Spinal Nerve and Root

Anatomy of the Spinal Nerves and Roots

The afferent (sensory) fibers (Fig. 4.1) from the peripheral nervous system enter the spinal cord in the dorsal roots and have their perikarya in the dorsal spinal root ganglia. The dorsal roots enter the cord in the dorsolateral sulcus. The efferent (motor) fibers arise from the motor neurons located in the ventral horns of the spinal cord and exit the cord as the ventral roots. The ventral and dorsal roots unite to form the mixed spinal nerve, which then travels through the intervertebral foramen. After emerging from the foramen, the spinal nerve divides into anterior and posterior primary rami. The smaller posterior primary rami supply the skin on the dorsal aspect of the trunk with sensory fibers and also send motor fibers to the longitudinal muscles of the axial skeleton. The anterior primary rami supply the limbs (see Chapter 3), nonaxial skeletal muscles, and skin of the lateral and anterior trunk and neck (by way of the lateral cutaneous and anterior cutaneous branches, respectively). The anterior primary rami also communicate with the sympathetic ganglia through white and gray rami communicantes.

Principles of Spinal Nerve and Root Localization

The identification of spinal nerve lesions requires a precise knowledge of each group of muscles supplied by a single anterior spinal root (myotome) and each cutaneous area supplied by a single posterior spinal root (dermatome) (Fig. 4.2). Differentiation from peripheral nerve or plexus lesions thereby depends on the segmental character of the sensory and motor signs and symptoms.

Sensory Symptoms

Irritative lesions of a dorsal root result in radicular pain or root pain, which has a characteristic lancinating, electric, or burning quality. This pain is abrupt, sharp, well localized, referred to a specific dermatome or myotome, and characteristically accentuated or precipitated by maneuvers that cause increased intraspinal pressure or stretching of the dorsal nerve root (e.g., coughing, straining, sneezing, Valsalva’s maneuver, or spine movements). Pain is often the first manifestation of a sensory radiculopathy and may be associated with paresthesias or dysesthesias in the area involved.

Destructive dorsal root lesions result in hypesthesia or anesthesia that is confined to the specific dermatome involved. Because of the overlap of cutaneous supply by adjacent nerve roots, sectioning of a single dorsal root results in little or no sensory loss. Therefore, the absence of sensory loss does not exclude the possibility of a lesion affecting a single dorsal root. When multiple dorsal root lesions are present, sensory loss is evident, the area of analgesia being larger than the area of anesthesia to light touch.

Motor Signs

Ventral root lesions result in weakness and atrophy in the myotomal distribution of the affected root. Fasciculations may be evident in the affected muscle.

Reflex Signs

Lesions of the dorsal or ventral root may interrupt the afferent or efferent arc, respectively, of a specific muscle stretch reflex. Therefore, with ventral or dorsal lesions, hypo- or areflexia occurs in the muscle subserved by the affected spinal root.

Etiologies of Spinal Nerve and Root Lesions

The spinal roots may be injured by direct (e.g., missile or penetrating wounds) or indirect (e.g., spinal traction) trauma and are frequently compressed by lesions in and about the intervertebral foramina (e.g., disc disease, spondylosis, a hypertrophied ligamentum flavum, or primary or metastatic tumors of the vertebrae or spinal nerves). The most common disc prolapse in the cervical region is at the C6–C7 interspace, resulting in signs and symptoms of C7 root involvement [14,16,25]. In the lumbar region, the most common disc prolapse is at the L4–L5 or L5–S1 level, resulting in signs and symptoms referable to the L5 or S1 roots, respectively. The neurologic signs and symptoms noted with irritation or damage of individual nerve roots by disc prolapse are outlined in Table 4.1.

A lumbosacral radiculopathy, most often involving the L2 level, and a polyradiculoneuropathy may occur as a complication of epidural analgesia or anesthesia [39]. A postradiation lumbosacral radiculopathy may occur years after irradiation of paraaortic lymph nodes in patients with neoplasms, especially testicular cancer [4]. These patients develop a motor disorder predominantly affecting the legs with mild sensory and sphincter abnormalities.

Certain generalized peripheral nervous system diseases have a predilection for the spinal roots (e.g., Guillain–Barré syndrome). Herpes zoster typically occurs in the distribution of sensory dermatomes, most often at a thoracic level [5,34]. Unilateral or bilateral radiculopathies may occur with Lyme disease, especially affecting the fifth cervical dermatome or lower thoracic levels [7,8,21]. Diabetes may cause thoracic root pain [10] or thoracoabdominal neuropathy [32,33], presenting with severe abdominal or chest pain, often not radicular in character. The presence of dysesthesias and abnormal findings on sensory examination of the trunk aid in the diagnosis of these diabetic neuropathies. Diabetic truncal neuropathy may result in sensory changes in a complete dermatomal band, in multiple dermatomal levels, in the distribution of the ventral or dorsal rami of the spinal nerves or branches of these rami, or in varying combinations of these distributions [32]. Diabetic truncal neuropathy may rarely present with focal, unilateral protrusion of the abdominal wall (pseudohernia), which may be associated with spontaneous, burning abdominal pain and hyperpathia or which may be painless [22,36].

Patients with acquired immunodeficiency syndrome may develop a distinctive syndrome of rapidly progressive flaccid paraparesis and areflexia that is frequently associated with sphincter disturbances [31]. This acute lumbosacral polyradiculopathy may have multiple causes, including cytomegalovirus infection, metastasis from systemic lymphoma, or unknown causes [2,6,19,29,31].

The Localization of Nerve Root Syndromes

Lesions Affecting the Cervical Roots

Lesions affecting the spinal nerves and roots [37,38] give rise to motor and sensory segmental defects and characteristic disturbances in muscle stretch reflexes. Each cervical segment is considered in more detail in this section. The individual spinal nerve root syndromes discussed are theoretical because clinical practice often presents lesions that affect multiple segments.

LESIONS AFFECTING C1

Because there is no dorsal root from C1, lesions of this root result in purely motor symptoms. This root supplies muscles that support the head, fix the neck, assist in neck flexion and extension, and tilt the head to one side. These, include the longus capitis, rectus capitis, obliquus capitis, longissimus capitis and cervicis, multifidi, intertransversarii, rotatores, semispinalis, and infrahyoid muscles. C1 lesions usually result in minor motor difficulties.

LESIONS AFFECTING C2

Sensory symptoms and signs due to C2 lesions are localized to the scalp posterior to the interaural line (the C2 dermatome). The motor supply of this segment involves the same muscles responsible for head and neck movements as those innervated by segment C1. In addition, the C2 nerve helps supply the sternocleidomastoid muscle (head rotation and flexion), which is predominantly innervated by the spinal accessory nerve (cranial nerve XI).

LESIONS AFFECTING C3

Sensory disturbances occur on the lower occiput, the angle of the jaw, and the upper neck [24]. Paresis may occur in the scalene and levator scapulae muscles of the neck (including the infrahyoids, semispinalis capitis and cervicis, longissimus capitis and cervicis, intertransversarii, rotatores, multifidi), and in the trapezius (shoulder elevation), this last muscle being predominantly innervated by the spinal accessory nerve (cranial nerve XI). Diaphragmatic paresis may also result because the phrenic nerve receives some of its fibers from the C3 segment.

Irritation of the C3 nerve root may cause a painful, burning, red ear (red ear syndrome) [12]. The increased ear temperature may be caused by antidromic release of vasodilator peptides. This red ear syndrome may also occur with temporomandibular joint dysfunction and with thalamic lesions [12].

LESIONS AFFECTING C4

Sensory signs and symptoms occur on the lower neck. Paresis occurs in the scalene and levator scapulae muscles (lateral neck flexion and scapular rotation, respectively), rhomboid muscles (scapular elevation and adduction), trapezius muscle (shoulder elevation), and some muscles of the neck. Diaphragmatic paresis may also occur because some fibers reach the phrenic nerve. There is no reflex impairment.

LESIONS AFFECTING C5

C5 nerve root involvement results in neck, shoulder, and upper anterior arm pain. Sensory disturbances occur on the lateral arm with these lesions. Paresis occurs predominantly and variably in the following muscles: levator scapulae, rhomboids, serratus anterior, supraspinatus, infraspinatus, deltoid, biceps, and brachioradialis (for methods of examination of each of these muscles, see Chapter 2). Diaphragmatic paresis may rarely occur owing to C5 fibers reaching the phrenic nerve. The biceps reflex (subserved by segments C5–C6) and the brachioradialis reflex (C5–C6) may be depressed.

LESIONS AFFECTING C6

This nerve root is often compressed by disc herniation at the C5–C6 vertebral level. A monoradiculopathy affecting the C6 nerve root is the second most common level of cervical radiculopathy after lesions of the C7 level [24]. C6 root involvement results in pain in the lateral arm and dorsal forearm. Sensory signs and symptoms occur on the lateral forearm, lateral hand, and the first and second digits. Paresis occurs predominantly in the following muscles: serratus anterior, biceps, pronator teres, flexor carpi radialis, brachioradialis, extensor carpi radialis longus, supinator, and extensor carpi radialis brevis (examination of these muscles is described in Chapter 2). The biceps reflex (segments C5–C6) and the brachioradialis reflex (segments C5–C6) may be depressed. An “inverted radial reflex” occurs when the lesion causes compression of the spinal cord at the C5–C6 level. A central disc prolapse or a horizontal bar due to degenerative disc disease is often responsible for this clinical finding. Damage of the corticospinal tract at the level of C5–C6 results in hyperreflexia at lower levels. Therefore, tapping the tendon of the brachioradialis muscle elicits no response by the brachioradialis but a brisk contraction of the finger flexors innervated by the C8–T1 segments.

LESIONS AFFECTING C7

This nerve root is often compressed by disc herniation at the C6–C7 vertebral level (the most common level of disc herniation) [16,25]. C7 root involvement results in pain in the dorsal forearm. In some patients, pain may be subscapular or located in the deep breast or chest [20]. Sensory disturbances occur on the third and fourth digits. Paresis occurs variably in the following muscles: serratus anterior, pectoralis major, latissimus dorsi, pronator teres, flexor carpi radialis, triceps, extensor carpi radialis longus, extensor carpi radialis brevis, and extensor digitorum (examination of these muscles is described in Chapter 2). The triceps reflex (C7–C8) may be depressed.

Pseudomyotonia is a term applied to the difficulty in opening the hand because of cervical osteoarthritis. Muscle relaxation is normal but attempts to extend the fingers produce paradoxical flexion of the fingers, probably as a result of misdirected regeneration of C7 nerve root fibers [30].

LESIONS AFFECTING C8

This nerve root is often compressed by disc herniation at the C7–T1 vertebral level. C8 root involvement results in pain in the medial arm and forearm. With C8 lesions [35], sensory signs and symptoms occur on the medial forearm and hand and on the fifth digit. Paresis occurs predominantly and variably in the following muscles: flexor digitorum superficialis, flexor pollicis longus, flexor digitorum profundus I to IV, pronator quadratus, abductor pollicis brevis, opponens pollicis, flexor pollicis brevis, all lumbricals, flexor carpi ulnaris, abductor digiti minimi, opponens digiti minimi, flexor digiti minimi, all interossei, adductor pollicis, extensor digiti minimi, extensor carpi ulnaris, abductor pollicis longus, extensor pollicis longus and brevis, and extensor indicis (see Chapter 2 for examination methods of these muscles). The finger flexor reflex (C8–T1) may be depressed. Sympathetic fibers destined for the superior cervical ganglia are interrupted, resulting in an ipsilateral Horner syndrome (ptosis, miosis, and anhidrosis).

There are frequent intradural communicating fibers between neighboring segments of the cervical posterior roots. These connections are most prominent between a specific cervical segment and the next caudal root. A lesion may therefore be falsely localized clinically to a segment one level higher than its actual location.

The theoretical root syndromes discussed earlier are also related to an “idealized” brachial plexus and do not take into consideration the possibility of a prefixed or postfixed plexus (see Chapter 3).

Lesions Affecting the Thoracic Roots

LESIONS AFFECTING T1

Sensory disturbances occur on the medial arm. Paresis occurs variably in the following muscles: abductor pollicis brevis, opponens pollicis, flexor pollicis brevis, all lumbricals and interossei, abductor digiti minimi, opponens digiti minimi, flexor digiti minimi, and adductor pollicis [13]. The finger flexor reflex (C8–T1) may be depressed. Sympathetic fibers destined for the superior cervical ganglia are interrupted, resulting in an ipsilateral Horner syndrome.

LESIONS AFFECTING SEGMENTS T2–T12

Lesions affecting the thoracic roots and spinal nerves are difficult to diagnose because thoracic and abdominal muscles are difficult to evaluate and there are no muscle stretch reflexes subserved by these levels. Therefore, clinical diagnosis relies predominantly on sensory symptoms and signs.

Thoracic nerves supply (by way of the intercostal nerves) the intercostal and abdominal muscles, which function predominantly in elevation and depression of the ribs, contraction of the abdomen, and flexion of the trunk. Thoracic nerve lesions result in intercostal muscle paralysis, which causes retraction of the costal interspace during inspiration and bulging of the interspace during cough or a Valsalva’s maneuver. Lower thoracic and upper lumbar root lesions may result in excessive protrusion of the abdomen during inspiration. When the abdominal muscles are affected there may be difficulty in rising from a recumbent position, and if these muscles are paralyzed unilaterally the umbilicus is pulled toward the normal side during inspiration or head elevation against resistance (while the patient is in the prone position). When there is bilateral lower abdominal muscle paresis below or at the T10 level, this maneuver results in elevation of the umbilicus (Beevor’s sign). Sensory disturbances are often predominantly or solely subjective. The patient complains of severe burning paresthesias or lightning-like pains. These occur in a unilateral or bilateral segmental distribution (radiating around the thorax or abdomen) and are precipitated by any maneuver that causes increased intraspinal pressure or stretching of the dorsal root (coughing, sneezing, Valsalva’s maneuver, neck flexion, spine movements). There may be sensory loss in the thoracic dermatome involved, but because of the overlapping cutaneous supply by adjacent nerve roots, complete section of a single dorsal root results in little or no sensory loss. An abdominal pseudohernia may be caused by herpes zoster truncal T12 radiculoneuropathy [15].

Axillary pain may be a heralding sign of neoplasm involving the upper thoracic root [28]. Pain in the armpit that is severe or progressive may be indicative of a malignant pathology, and evaluation should target the upper thoracic root regions.

A benign condition of unknown etiology, termed notalgia paresthetica, has been described in which burning, pruritus, and paresthesias develop over an area (approximately the size of the palm of the hand) at the medial margin of the scapula [23]. Decreased sensitivity to pain may occur in this area, which is likely in the territory of the dorsal branches of roots T2 to T6.

Lesions of the Lumbar and Sacral Roots

LESIONS AFFECTING L1

Sensory signs and symptoms occur mainly in the inguinal region. Lower abdominal paresis (internal oblique, transversus abdominis) may occur but is difficult to demonstrate.

LESIONS AFFECTING L2

Sensory disturbances occur on the anterior thigh. Paresis may be present in the pectineus (thigh adduction, flexion, and eversion), iliopsoas (thigh flexion), sartorius (thigh flexion and eversion), quadriceps (leg extension), and thigh adductors. The cremasteric reflex (L2) may be depressed. With upper lumbar root lesions (L2–L4), the result of bent-knee pulling test is often positive [9]. The examiner pulls the half-prone patient’s knee backward while putting forward pressure on the buttock; the test result is positive when lumbar radicular pain is elicited.

LESIONS AFFECTING L3

Sensory signs and symptoms occur on the lower anterior thigh and medial aspect of the knee. Paresis occurs variably in the pectineus (thigh adduction, flexion, and eversion), iliopsoas (thigh flexion), sartorius (thigh flexion and eversion), quadriceps (leg extension), and thigh adductors. The patellar reflex (L2–L4) may be depressed.

LESIONS AFFECTING L4

L4 root involvement causes lower back, buttock, anterolateral thigh, and anterior leg pain. Sensory disturbances occur on the knee and the medial leg. Paresis occurs variably in the quadriceps (leg extension), sartorius (thigh flexion and eversion), and tibialis anterior (foot dorsiflexion and inversion). The patellar reflex (L2–L4) may be depressed. Rarely, neurogenic hypertrophy of the tibialis anterior muscle may occur with a chronic L4 lesion, perhaps because of excessive spontaneous muscle activity [17].

A recent study evaluated four office tests of quadriceps strength in symptomatic adults with radiographic evidence of L3 or L4 nerve root compression [26]. The study observed the performance of each test for its ability to detect quadriceps weakness when compared to the asymptomatic side. To determine the potential influence of radicular pain on the performance of the four tests, a control group of patients older than 40 years with clinical and radiographic L5 or S1 radiculopathies underwent identical testing of quadriceps strength. The L3 and L4 nerve roots innervate the quadriceps; therefore, quadriceps weakness may be a consequence of L3 or L4 radiculopathies. Thirty-three consecutive patients with L3 or L4 radiculopathies and 19 with L5 or S1 radiculopathies were studied. The four tests of quadriceps strength included: (a) single leg sit-to-stand test (with the seated patient asked to extend one leg, hold that foot above the floor, and rise to a standing position with the other leg; the patient could hold the examiner’s hand for balance); (b) step-up test (with the patient stepping up onto a standard 7-inch step-stool, again holding the examiner’s hand for balance); (c) knee-flexed manual muscle testing (with the patient supine, the hip flexed to 90 degrees, the knee maximally flexed, the patient attempts to extend the knee against the examiner’s resistance); and (d) knee-extended manual muscle testing (as in the knee-extended manual test but with the knee extended and the examiner trying to overcome knee extension). In L3 and L4 radiculopathies, unilateral quadriceps weakness was detected by the single leg sit-to-stand test in 61%, by knee-flexed manual muscle testing in 42%, by step-up test in 27% and by knee-extended manual muscle testing in 9% of patients. The sit-to-stand test detected weakness in all but one case in which weakness was detected by another test. All patients with L5 or S1 radiculopathies could perform the sit-to-stand test. It was concluded that in L3 and L4 radiculopathies, unilateral quadriceps weakness was best detected by a single leg sit-to-stand test. Patients of similar age with radicular pain caused by L5 or S1 radiculopathies could perform this test. As the interrater reliability of the single leg sit-to-stand test is high, clinicians should consider utilizing this test for assessing the quadriceps strength in cases of L3 and L4 radiculopathies [26].

LESIONS AFFECTING L5

L5 root involvement causes lower back, buttock, lateral thigh, and anterolateral calf pain. Sensory signs and symptoms occur on the lateral leg, the dorsomedial foot, and the large toe. Paresis occurs in the gluteus medius, gluteus minimus, tensor fasciae latae (adduction and internal rotation of thigh), semimembranosus and semitendinosus (knee flexion), tibialis posterior (plantar flexion and inversion of foot), tibialis anterior (dorsiflexion and inversion of foot), peronei (foot plantar flexion and eversion), flexor digitorum longus (plantar flexion of foot and all toes except the large toe), extensor digitorum brevis (extension of the large toe and three medial toes), extensor hallucis longus (extension of great toe and foot dorsiflexion), and extensor digitorum longus (extension of four lateral toes and foot dorsiflexion). With L5 root lesions, both the patellar (L2–L4) and Achilles (S1–S2) reflexes are spared.

LESIONS AFFECTING S1

S1 root involvement causes lower back, buttock, lateral thigh, and calf pain. Sensory disturbances occur on the little toe, lateral foot, and most of the sole of the foot. Paresis occurs in the gluteus maximus (hip extension), biceps femoris (knee flexion), gastrocnemius and soleus (plantar flexion of foot), flexor hallucis longus (plantar flexion of foot and terminal phalanx of great toe), flexor digitorum longus (plantar flexion of foot and all toes except the large toe), all of the small muscles of the foot, and extensor digitorum brevis (extension of large toe and three medial toes). Rarely, an S1 radiculopathy may result in unilateral calf enlargement [18], likely because of a combination of increased amounts of connective tissue and fat and a varying degree of muscle fiber hypertrophy and atrophy [27]. The Achilles reflex (S1–S2) is depressed.

LESIONS AFFECTING S2–S5

Sensory disturbances occur on the calf, posterior thigh, buttock, and perianal region. Bladder and bowel control may be impaired. The external anal sphincter may fail to contract in response to pricking of the skin or mucous membrane of the perianal region (absent anal wink).

The Localization of Lumbosacral Disc Disease

Herniation of a lumbar intervertebral disc may result in root compression (Table 4.1). Almost all lumbar herniations occur between the fourth and fifth lumbar or the fifth lumbar and first sacral interspaces. Not only the interspace level but also the location of the protruded disc determines which roots are predominantly affected (Fig. 4.3). For example, an L4–L5 protrusion that occurs posterolaterally affects the L5 root destined to leave the canal in the L5–S1 foramen. A very lateral L4–L5 protrusion, however, may affect the L4 root traversing the L4–L5 interspace, and a very medial lesion may affect the S1 root in its downward course. Central disc herniations are less common and may actually affect roots on both sides, resulting in bilateral pain, autonomic paralysis of the bladder and bowel, saddle anesthesia, and bilateral lower extremity weakness (cauda equina syndrome) (see Chapter 5).

Similarly, a very lateral disc protrusion at L5–S1 affects the L5 root (which leaves the canal in the L5–S1 interspace), whereas the usual posterolateral protrusion causes symptoms referable predominantly to an S1 distribution.