innervations and pain patterns of Cx Spine

Innervation and CHAPTER Pain Patterns of the Cervical Spine Nikolai Bogduk Two types of pain may arise from the cervical spine. One type occurs when nerve end- ings in the innervated tissues of the cervical spine are stimulated. The pain is felt lo- cally in the neck but may also be referred to distant regions such as the head, the chest wall, and the upper limb girdle and into the upper limb itself. Since it arises from the somatic structures of the cervical spine, this type of pain is called somatic pain; when it is referred to other regions, it is known as somatic referred pain. The other type of pain occurs when a cervical spinal nerve root is irritated. It is felt not in the neck but in the upper limb or upper limb girdle. To specify its origin and to distinguish it from somatic referred pain, this type of pain is known as cervical radicular pain. SOMATIC PAIN Neck pain can arise from any structure in the cervical spine that receives a nerve sup- ply. Consequently, an appreciation of the innervation of the cervical spine forms a foundation for interpreting the differential diagnosis of cervical pain syndromes. The posterior elements of the neck are those structures that lie behind the inter- vertebral foramina and nerve roots. These structures are all innervated by the dorsal rami of the cervical spine nerves.' The lateral branches of the cervical dorsal rami supply the more superficial posterior neck muscles, such as iliocostalis cervicis, longis- simus cervicis and capitis, and splenius cervicis and capitis.' The medial branches of the cervical dorsal rami supply the deeper and more medial muscles of the neck, such as the semispinalis cervicis and capitis, multifidus, and the interspinales. 1 These nerves also innervate the cervical zygapophyseal joints 1,2 (Figure 4-1). The suboccipital muscles are innervated by the C1 and C2 dorsal rami.' The anterior elements of the neck are in front of the cervical spinal nerves and in- clude the cervical intervertebral discs, the anterior and posterior longitudinal liga- ments, the prevertebral muscles, and the atlantooccipital and atlantoaxial joints and their ligaments. The prevertebral muscles of the neck (longus cervicis and capitis) are innervated by the ventral rami of the C1 to C6 spinal nerves.! Other muscles in the neck also innervated by the cervical ventral rami are the scalenes, the trapezius, and 61

62 Chapter 4 Innervation and Pain Patterns of the Cervical Spine Ib ni Figure 4-1 Deep dissection of the left cervical dorsal rami. The superficial posterior neck muscles have been resected. The lateral branches (tb) of the dorsal rami and the nerves to the intertrans- versarii (ni) have been transected, leaving only the medial branches (m) intact. The CI dorsal ramus supplies the obliquus superior (os), obliquus inferior (oi), and rectus capitis (rc) muscles. The medial branches of the C2 and C3 dorsal rami, respectively, form the greater occipital (gon) and third occipital (ton) nerves. Communicating loops (c) connect the CI, C2, and C3 dorsal rami. Three medial branches (nnS) of the C2 and C3 dorsal rami innervate the semispinalis capitis, whereas the C3 to C8 medial branches send articular branches (a) to the zygapophyseal joints before innervating the multifidus (M) and semispinalis cervicis (SSCe), and those at C4 and C5 form superficial cutaneous branches (s). Tp, transverse process of atlas; Sp' spinous process of Tl. IFrom Bogduk N: Spine 7:319, 1982.1 the sternocleidomastoid. Although the latter two muscles receive their motor inner- vation from the accessory nerve, their sensory supply is from the upper two or three cervical ventral rami.' The atlantooccipital and lateral atlantoaxial joints are inner- vated, respectively, by the Cl and C2 ventral rami.i The ligaments of the atlantoaxial region are innervated by the Cl to C3 sinuver- tebral nerves, which also innervate the dura mater of the upper spinal cord and the posterior cranial fossa\" (Figure 4-2). As they cross the back of the median atlantoaxial joint, these nerves furnish branches to that joint,\" At lower cervical levels (C3 to C8), the dura mater is innervated by an extensive plexus of nerves derived from the cervical sinuvertebral nerves.' Fibers within the plexus extend along the dural sac for several segments above and below their segment of origin. Previous descriptions of the innervation of the cervical intervertebral discs need to be modified in light of recent observations on the structure of these discs. Unlike lumbar discs, the cervical discs lack a uniform, concentric annulus fibrosus.? Rather,

Somatic Pain 63 Figure 4-2 Distribution of the upper three cervical sinuvertebral nerves and the innervation of the at- lantooccipital and atlantoaxial joints. Articular branches {arrtnus} to the atlantooccipital and atlantoaxial joints arise from the Cl and C2 ventral rami, respectively. The Cl to C3 sinu- vertebral nerves (svn) pass through the foramen magnum to innervate the dura mater over the clivus. En route, they cross and supply the transverse ligament of the atlas (TL). The dura mater of the more lateral parts of the posterior cranial fossa is innervated by meningeal branches of the hypoglossal (xii) and vagus (x) nerves. the annulus is a crescentic structure, thick anteriorly but tapering in thickness later- ally on each side toward the anterior edge of the uncinate process. Posteriorly it is represented only by a thin bundle of paramedian fibers. An annulus is lacking at the posterolateral regions of the discs. These regions are simply covered by the alar fibers of the posterior longitudinal ligament that extend laterally to the posterior edge of the uncinate process (Figure 4-3). These differences in structure of the cervical interver- tebral disc do not affect the pattern of innervation of the discs, but they do affect the target tissues. The anterior longitudinal ligament of the cervical spine is accompanied by a plexus of nerves/ whose components are derived from the cervical sympathetic trunks/ and from the vertebral nerves that accompany the vertebral arteries\" (Figure 4-3). Branches of this plexus innervate the anterior longitudinal ligament and also penetrate the ligament to enter the annulus fibrosus anteriorly and laterally. In the anterior and anterolateral regions of the disc, nerve fibers renetrate at least the outer third and up to the outer half of the annulus fibrosus.7,9,1 The posterior longitudinal ligament is accompanied by a similar plexus derived from the cervical sinuvertebral nerves.\" Branches of this plexus supply the posterior longitudinal ligament, but the absence of a substantive annulus posteriorly in the cer- vical intervertebral discs means that the disc itself receives no posterior innervation. The apparent distribution of sinuvertebral nerves to the discs posteriorly\" is, perforce, restricted to the posterior longitudinal ligament. Whether the paramedian fibers of the annulus fibrosus receive an innervation has yet to be demonstrated. In addition to the joints and muscles of the cervical spine, the major arteries of the neck also receive a sensory innervation. The source of innervation of the internal ca-

64 Chapter 4 Innervation and Pain Patterns of the Cervical Spine st Figure 4-3 The innervation of a cervical intervertebral disc. The anterior longitudinal ligament (all) is accompanied by a plexus of nerves derived from the cervical sympathetic trunks (st) and the nerves that accompany the vertebral artery (va). Branches of this plexus penetrate the ante- rior and anterolateral aspects of the annulus fibrosus. The posterior longitudinal ligament (pll) is supplied by a plexus derived from the sinuvertebral nerves. rotid artery has not been established, but that of the vertebral artery is the vertebral nerve,\" through which afferents return to the cervical dorsal root ganglia. 11 The somatic structures that receive an innervation and are therefore potential sources of cervical pain are the cervical zygapophyseal joints; the posterior, preverte- bral, and anterolateral neck muscles; the atlantooccipital and atlantoaxial joints and their ligaments; the cervical dura mater; and the cervical intervertebral discs and their ligaments. The major arteries of the neck, disorders of which are important in the dif- ferential diagnosis of neck pain, should be added to this list. SOMATIC REFERRED PAIN Referred pain is pain perceived in a region separate from the location of the primary source of the pain. Strictly and more explicitly, referred pain is pain perceived in a ter- ritory innervated by nerves other than the ones that innervate the actual source of pain. As a rule, both sets of nerves usually stem from the same spinal segment, such that the source of pain may be innervated by the dorsal ramus of spinal nerve but the pain is referred into regions innervated by the ventral ramus of the same spinal nerve. In some instances the pain may be referred into regions innervated by spinal nerves adjacent to the one that innervates the source of pain. In such cases it is not clear whether the pattern of referral is due to multisegmental innervation of the source of pain or to multisegmental distribution within the spinal cord of afferents from the primary source.

Somatic Referred Pain 65 The term somatic referred pain pertains to referred pain that is elicited by stimulation of nociceptive, afferent fibers from somatic tissues, such as joints, ligaments, bones, and muscles. The term is used to distinguish referred pain aris- ing from these tissues from pain arising from viscera. When pain is referred from vis- cera, the term visceral referred pain can be used. Both types of referred pain are gen- erated by similar mechanisms, and the terms simply distinguish the origin of the pain. The most plausible mechanism of somatic referred pain is convergence, in which primary afferent fibers from a particular structure synapse on second-order neurons in the spinal cord that also happen to receive afferents from another region. Under these conditions pain elicited by the structure can be misperceived as arising from the region whose afferents converge on the second-order neuron. In the case of cervical somatic referred pain, afferents from the cervical spine converge on common neurons with afferents from peripheral regions such as the head, chest wall, and upper limb. Consequently, a nociceptive signal rising from the cervical spine may be perceived as rising from the head, the chest wall, or the upper limb. In the context of somatic referred pain, convergence has attracted little formal study from physiologists, although the few studies that have been conducted val- idate the concept. In animal experiments, convergence has been demonstrated be- tween trigeminal afferents and afferents in the Cl spinal nerve t2 and also between afferents from the superior sagittal sinus and afferents in the greater occipital nerve. t3 In the lumbar spinal cord, afferents from spinal structures relay to neurons that subtend large regions of the lower limbs and trunk.!\" Otherwise, the car- dinal evidence concerning cervical somatic referred pain stems from clinical experiments. Stimulation of the cervical interspinous muscles with noxious injections of hyper- tonic saline produces somatic referred pain in normal volunteers. 15- 17 Stimulation of upper cervical levels produces referred pain in the head. Stimulation of lower cervical levels produces pain in the chest wall, shoulder girdle, and upper limb. Dis- tension of the cervical zygapophyseal joints with contrast medium in normal volun- teers produces referred pain that is perceived in the head or shoulder girdle, depend- ing on which segmental level is stimulated.l\" Similar patterns are produced when the nerves supplying these joints are stimulated electrically.'? Earlier studies showed that electrical and mechanical stimulation of the lower cervical intervertebral discs pro- duces pain in the posterior chest wall and scapular regiol1;20 and that pressure on the posterior longitudinal ligament produces pain in the anterior chest.i! More recent studies have shown that the patterns of referred pain from the cervical intervertebral discs resemble those from the cervical zygapophyseal joints of the same segmental level.22,23 All of these experimental and clinical observations indicate that noxious stimuli from the cervical spine are capable of causing pain in the head, upper limb, and chest wall. None of the experiments in normal volunteers and patients involved spinal nerves and nerve roots. Therefore nerve root irritation cannot have been the cause of pain. Convergence in the central nervous system is the only mechanism postulated to date that explains these phenomena. The capacity of cervical pain to be referred to the head, upper limb, or chest wall can pose diagnostic difficulties. For instance, in patients with pain referred to the head, the presenting complaint could be headache rather than neck pain, and this headache may be misinterpreted as tension headache if the cervical cause is not rec- ognized. Referred pain to the anterior chest wall may mimic angina. 24.25

66 Chapter 4 Innervation and Pain Patterns of the Cervical Spine PATTERNS OF REFERRED PAIN The early experiments on somatic referred pain were undertaken to establish charts of referred pain patterns.16•17 It was noted that referred pain tends to follow a seg- mental pattern in that stimuli to lower levels in the neck resulted in the referral of pain to more caudal areas in the upper limb or chest wall. The apparent patterns of referred pain differed from those of dermatomes, and to distinguish this different pattern, the concept of sclerotomes was introduced.I? The term sclerotome was invoked to complement those of dermatome and myotome. Just as dermatomes represented the segmental innervation of skin and myotomes rep- resented the segmental innervation of muscles, sclerotomes were supposed to repre- sent the segmental innervation ofskeletal tissues.i? This notion, however, is fallacious. Dermatomes and myotomes have an anatomical basis; sclerotomes do not. Der- matomes can be determined by dissection, by tracing areas of numbness after section of segmental nerves, and by mapping the distribution of vesicles in herpes zoster. They have a physical substrate. Similarly, myotomes can be mapped electromyo- graphically by stimulating segmental nerves and by tracing denervation of muscles af- ter section of spinal nerves. Like dermatomes, they can be determined objectively. Sclerotomes were not objectively determined. They were simply based on maps of ar- eas in which volunteers perceived referred pain. As such, they are entirely subjective. Although they may reflect some sort of pattern of innervation of peripheral tissues, this pattern has not been established objectively. Moreover, there is no evidence that referred pain is perceived only in skeletal tissues. Somatic referred pain is perceived also in muscles. Consequently, there are no grounds for segregating sclerotomes from myotomes. Furthermore, referred pain patterns may be based as much on patterns of central nervous connections, or more, as on the segmental distribution of peripheral nerves. If one consults the literature carefully, it emerges that what have been portrayed as maps of sclerotomes are essentially idealized representations.i\" They were not de- rived from quantitative analysis of data. Indeed, when one performs such an analysis, variance rather than consistency appears to be the rule. In those studies that provided quantitative data 15-17 different individuals reported different distributions of referred pain, even when exactly the same structures and segmental levels were stimulated. Moreover, the distributions of referred pain reported in different studies differed markedly (Figure 4-4). Because of these inconsistencies, maps of sclerotomes serve little clinical purpose. They do not allow the segmental location of a source of pain to be determined from the pattern of distribution of pain. Maps of the distribution of referred pain from spe- cific cervical structures have proved more useful. Distinctive patterns of referred pain occur when the cervical zygapophyseal joints are experimentally stimulated in normal volunteers18•19 (Figure 4-5); these patterns have been found to be valid in identifying symptomatic zygapophyseal joints in pa- tients with neck painP However, these pain patterns are not diagnostic of zygapo- physeal joint pain; they indicate only the segmental origin of the pain. Recent studies of discography have shown that the pain patterns of the cervical intervertebral discs are essentially the same as those for zygapophyseal joints with the same segmental number as the discy·23 Thus the virtue of pain charts lies not in establishing which structure is the source of somatic referred pain, but only in pinpointing the segment involved. In this regard, pain charts are not infallible. Their utility decreases if pa- tients have widespread pain or unusual patterns of pain.

Patterns of Referred Pain 67 C5 C7 Figure 4-4 Patterns of referred pain induced in normal volunteers by stimulation of the interspinous muscles at the levels indicated. The left-hand figures are based on the studies of Kellgren.!? The right-hand figures are based on the studies of Feinstein et alY Comparison of the two sets of figures reveals the variation in patterns of referred pain from the same cervical struc- tures and segmental levels.

68 Chapter 4 Innervation and Pain Patterns of the Cervical Spine Figure 4-5 Patterns of referred pain produced by stimulating the cervical zygapophyseal joints in normal volunteers. (Modified from Dwyer A, Aprill C, Bodguk N: Spine 14:453, 1990.) RADICULAR PAIN There is little scientific information on the nature and mechanism of cervical ra- dicular pain. 28 What is known about radicular pain is based largely on studies of lumbar radicular pain. Moreover, confusion persists between radicular pain and radiculopathy. Radiculopathy is a neurological condition in which conduction is impaired along fibers of a nerve root or spinal nerve. If conduction is blocked in sensory nerves, the resultant features are numbness and loss of proprioception, depending on which par- ticular afferents are affected. If conduction is blocked in motor nerves, the resultant feature is segmental weakness in the muscles innervated by the affected nerve. Dimin- ished reflexes can occur if either the sensory or motor arm of the reflex is impaired. Paresthesia is a sign of ischemia and occurs when the blood supply to a segmental nerve is impaired.i\" Radiculopathy, per se, does not cause pain; it results in loss of neurological func- tion. However, radiculopathy can occur in association with pain, and it is for this rea- son that confusion can arise. The presence of radiculopathy does not imply that the associated pain is radicular in origin. Radiculopathy can occur in association with somatic referred pain. This combi- nation can arise when one lesion produces somatic pain and another lesion produces radiculopathy, or it can arise when the one lesion is responsible for both but by dif- ferent mechanisms. For example, an osteoarthritic zygapophyseal joint may produce

Radicular Pain 69 local and referred somatic pain, but if the joint develops an osteophyte that com- presses the adjacent spinal nerve, it will also produce radiculopathy. Resecting the os- teophyte will relieve the radiculopathy but will not relieve the somatic pain. Nevertheless, lesions affecting the cervical spinal nerves or their roots can pro- duce radicular pain. However, the clinical features of this form of pain are poorly de- fined. The features of lumbar radicular pain are better understood. Compression of lumbar nerve roots does not cause radicular pain30 j it produces only paresthesia and numbness. For radicular pain to be produced, either the dorsal root ganglion has to be compressed or the compressed nerve roots must have been previously affected by inflammation. 31-H In that event, however, radicular pain is dis- tinctive in character. It is a shooting, lancinating pain that travels along the affected limb in a narrow band.33 In this regard, radicular pain differs from somatic pain be- cause the latter is a dull, deep aching pain perceived in wide areas whose location is relatively static. The extent to which these features can be transcribed from lumbar radicular pain to cervical radicular pain is not known. The only experiments that have been con- ducted directly on cervical spinal nerves involved needling these nerves in volun- teers.l\" Such acute stimuli did not serve to typify the character of the pain evoked, but they did provide maps of cervical radicular pain. The distribution of pain from a given cervical spinal nerve varies considerably from individual to individual.34 The pain can be perceived in various regions across the back of the shoulder girdle and anterior chest wall and into the upper limb. Proxi- mally, there is no distinctive pattern that can be discerned as representative of a given spinal nerve. Such patterns emerge only peripherally but even then with little distinc- tion between segments (Figure 4-6). When the C4 spinal nerve is stimulated, the area where most subjects feel pain is centered over the lateral aspect of the neck and the top of the shoulder girdle. A similar distribution applies to C5, but the area tends to extend further distally into the upper limb, over the deltoid muscle. Pain from C6 ex- tends from the top of the shoulder along the cephalic border of the upper limb and into the index finger and thumb. Pain from C7 is somewhat similar but tends to con- centrate more posteriorly along the cephalic border and extend to the middle and ring fingers. Although differences in the average pattern can be discerned (Figure 4-6), these typical regions overlap so much that in a given case, the segmental origin of the pain cannot be confidently determined simply from the distribution of pain. A further confounding factor is that the distribution of cervical radicular pain is not unlike the distribution of somatic referred pain. Therefore in a patient with pain in the upper limb, the pattern of pain cannot be used to distinguish radicular pain from somatic referred pain. Unless and until studies are conducted on the nature of pain and its differences in patients with proven radicular pain and patients with proven somatic referred pain, no valid statement can be made concerning their distinction. Although readers might be accustomed to ascribing the pain seen in a patients' radiculopathy to nerve root ir- ritation, they should be circumspect in continuing to do so. They should realize that they can readily diagnose radiculopathy on the basis of the distribution of paresthesia, numbness, weakness, and loss of reflexes, but this action tells them nothing of the ori- gin of the pain. Although they may have been taught to infer that the pain is radicular, this inference is based on traditional teaching and not on experimental data or valid observations. Although experiments in normal volunteers have produced somatic referred pain to distal regions of the upper limb (Figure 4-4), modem clinical studies have not re- ported such patterns of distant referral in patients. Somatic referred pain from the

70 Chapter 4 Innervation and Pain Patterns of the Cervical Spine (\\ Figure 4-6 Maps of the distribution of pain evoked by mechanical stimulation of the C4, CS, C6, and C7 spinal nerves. [From Bogduk N: Medical manogement of acute cervical radicular pain: an evtdence-bosed approach, Newcastle, Australia, 1999, Newcastle Bone and Joint Institute. Based on Slipman CW, Plastaras CT, Palmitier RA et al: Spine 23:2235, 1998.) cervical zygapophyseal joints18,35,36 and from the cervical intervertebral discs2,23 is usually located proximally-over the shoulder girdle and into the upper arm. Referred pain into the forearm or hand has not been reported from these structures and would appear to be uncommon. Accordingly, the following two operating rules can be con- structed on the basis of available published data: 1. Pain over the shoulder girdle and upper arm could be either somatic referred pain or radicular pain. 2. Pain in the forearm and hand is unlikely to be somatic referred pain and is more likely to be radicular in origin.

References 71 These inferences are corroborated by surgeons with experience of operating on patients under local anesthesia. Some have explicitly stated that \"the pain in the neck, rhomboid region, and anterior chest was referred pain from the disc itself, while arm pain was usually the result of nerve compression\"? Others concur that arm pain is caused by nerve root irritation but that more proximal pain is referred from the neck.38 Until better data become available, these rules serve to assist practitioners in as- sessing patients with radiculopathy. The objective is to prevent radicular pain from being overdiagnosed and misdiagnosed. In cases of doubt, it is preferable that the doubt be recorded rather than a misdiagnosis be perpetrated and perpetuated. References 1. Bogduk N: The clinical anatomy of the cervical dorsal rami, Spine 7:319, 1982. 2. Lazorthes G, Gaubert J: L'innervation des articulations interapophysaire vertebrales, Comptes Rendues de l'Association des Anatomistes, 43:488, 1956. 3. Williams PL, editor: Gray's anatomy, ed 38, Edinburgh, 1995, Churchill Livingstone, p 808. 4. Kimmel DL: Innervation of the spinal dura mater and dura mater of the posterior cranial fossa, Neurology 10:800, 1960. 5. Groen GJ, Baljet B, Drukker J: The innervation of the spinal dura mater: anatomy and clinical implications, Acta Neurocbir 92:39, 1988. 6. Mercer S, Bogduk N: The ligaments and annulus fibrosus of human adult cervical inter- vertebral discs, Spine 24:619, 1999. 7. Groen GJ, Baljet B, Drukker J: Nerves and nerve plexuses of the human vertebral column, Am J Anat 188:282, 1990. 8. Bogduk N, Lambert G, Duckworth JW: The anatomy and physiology of the vertebral nerve in relation to cervical migraine, Cephalalgia 1:11, 1981. 9. Bogduk N, Windsor M, Inglis A: The innervation of the cervical intervertebral discs, Spine 13:2,1988. 10. Mendel T, Wink CS, Zimny ML: Neural elements in human cervical intervertebral discs, Spine 17:132, 1992. 11. Kimmel DL: The cervical sympathetic rami and the vertebral plexus in the human foetus, J Comp NeuroI112:141, 1959. 12. Kerr FWL: Structural relation of the trigeminal spinal tract to upper cervical roots and the solitary nucleus in the cat, Exp Neuro/4:134, 1961. 13. Angus-Leppan H, Lambert GA, MichalicekJ: Convergence of occipital nerve and superior sagittal sinus input in the cervical spinal cord of the cat, Cephalalgia 17:625, 1997. 14. Gillette RG, Kramis RC, Roberts \\V]: Characterization of spinal somatosensory neurons 15. having receptive fields in lumbar tissues of cats, Pain 54:85, 1993. J Neru Ment Dis Campbell DG, Parsons CM: Referred head pain and its concomitants, 99:544, 1944. 16. Kellgren JH: On the distribution of pain arising from deep somatic structures with charts of segmental pain areas, Clin Sci 4:35, 1939. 17. Feinstein B, Langton JBK, Jameson RM, Schiller F: Experiments on referred pain from deep somatic tissues, J Bone Joint Surg 36A:981, 1954. 18. Dwyer A, Aprill C, Bogduk N: Cervical zygapophyseal joint pain patterns. I. A study in normal volunteers, Spine 15:453, 1990. 19. Fukui S, Ohseto K, Shiotani M et al: Referred pain distribution of the cervical zygapo- physeal joints and cervical dorsal rami, Pain 68:79, 1996. 20. Cloward RB: Cervical diskography: a contribution to the aetiology and mechanism of neck, shoulder and arm pain, Ann Surg 130:1052, 1959. 21. Murphey F: Sources and patterns of pain in disc disease, Clin Neurosurg 15:343,1968. 22. Schellhas KP, Smith MD, Gundry CR, Pollei SR: Cervical discogenic pain: prospective correlation of magnetic resonance imaging and discography in asymptomatic subjects and pain sufferers, Spine 21:300, 1996.

72 Chapter 4 Innervation and Pain Patterns of the Cervical Spine 23. Grubb SA, Kelly CK: Cervical discography: clinical implications from 12 years of experi- ence, Spine 25:1382, 2000. 24. Booth RE, Rothman RH: Cervical angina, Spine 1:28, 1976. 25. Brodsky AE: Cervical angina: a correlative study with emphasis on the use of coronary ar- teriography, Spine 10:699, 1985. 26. Inman vr, Saunders JBD: Referred pain from skeletal structure, J Nero Ment Dis99:660, 27. 1944. Dwyer A, Bogduk N: Cervical zygapophyseal joint pain patterns. n. A clinical Aprill C, evaluation, Spine 15:458, 1990. 28. Bogduk N: Medical management of acute cervical radicular pain: an evidence-based ap- proach, Newcastle, Australia, 1999, Newcastle Bone and Joint Institute. 29. Ochoa JL, Torebjork HE: Paraesthesiae from ectopic impulse generation in human sen- sory nerves, Brain 103:835, 1980. 30. MacNab I: The mechanism of spondylogenic pain. In Hirsch C, Zotterman Y, editors: Ceruical pain, Oxford, England, 1972, Pergamon. 31. Howe JF: A neurophysiological basis for the radicular pain of nerve root compression. In Bonica JJ, Liebeskind JC, Albe-Fessard DG, editors: Advances in pain research and therapy, vol 3, New York, 1979, Raven Press. 32. Howe JF, Loeser JD, Calvin WH: Mechanosensitivity of dorsal root ganglia and chroni- cally injured axons: a physiological basis for the radicular pain of nerve root compression, Pain 3:25, 1977. 33. Smyth MJ, Wright V: Sciatica and the intervertebral disc: an experimental study, J Bone Joint Surg 40A:1401, 1959. 34. Slipman CW; Plastaras CT, Palmitier RA et al: Symptom provocation of fluoroscopically guided cervical nerve root stimulation: are dynatomal maps identical to dermatomal maps? Spine 23:2235, 1998. 35. Barnsley L, Lord SM, Wallis BJ, Bogduk N: The prevalence of chronic cervical zygapophysial joint pain after whiplash, Spine 20:20, 1995. 36. Lord S, Bamsley L, Wallis BJ, Bogduk N: Chronic cervical zygapophysial joint pain after whiplash: a placebo-controlled prevalence study, Spine 21:1737, 1996. 37. Murphey F, Simmons JCH, Brunson B: Surgical treatment of laterally ruptured disc: re- view of 648 cases, 1939 to 1972,J Neurosurg 38:679,1973. 38. Yamano Y: Soft disc herniation of the cervical spine, Int Orthop 9:19,1985.