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Traversing nerve root of spinal nerve

The traversing nerve root is the portion of a spinal nerve root that descends obliquely within the spinal canal after exiting the thecal sac, coursing downward to exit at the intervertebral foramen below its corresponding vertebral level. For example, the L5 traversing nerve root lies within the L4–L5 level and exits through the L5–S1 neural foramen.

Traversing roots are critical in spine imaging and surgery because they are the most frequently compressed nerve roots in conditions such as disc herniation, spinal stenosis, or foraminal narrowing.

Synonyms

  • Descending nerve root

  • Intraspinal course of spinal nerve root

  • Traversing rootlet

Origin, Course, and Exit

  • Origin: Arises from the spinal cord as part of the ventral (motor) and dorsal (sensory) rootlets, merging to form the spinal nerve root within the thecal sac

  • Course:

    • Travels obliquely downward and laterally within the spinal canal

    • Passes close to the posterior surface of the vertebral body and intervertebral disc

    • Lies in the lateral recess of the spinal canal

  • Exit: Leaves the spinal canal through the intervertebral foramen one level below its corresponding vertebra (e.g., L5 traversing nerve root exits at the L5–S1 foramen)

Relations

  • Anteriorly: Posterior vertebral body and intervertebral disc

  • Posteriorly: Ligamentum flavum and lamina

  • Medially: Thecal sac and dural root sleeve

  • Laterally: Pedicle and neural foramen structures

Function

  • Carries motor fibers from the ventral root to supply muscles of the corresponding myotome

  • Carries sensory fibers from the dorsal root to supply dermatomal sensation

  • Conducts reflex arcs and pain signals in conditions such as radiculopathy

Clinical Significance

  • Commonly compressed by posterolateral or paracentral disc herniations

  • Implicated in radicular pain, paresthesias, and weakness in specific dermatomal/myotomal patterns

  • Important landmark in spine surgery and interventional procedures (e.g., epidural injections, decompressions)

  • MRI is essential in evaluating traversing root compression, edema, or enhancement

MRI Appearance

T1-weighted images:

  • Nerve root appears as a linear low-to-intermediate signal intensity structure within bright epidural fat

T2-weighted images:

  • Nerve root shows intermediate to mildly hyperintense signal compared to muscle

  • Compressed or inflamed roots may appear brighter and thickened

STIR (Short Tau Inversion Recovery):

  • Normal root appears as low signal

  • Inflamed or edematous root appears bright hyperintense

T1 Fat-Sat Post-Contrast:

  • Normal root shows minimal or no enhancement

  • Pathologic roots (neuritis, tumor infiltration, infection) demonstrate abnormal focal or diffuse enhancement

3D T2 SPACE / CISS:

  • Traversing root shows intermediate to mildly hyperintense signal compared to muscle

  • Surrounded by bright CSF, which provides excellent contrast

  • Sequence allows precise visualization of the nerve root course within the thecal sac and lateral recess

  • Ideal for detecting disc herniation contact, root compression, and small intradural lesions

CT Appearance

Non-Contrast CT:

  • Traversing root not directly visualized due to limited soft tissue resolution

  • Indirectly evaluated by narrowing of lateral recess or foramen, presence of disc herniation, or bony stenosis

Post-Contrast CT (CT Myelography):

  • Nerve root may be outlined by intrathecal contrast

  • Root compression or displacement visible as contour irregularity or absence of contrast surrounding the root

  • Useful when MRI is contraindicated

MRI image

Traversing nerve root of spinal nerve MRI sag anatomy image-img-00000-00000

MRI image

Traversing nerve root of spinal nerve MRI axial anatomy image-img-00000-00000

MRI image

Traversing nerve root of spinal nerve coronal mri image