Differential Diagnosis in Neurology

6.1. Introduction

Radicular pain is elicited from depolarization of A-delta afferents (1-4 μ). It is characterized as lancinating and is well localized. It may be followed by a deep aching pain with a burning quality which is mediated by 1μ C-fibers. Sympathetic discharge elicited by somatic sympathetic reflexes may amplify and drive this pain in chronic conditions. The usual radicular pain radiates within a specific dermatome. It may be appreciated in only one part of the anatomical confines of the dermatome. Thus an L5 root may be felt just or predominately in the lateral thigh or great toe and not the lower back. The pain is usually increased by mechanical maneuvers that increase intraspinal pressure or stretch the nerve root. The nervi nervorum of the nerve sheath, is sensitized so that a mechanical rather than tissue destructive stimulus will depolarize the nociceptors of the nerve sheath and the radicle itself (basis for the straight leg raising test and other provocative stretch maneuvers).

There is segregation of spinal root afferents at the dorsal root entry zone (DREZ). Medial dorsal root fibers carry proprioception vibration and light touch. Lateral dorsal root fibers (closest to the dorsolateral extrusion area of the discs) carry lancinating pain (A-delta fiber mediated), cold (A-delta fiber), temperature and burning pain (C-fiber). Lesions affecting the dorsal root entry zone are in general painful. Lesions peripheral to the DRG in the neuronal exit foramina cause numbness. Lesions of the medial branch of the posterior primary division are associated with numbness and paraesthesias 1 1/2 inches laterally from the spinous process. Meningeal branches innervate the dura at that level. These are the recurrent branches of Spurling. The L5 branch is particularly relevant as it refers sensations to the top of the thigh (often mistaken for L1, L2, L3 patterns).

A ventral root lesion causes segmental weakness. If axons are severed, there will be associated atrophy and fasciculation. Early with compressive or irritative lesions, reflexes may be enhanced. This has been posited to be caused by differential susceptibility of inhibitory afferents in the ventral root. In the lower extremity the examiner must always be aware of higher compressive or intrinsic spinal cord lesions that may disinhibit the corticospinal tract and increase reflexes below the lesion level.

• General features:
  • Contents of the neural foramina are the spinal nerve roots, recurrent meningeal nerves and radicular blood vessels. The anatomical boundaries are the pedicles inferiorly and superiorly, anteriorly the intervertebral disc and vertebral body and posteriorly by the facet joint. The superior and inferior facets of the joints are lined by synovium. The dorsal root ganglia (DRG) may compress roots at higher levels as they can be more intraspinal.
  • The blood supply to a spinal nerve root derives from the corresponding radicular artery. At the root entry zone, blood vessels lie on the surface of rootlets and in interradicular spaces the capillary density is high in the ventral root entry zone.
  • Facet joints are innervated by branches of the posterior primary ramus of the spinal nerve root
  • The posterior longitudinal ligament which overlies the disk has nociceptive innervation
  • Disc contents illicit an inflammatory response in surrounding tissue that induces the production of IL-1, IL6 and tumor necrosis factor α ( alpha) TNFα. The later has been shown to directly depolarize nociceptive C-fiber and A-delta afferents.
  • The outer annulus of a disc may have nociceptive innervation and may be able to refer pain.
  • Dorsal ganglia have a less tight blood nerve barrier than the roots or axons

Dorsal Root Entry Zone

• Medial dorsal root fibers carry proprioception, vibration, light touch and tap
• Lateral dorsal root fibers carry lancinating pain (A-delta), cold (A-delta; 1-4 u), temperature (heat; C-fibers)
• Clinically:
  • Lesions affecting the dorsal root entry zone (DREZ) are painful
  • Lesions peripheral to the DRG cause numbness
• Lesions of the medial branch of the posterior primary division; numbness, paresthesias and pain 1.5 inches from spinal process laterally
• Meningeal branches (dural innervation of the meninges)
  • Recurrent nerves of Spurling occur at each level
  • Particularly relevant at L5 (referred sensation to the top of the thigh)

Ventral Root Lesions

• Dermatomal or segmental weakness
• Atrophy
• Fasciculation of involved muscle groups
• Absent or depressed segmental reflexes:
  • Early with compressive or irritative lesion; the reflexes may be enhanced; then it is lost with further pathology (pressure or destruction)
  • Reflexes may be increased with spinal cord disinhibition (B12; compressive higher lesions such as spinal stenosis)
  • There are putative inhibitory afferents in the ventral horn

Spinal Nerves and Roots

• Anatomy of a spinal nerve
  • Posterior primary rami of a spinal nerve:
    • Supplies the skin of the dorsal trunk
    • Innervates the longitudinal muscles of the axial skeleton
  • Anterior primary rami:
    • Motor innervation of the limbs
    • Non-axial skeletal muscles
    • Skin of the lateral and anterior trunk and neck (lateral cutaneous and anterior cutaneous branches)
    • Communicate with the sympathetic ganglion by way of white and grey rami communicantes
  • Dorsal root irritative lesions:
    • Radicular pain
    • A-delta fibers
    • Lancinating, sharp, well localized
    • Abrupt in onset
    • Radiates to a specific dermatome
    • Increased by mechanical maneuvers that increase intra spinal pressure or stretch the nerve (if it is sensitized it is mechanosensitivity; fires to a mechanical rather than a nociceptive stimulus)
    • Pain may be the first symptom of a sensory radiculopathy; often intermixed with deep ache or paresthesia
  • Destructive dorsal root lesion:
    • Hypesthesia of a core region supplied by that root
    • Overlap of adjacent roots; therefore, little sensory loss
    • Multiple root involvement; area of analgesia larger than anesthesia
    • CRPS I/II; the area of involvement does not respect the affected dermatome; it is extraterritorial
    • Loss of motor function in involved myotome; atrophy
    • Reflex loss in involved myotome is usual; the following exceptions occur:
      • Early lesions may increase reflex (loss of inhibition)
      • GBS (triceps spared)
      • Acute intermittent porphyrias (ALP and variegate porphyrin-ankle jerks spared)
      • B12 deficiency: may have hyperactive reflexes in face of peripheral muscle atrophy

Mechanical Degenerative Root Disease

• Differential Diagnosis of Benign Processes
  • Herniated nucleus pulposus
  • Cervical and lumbar spondylosis
  • Spondylolysis (defect of pars interarticularis)
  • Spondylolisthesis (malalignment of vertebrae)
  • Osteophytes within the foraminal exit foramina
  • Lateral recess syndrome L5 (the longest exit foramina)
  • Degenerative facet disease with tropism (rotation of the inferior facet into the exit foramina)
  • Spinal stenosis (degenerative)
  • Synovial cyst of the facet joint