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Spinal nerve S4

The spinal nerve S4 is one of the sacral spinal nerves, arising from the fourth sacral spinal segment. It is a mixed nerve containing sensory, motor, and autonomic fibers. The S4 roots emerge from the conus medullaris as part of the cauda equina, traverse the sacral canal, and exit through the anterior and posterior sacral foramina of the sacrum. S4 contributes to the sacral plexus and pelvic splanchnic nerves, playing a critical role in pelvic floor motor control, perineal sensation, and autonomic regulation of pelvic organs.

Synonyms

  • Fourth sacral spinal nerve

  • S4 nerve root

  • Sacral segment 4 nerve

Origin, Course, and Branches

  • Origin:

    • Arises from dorsal (sensory) and ventral (motor) rootlets of the S4 spinal segment in the conus medullaris

    • Joins to form the mixed spinal nerve within the sacral canal

  • Course:

    • Travels within the sacral canal

    • Exits through the anterior and posterior sacral foramina of S4

    • Contributes to sacral plexus and pelvic plexus

  • Branches:

    • Pelvic splanchnic nerves (parasympathetic, S2–S4)

    • Contributions to pudendal nerve

    • Contributions to coccygeal plexus

    • Direct muscular branches to pelvic floor (levator ani, coccygeus)

Relations

  • Anteriorly: Pelvic organs (rectum, bladder, uterus/prostate)

  • Posteriorly: Sacral bone and posterior sacral foramina

  • Superiorly: S3 spinal nerve

  • Inferiorly: S5 spinal nerve and coccygeal nerve

Function

  • Motor: Contributes to levator ani, coccygeus, external anal sphincter, urethral sphincter, and pelvic floor muscles

  • Sensory: Supplies perianal skin, perineum, and parts of external genitalia (via pudendal and coccygeal plexus)

  • Autonomic (parasympathetic): Pelvic splanchnic contribution to bladder, rectum, and genital organs

Clinical Significance

  • Injury or compression of S4 may cause pelvic floor weakness, incontinence, perianal sensory loss

  • Important in cauda equina syndrome and sacral plexopathy

  • Target in sacral neuromodulation for urinary/fecal incontinence

  • May be affected by sacral fractures, tumors, or infections involving the sacral canal

MRI Appearance

T1-weighted images:

  • S4 nerve roots appear as low-to-intermediate signal linear structures against bright epidural fat

T2-weighted images:

  • Roots show intermediate to mildly hyperintense signal compared to muscle

  • Pathology (edema, compression, neuritis) increases signal

STIR (Short Tau Inversion Recovery):

  • Normal roots show low signal

  • Inflamed or compressed roots appear bright hyperintense

T1 Fat-Sat Post-Contrast:

  • Normal S4 roots enhance minimally

  • Radiculitis, tumor infiltration, or inflammation show focal/diffuse enhancement

3D T2 SPACE / CISS:

  • Roots appear intermediate to mildly hyperintense compared to muscle

  • Surrounded by very bright CSF in sacral canal, providing excellent contrast

  • Allows clear visualization of root trajectory to anterior/posterior sacral foramina and subtle compressive lesions

CT Appearance

Non-Contrast CT:

  • Nerve roots not directly visible; location inferred in S4 sacral foramina

  • Sacral bone margins and foraminal fat help define pathway

Post-Contrast CT:

  • Nerve itself does not enhance

  • Pathological processes (nerve sheath tumors, inflammatory infiltration, metastasis) appear as enhancing or soft tissue lesions in sacral canal or foramina

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