REFERRED PAIN AND RADICULAR PAIN

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There is a clear difference between radicular pain and referred pain.  Medical studies have been documenting this for many decades, but confusion persists among clinicians.  Perhaps the most prominent and widely published authority on this is Nikolai Bogduk, MD, PhD, in Australia.  In his 2003 article “The anatomy and physiology of neck pain,” Phys Med Rehabil Clin N Am, 14 (2003) 455-472, he applies basic anatomy and pathophysiology to itemize the anatomic structures that can invoke neck pain.  More recently he has applied the same science to back pain in his article “On the definitions and physiology or back pain, referred pain, and radicular pain,” Pain, 147 (2009) 17-19.  This article consists primarily of quotations from Dr. Bogduk’s articles.

Nociceptive pain is pain that is invoked by noxious stimulation of structures in the spine.  Volunteer studies show that the posterior surface of discs is the most potent source of experimentally-induced pain.  Noxious stimulation causes dull, aching pain.  When it occurs clinically, this type of pain should be inferred to be nociceptive pain.

Somatic referred pain is perceived in regions innervated by nerves other than those that innervate the site of noxious stimulation – the core of the definition of referred pain.  Somatic pain does not involve the stimulation of nerve roots.  It is produced by noxious stimulation of nerve endings within spinal structures such as discs, facet joints, or sacroiliac joints.  The mechanism does not involve nerve roots; it involves convergence on second-order neurons in the spinal cord.  As a general rule, somatic referred pain is perceived in regions that share the same segmental innervation as the source.  But somatic pain is not caused by compression of nerve roots, so there will be no neurological signs.

Somatic referred pain is dull, aching, and gnawing.  It can expand into a wide area that is difficult to localize.  Once established, it tends to be fixed in location.  Subjects often find it difficult to define the boundaries of the affected area but can confidently identify its center or core.  But patterns are not consistent among subjects in the studies and are not dermatomal.

Radicular pain is distinctive, having a lancinating quality and traveling along the length of the limb in a band no wider than two or three inches.  This is the only pain that has been produced by stimulating nerve roots.  Therefore, it is the only type of pain that should be interpreted as radicular pain.

Squeezing or pulling normal nerve roots does not produce radicular pain.  Mechanical stimulation evokes pain only if the nerve root has previously been inflamed.

In laboratory animal studies, squeezing normal nerve roots evokes only a momentary discharge, but squeezing a dorsal root ganglion or an inflamed dorsal root evokes pain in certain fibers.  Radicular pain, therefore, is not due to a discharge exclusively in nociceptive afferents; it is due to a heterospecific discharge in the affected nerve.  The evoked sensation is unpleasant, but is not exactly pain.

Dr. Bogduk points out that the term “sciatica” is arcane, stemming from an era when the mechanisms of referred pain were no understood and any referred pain was attributed to irritation of the peripheral nerve that passed through the region of the pain.  The taxonomy of the International Association for the Study of Pain (IASP) recommends replacement by the term “radicular pain.”

In his 2003 study on cervical pain, Dr. Bogduk found that radicular pain is commonly associated with paresthesia, which is consistent with certain fibers being included in the discharge.  But, unlike the sensory loss of cervical radiculopathy, the pattern of cervical radicular pain is not dermatomal.  Radicular pain is perceived deeply, through the shoulder girdle and into the arm.  Radicular pain from C5 tends to remain in the arm, but pain from C6, C7, and C8 extends into the forearm and hand.  These patterns of distribution indicate that the pain is not restricted to cutaneous afferents.  It involves afferents from deep tissues, such as muscles and joints, as well.  Because the segmental innervation of deep tissues is not the same as that of skin, radicular pain cannot be, and is not, dermatomal in distribution.  In particular, muscles of the shoulder girdle are innervated by C6 and C7, well away from the dermatomes of these nerves.

Radiculopathy is a neurological state in which conduction is blocked along a spinal nerve or its roots.  When sensory fibers are blocked, numbness is the symptom and sign.  When motor fibers are blocked, weakness ensues.  Diminished reflexes occur as a result of either sensory or motor block.  The numbness has a dermatomal distribution and the weakness is myotomal.  But radiculopathy is not defined by pain.  Radiculopathy can occur in the absence of pain, and radicular pain can occur in the absence of radiculopathy.

Careful clinical examination remains the best tool for diagnosing a radiculopathy.  But this does not mean that the segmental origin of radicular pain can be determined from its distribution.  For example, the patterns of L4, L5, and S1 radicular pain cannot be distinguished from one another.  It is the dermatomal distribution of numbness, not the distribution of pain, that allows the segment of origin to be determined.

Points:

  • Radicular pain is not as common as back pain or somatic referred pain. Radicular pain, as properly defined, comprises only 12% or less of all back pain. Studies that claim otherwise bear a strong possibility of mistaking somatic referred pain as radicular pain.  Imaging can often establish a causative lesion, but not so for somatic referred pain.
  • A person with aching back pain that spreads into the buttock and thigh, but no lancinating pain and no neurological symptoms, has nociceptive back pain and somatic referred pain.
  • It is possible to have nociceptive back pain, such as through internal disc disruption. Chemical irritation of the nerve root will cause radicular pain.  Radiculopathy may ensue as the nerve root becomes swollen and conduction block occurs.  Discectomy might remove the disc herniation and relieve the radicular pain, but it will not remove the back pain or any somatic referred pain.
  • Patients can have back pain; they can also have sciatica; but the two symptoms have separate mechanisms and causes.
  • Thus, although disc herniation is the most common cause of radicular pain, it is not the most common cause of back pain.

Referred cervical pain can be diagnosed clinically.  Pain from cervical facet joints tends to follow relatively constant and recognizable segment patterns.

  • From the C2-3 level, pain is referred up into the head.
  • From C3-4 and C4-5, pain is located over the posterior neck.
  • From C5-6, pain spreads over the supraspinous fossa of the scapula.
  • From C6-7, pain spreads further caudally (lower) over the scapula.

These pain patterns have been produced by mechanical stimulation of cervical intervertebral discs.  The pattern of pain is determined by the nerve supply of the structure.  Thus, any structure innervated by the same cervical segmental nerves will have the same distribution of pain.  Clinically, discogenic pain cannot be distinguished from facet joint pain.

Other structures that have been shown to be able to produce neck pain and headache on normal volunteers are the atlanto-occipital and the lateral atlanto-axial joints.  Pain from these structures does not occur in a unique distribution.  Along with the C2-3 joints, these structures all produce pain in the suboccipital region.

Fractures are an accepted cause of pain, but not all fractures are necessarily painful.  Spondylosis and osteoarthritis are often applied diagnoses in patients with neck pain, but neither diagnosis is valid.  Radiographic features of spondylosis occur with increasing frequency with increasing age in asymptomatic individuals.  Most commonly they affect the C5-6 and C6-7 segments, but these changes are weakly, if at all, associated with pain.  Consequently, finding spondylosis or osteoarthritis on an x-ray does not constitute making a causative diagnosis or finding the source of pain.

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