Definitions and Physiology of Back Pain, Referred Pain, and Radicular Pain

Introduction to Lumbar Pain Taxonomy

• Despite efforts by the International Association for the Study of Pain (IASP), significant confusion remains among clinicians regarding definitions of back pain, referred pain, radicular pain, and radiculopathy.
• This confusion is being inherited by basic scientists developing animal models for back pain.
• The definitions of these entities are based on seminal studies conducted over $50$, $60$, and $70$ years ago, yet this pioneering work has not been properly integrated into medical education, clinical practice, or current publications.

Nociceptive Back Pain

• Definition: Nociceptive back pain is pain evoked by the noxious stimulation of structures within the lumbar spine.
• Clinical Characteristics:
  • The quality of the pain is consistently described as dull and aching.
  • These characteristics have been determined through experimental studies on normal volunteers.
• Evidence from Experimental Stimulation:
  • Muscles and Ligaments: Hypertonic saline injections into back muscles (Kellgren, $1938$) or interspinous ligaments (Kellgren, $1939$) evoked dull, aching pain. These findings were replicated by others (Bogduk, $1980$; Feinstein et al., $1954$).
  • Zygapophysial and Sacroiliac (SI) Joints: Distending these joints with contrast medium injections resulted in similar dull, aching pain (McCall et al., $1979$; Mooney and Robertson, $1976$; Fortin et al., $1994$).
  • Dura Mater: Mechanical (Smyth and Wright, $1959$) and chemical stimulation (El Mahdi et al., $1981$) of the dura mater also produced pain.
  • Intervertebral Discs: Studies on patients undergoing surgery under local anesthesia (Falconer et al., $1948$; Kuslich et al., $1991$; Wiberg, $1947$) identified the posterior surface of lumbar intervertebral discs as the most potent source of experimentally induced back pain.

Somatic Referred Pain

• Definition: Pain perceived in regions innervated by nerves other than those innervating the actual site of noxious stimulation.
• Etymology: Named "somatic referred pain" to distinguish it from visceral referred pain and radicular pain, as the source lies in the somatic tissues of the lumbar spine.
• Anatomical Source: It results from noxious stimulation of nerve endings in spinal structures like discs, zygapophysial joints, or sacroiliac joints. It does not involve the stimulation of nerve roots.
• Neurophysiological Mechanism:
  • The proposed mechanism is the convergence of nociceptive afferents on second-order neurons in the spinal cord that also subtend regions of the lower limb.
• Clinical Features:
  • Quality: Described as dull, aching, and gnawing; sometimes perceived as an expanding pressure.
  • Localization: Often perceived in wide areas that are difficult to localize. Once established, the pain tends to be fixed in location. Patients can typically identify the center or core of the pain but struggle to define its boundaries.
  • Distribution:
    • It generally occurs in regions sharing the same segmental innervation as the source.
    • The pattern is not dermatomal. It corresponds more closely to the segmental innervation of deep tissues like muscles and joints.
    • Though typically centered in the gluteal region and proximal thigh, it can extend as far down as the foot.
  • Absence of Neurological Signs: Because it is not caused by nerve root compression, there are no neurological deficits (e.g., no numbness or weakness).
• Diagnostic Confirmation: Patterns have been evoked by stimulating Z-joints or discs and relieved in patients by anesthetizing the zygapophysial joints (Fairbank et al., $1981$; Mooney and Robertson, $1976$; Schwarzer et al., $1994$).

Radicular Pain

• Definition: Pain evoked by ectopic discharges emanating from a dorsal root or its ganglion.
• Pathophysiology:
  • The most common cause is disc herniation.
  • Inflammation of the affected nerve is considered a critical process (Bogduk, $2005$).
  • Squeezing or pulling normal nerve roots does not produce radicular pain. Experimental clinical evidence (Smyth and Wright, $1959$) shows that mechanical stimulation only evokes pain if the nerve roots have been previously inflamed.
  • Compression alone may only be painful if it involves the dorsal root ganglion, which has been verified in animal studies.
• Neural Correlates:
  • Laboratory studies (Howe, $1979$; Howe et al., $1977$) show that squeezing an inflamed dorsal root or a dorsal root ganglion evokes heterospecific discharges in $A\beta$, $A\delta$, and $C$ fibers.
  • Consequently, radicular pain is not just nociceptive but involves multiple fiber types, explaining why the sensation is described as "electric" or "shocking."
• Clinical Characteristics:
  • Quality: Lancinating, shocking, or electric.
  • Distribution: Travels along the length of the lower limb in a narrow band no more than $2\text{--}3\,inches$ ($5\text{--}7.6\,cm$) wide.
• Terminology: The term "sciatica" is considered archaic and should be replaced by "radicular pain" per IASP taxonomy.

Radiculopathy: A Distinct Neurological State

• Definition: A neurological state where conduction is blocked along a spinal nerve or its roots.
• Clinical Signs and Symptoms:
  • Sensory Block: Results in numbness, which has a dermatomal distribution.
  • Motor Block: Results in weakness, which has a myotomal distribution.
  • Reflexes: Diminished reflexes can result from either motor or sensory block.
• Key Distinction: Radiculopathy is defined by objective neurological signs, not by pain. While radiculopathy and radicular pain often coexist, they can occur independently.
• Diagnosis: Clinical examination is the primary diagnostic tool. Electrophysiological testing is rarely necessary for diagnosis.
• Segmental Localization: The distribution of radicular pain alone cannot reliably distinguish between $L4$, $L5$, and $S1$ segments (Norlen, $1944$; van Akkerveeken, $1989$). Segmental origin can only be determined by mapping the dermatomal distribution of numbness when radiculopathy is present.

Clinical Differentiation and Management Implications

• Prevalence: Radicular pain is relatively uncommon, with a strict prevalence of approximately $12\%$ or less (Deyo and Tsui-Wu, $1987$). Somatic referred pain is far more common.
• Imaging:
  • Radicular Pain/Radiculopathy: Imaging (MRI, CT) is justified as it can identify the causative lesion (e.g., herniation).
  • Somatic Referred Pain: Imaging is generally unable to reveal the cause. Findings such as disc bulges or degenerative changes are often incidental and carry a high risk of false-positive interpretations, potentially leading to unnecessary surgery.
• Surgical Outcomes: Discectomy is highly successful for relieving leg pain (radicular pain) but often leaves the original back pain and somatic referred pain unresolved, as these have separate mechanisms.
• Neurological Features:
  • Somatic Pain: Should not present with neurological symptoms. Allodynia is not a feature.
  • Radicular Pain: Allodynia is theoretically possible but is typically not a feature unless there is actual nerve damage (neuropathy) rather than simple compression/inflammation.

Discussion of Misleading Terminology

• "Low back pain – sciatica": This term is misleading because it implies a single condition. In reality, back pain and sciatica have separate causes and mechanisms. Disc herniation is the leading cause of radicular pain but is not a common cause of isolated back pain.
• "Pseudo-sciatica" or "Pseudo-radicular pain": These terms are considered superfluous and unhelpful. There is nothing "false" about the pain; it is simply somatic referred pain or peripheral nerve entrapment.
• Combined Presentations: A patient may have multiple issues simultaneously (e.g., internal disc disruption causing somatic referred pain, plus a herniation leaking inflammatory chemicals causing radicular pain and radiculopathy). Each requires separate diagnostic and therapeutic consideration.