal. Discussion 1. The key symptoms and signs in this case are: Paralysis, with decreased tone and absent reflexes in the entire right arm and hand Absent light touch, pinprick, and vibration sense in the entire right arm up to the deltoid Pain and swelling in the right arm Weakness in one arm can be caused by peripheral nerve lesions or by lesions in the arm region of the motor cortex (see KCC 6.3; Figure 6.14E). However, it is unlikely that a cortical lesion would produce complete paral- ysis and sensory loss in the entire arm ending sharply at the shoulder, with no face or leg weakness at all. In addition, no single peripheral nerve lesion could produce this pattern. Therefore, the lesion must involve the entire right brachial plexus, or all right-sided nerve roots from C5 through T1. 2. The history of right apical lung tumor supports the possibility of a lesion invading the right brachial plexus from below (see the description of Pan- coast’s syndrome in KCC 9.1), as does the presence of swelling in the arm, suggesting obstruction of venous return. Because of the past history of ra- diation therapy, radiation plexitis is another possibility, in which numbness and sometimes weakness can develop in a limb months to years after treat- ment due to radiation-induced nerve injury.  Major Plexuses and Peripheral Nerves 371 3. A lesion of the proximal portion of the lower brachial plexus involving the T1 nerve root can cause a Horner’s syndrome (see Figures 6.13, 13.10; KCC 13.5). This condition could be difficult to appreciate in this patient because of his prior history of right eye trauma. Clinical Course and Neuroimaging A brachial plexus MRI (Image 9.1, page 372) showed extensive invasion of the apical lung mass into the region of the right brachial plexus. The cancer in this patient, unfortunately, was no longer amenable to treatment. However, his pain was managed by a multidisciplinary team using oral, intravenous, and epidural medications to provide adequate pain relief. CASE 9.2 A NEWBORN WITH WEAKNESS IN ONE ARM MINICASE A 3-week-old infant girl was brought to the pediatrician be- rotated at the infant’s side with decreased spontaneous cause of left arm weakness. She was born at 42 weeks (2 weeks movements. She did not abduct the left arm or flex it at the past the due date) weighing 10 pounds 11 ounces, and the de- elbow but did have spontaneous opening and closing of the livery was complicated by shoulder dystocia (difficulty deliv- hand with normal grip strength, normal elbow extension, and ering the shoulder) resulting in significant traction on the left some wrist flexion. The left biceps reflex was absent, and neck and shoulder during delivery. Left arm weakness was other reflexes were 2+ throughout. noted at birth that improved slightly but was still present at LOCALIZATION AND DIFFERENTIAL DIAGNOSIS the appointment. Exam was normal except for the left up- per extremity, which had decreased tone and lay internally On the basis of the symptoms and signs shown in bold above, where is the lesion? What is the most likely diagnosis? Discussion The key symptoms and signs in this case are: Weakness of left arm external rotation, abduction, and elbow flexion, with decreased tone and absent biceps reflex The patient has typical findings consistent with a left brachial plexus upper trunk injury (Erb–Duchenne palsy), affecting C5 and C6 innervated muscles, caused by left shoulder traction at birth (KCC 9.1). Clinical Course A physical therapy program was initiated to preserve range of motion dur- ing recovery. At age 7 weeks the patient was able to lift her arm off the table and had some external rotation of the arm, as well as slight spontaneous elbow flexion. The left biceps reflex was still absent. By age 4 months she was able to reach for objects well with either hand, although she preferred to use her right hand, and she had 4+/5 left biceps strength when pulled to a seated position. Continued improvement was anticipated. 372 Chapter 9 CASE 9.1 COMPLETE PARALYSIS AND LOSS OF SENSATION IN ONE ARM IMAGE 9.1 Right Apical Lung Cancer Extending into the Region of the Brachial Plexus T1-weighted coronal MRI scan of the chest. Region of left Tumor brachial plexus Left lung R L CASE 9.3 A BLOW TO THE MEDIAL ARM CAUSING HAND WEAKNESS AND NUMBNESS MINICASE A 38-year-old alcoholic man was seen to fall, catching his right arm on a garbage can. He was brought to the emergency room. He had an abrasion and tenderness of the right up- per medial arm. Neurologic exam was normal except for 4/5 strength of right thumb opposition, second and third fin- ger flexors, and wrist flexion and abduction. There was also decreased pinprick and light touch sense along the lateral surface of the right hand and first, second, and third fin- gers (Figure 9.11). LOCALIZATION AND DIFFERENTIAL DIAGNOSIS On the basis of the symptoms and signs shown in bold above, where is the lesion? What is the most likely diagnosis? FIGURE 9.11 Region of Sensory Loss