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Cauda equina

The cauda equina (“horse’s tail”) is a bundle of lumbar, sacral, and coccygeal nerve roots that descend within the lumbar spinal canal below the termination of the spinal cord at the conus medullaris (around L1–L2 level in adults). These nerve roots extend inferiorly through the lumbar cistern, suspended in cerebrospinal fluid (CSF), before exiting individually through their respective intervertebral foramina.

Anatomically, the cauda equina is composed of the L2–L5 lumbar roots, all sacral roots, and the coccygeal root. They are arranged in a fan-like distribution within the dural sac. The roots of the cauda equina control motor and sensory functions of the pelvis, bladder, bowel, lower limbs, and perineum.

Clinically, compression of these nerve roots results in cauda equina syndrome, a neurosurgical emergency characterized by severe back pain, bilateral lower limb weakness, saddle anesthesia, and bowel/bladder dysfunction. The cauda equina is also important in cases of lumbar disc herniation, spinal stenosis, trauma, tumors, arachnoiditis, and inflammatory neuropathies.

Synonyms

  • Horse’s tail of spinal cord

  • Lumbar and sacral nerve root bundle

  • Distal spinal nerve roots

Function

  • Provides motor and sensory innervation to lower limbs, perineum, and pelvic organs

  • Controls bladder and bowel function through parasympathetic and somatic fibers

  • Serves as a critical pathway for reflexes and voluntary motor control

MRI Appearance

T1-weighted images:

  • Nerve roots appear as linear to dot-like intermediate-to-low signal structures within the hyperintense CSF of the lumbar cistern

  • Distinction from surrounding fat and vertebral structures is clear

T2-weighted images:

  • CSF appears bright hyperintense, while cauda equina roots appear as hypointense linear bundles

  • Excellent for detecting compression, displacement, or clumping of roots

STIR:

  • Suppresses fat, enhancing visibility of the nerve roots against vertebral and epidural fat

  • Highlights inflammation, edema, or root pathology

T1 Fat-Saturated Post-Contrast (Gadolinium):

  • Nerve roots normally show no enhancement

  • Pathological conditions (arachnoiditis, neoplasms, inflammatory neuropathy) show abnormal root enhancement

  • Detects leptomeningeal disease, nerve sheath tumors, and radiculitis

MRI Non-Contrast 3D Imaging:

  • Provides high-resolution multiplanar visualization of root orientation, crowding, and displacement

  • Valuable in pre-surgical planning and stenosis assessment

CT Appearance

CT Pre-Contrast:

  • Cauda equina nerve roots are not directly visible but inferred as soft tissue densities within the thecal sac

  • Bony detail allows assessment of stenosis, fractures, or foraminal narrowing

CT Post-Contrast (CT Myelography):

  • With intrathecal contrast, cauda equina roots appear as linear filling defects within opacified CSF

  • Useful alternative in patients unable to undergo MRI

  • Excellent for detecting arachnoid adhesions, root avulsions, or compressive lesions

MRI images

Cauda equina MRI sagittal anatomy image -img-00000-00000

MRI images

Cauda equina MRI axial anatomy image -img-00000-00000

CT images

Cauda equina CT axial image

CT images

Cauda equina ct sag image