How Herniated Discs Cause Radiculopathy
A herniated disc causes radiculopathy through two distinct mechanisms: direct mechanical compression of the nerve root and chemical irritation from inflammatory mediators released by the herniated nucleus pulposus material. 1, 2
Mechanical Compression Mechanism
The herniated disc material physically compresses neural structures, with the specific location determining the clinical presentation:
Paramedian herniations (most common for radiculopathy) compress the nerve root at the lateral recess or anterolateral corner of the spinal canal, producing isolated radicular symptoms in 64% of cervical cases and 12% of radiculopathy cases overall 3
Lateral herniations compress the nerve root at the inlet of the neural foramen, accounting for 88% of pure radiculopathy presentations without myelopathy 3
The herniated material typically affects the nerve root exiting under the pedicle of the adjacent inferior vertebral body, though non-adjacent radiculopathy can occasionally occur 4
In the cervical spine, soft disc herniations or spondylosis with foraminal narrowing from facet/uncovertebral joint hypertrophy create the compressive pathology 1
Chemical Inflammation Mechanism
Beyond simple mechanical pressure, the biochemical component plays a critical role:
Nuclear material leaking from annular tears releases inflammatory mediators that chemically irritate the nerve root, even in the absence of significant mechanical compression 2
This chemical injury is most prominent in acute disc pathology, where the inflammatory cascade contributes substantially to radicular pain 2
The chemical component explains why focal disc abnormalities can be observed in asymptomatic patients, and why symptomatic discs can become asymptomatic as inflammation resolves 2
Clinical Correlation Patterns
The relationship between imaging findings and symptoms requires careful interpretation:
57% of patients with low back pain and 65% with radiculopathy demonstrate disc herniation on MRI, compared to only 20-28% of asymptomatic individuals 1
However, MRI alone should not diagnose radiculopathy due to frequent false-positive and false-negative findings—clinical correlation is mandatory 1
The size, type, and location of herniation do not reliably predict patient outcomes 1
Temporal Evolution and Treatment Windows
Understanding the natural history informs treatment decisions:
75-90% of cervical radiculopathy cases resolve with conservative management, as disc herniations can resolve radiologically and symptoms improve before radiographic changes 1, 2
A critical window of opportunity exists in acute injury where treatments addressing the chemical component are most effective 2
After this acute phase, chronic radicular pain may involve irreversible neural structural damage and increased somatosensory system sensitivity, responding poorly to mechanical or chemical interventions 2
Common Pitfalls to Avoid
Do not assume ipsilateral correlation: Herniated discs can occasionally cause contralateral radiculopathy through mechanisms including migrated epidural fat displacement 5
Do not rely solely on imaging: Physical examination has limited diagnostic accuracy for cervical radiculopathy, and imaging must always be interpreted with clinical findings 1
Do not delay intervention in progressive deficits: While most cases resolve conservatively, rapidly progressive neurological impairment, cauda equina syndrome, or severe myelopathy warrant urgent surgical consideration 1