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Exiting nerve root of spinal nerve S4

The exiting nerve root of spinal nerve S4 is one of the lowest sacral nerve roots. It leaves the dural sac at the S4 level, travels a short distance within the sacral canal, and exits the vertebral column through the fourth sacral foramen. It carries motor, sensory, and parasympathetic fibers and contributes significantly to pelvic floor, perineal, and visceral functions.

This root is clinically important because of its involvement in bladder, bowel, and sexual function, as well as in pelvic plexus innervation. Damage or pathology of S4 can lead to profound pelvic dysfunction.

Synonyms

  • Sacral nerve root S4

  • Fourth sacral exiting nerve root

  • Sacral spinal nerve (S4 segment)

Origin, Course, and Exit

  • Origin:

    • Arises from dorsal and ventral rootlets of the S4 spinal cord segment at the conus medullaris

  • Course:

    • Rootlets converge to form dorsal and ventral roots within the dural sac

    • Roots travel a short intrathecal course in the sacral canal

    • Combine into a mixed spinal nerve root at the foramen

  • Exit:

    • Leaves the sacral canal through the fourth sacral foramen

Relations

  • Anteriorly: Sacral body and pelvic viscera (rectum, bladder, vagina/prostate)

  • Posteriorly: Sacral lamina and ligaments

  • Superiorly: Exiting S3 nerve root

  • Inferiorly: Exiting S5 nerve root

  • Laterally: Pelvic splanchnic nerves and components of sacral plexus

Function

  • Motor contributions:

    • Innervates parts of the pelvic floor and external anal sphincter

    • Contributes to levator ani and coccygeus muscles

  • Sensory contributions:

    • Provides sensation to perianal skin, anal canal, and parts of external genitalia

  • Autonomic contributions:

    • Carries parasympathetic fibers contributing to pelvic splanchnic nerves (nervi erigentes) for bladder, rectal, and sexual function

Clinical Significance

  • Pelvic dysfunction: Injury may cause urinary and fecal incontinence, sexual dysfunction

  • Radiculopathy: May be affected by sacral tumors, trauma, or inflammatory lesions

  • Surgical relevance: Must be preserved during pelvic, colorectal, and gynecological procedures

  • Oncology: Important in staging pelvic cancers due to perineural tumor spread

  • Imaging: Key landmark in sacral canal MRI for evaluating cauda equina and sacral plexus pathology

MRI Appearance

T1-weighted images:

  • Exiting S4 root appears as a fine linear structure of low-to-intermediate signal

  • Surrounded by bright epidural and perineural fat in the sacral canal and foramina

T2-weighted images:

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

  • Surrounded by bright CSF within the dural sac, then by fat at its exit through the foramen

  • Pathological changes (edema, neuritis, infiltration) show increased signal intensity

T1 Fat-Sat Post-Contrast:

  • Normal root: minimal or no enhancement

  • Pathology (infection, tumor, neuritis): focal or diffuse enhancement

3D T2 SPACE / CISS:

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

  • Clearly outlined by very bright CSF proximally and fat distally

  • Provides excellent visualization of its short course and foraminal exit, especially in cases of entrapment, displacement, or perineural spread

MRI image

Exiting nerve root of spinal nerve S4 MRI coronal anatomy image-img-00000-00000