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Sacral splanchnic nerves

The sacral splanchnic nerves are sympathetic autonomic nerves arising from the sacral part of the sympathetic trunk. They descend into the pelvis to join the inferior hypogastric plexus, where they intermingle with parasympathetic fibers from the pelvic splanchnic nerves. Their fibers primarily provide sympathetic innervation to the pelvic viscera, regulating smooth muscle tone, glandular secretion, and vascular control.

They play a crucial role in autonomic control of bladder, rectum, and reproductive organs, and are important in understanding pelvic pain syndromes, autonomic neuropathies, and surgical procedures in the pelvis.

Synonyms

  • Sympathetic sacral nerves

  • Pelvic sympathetic splanchnic nerves

  • Sacral sympathetic efferents

Origin, Course, and Connections

  • Origin:

    • Arise from the sacral sympathetic trunk (ventral rami of sacral nerves via rami communicantes)

  • Course:

    • Pass anteromedially from the sympathetic trunk in the sacral region

    • Converge toward the midline to reach the inferior hypogastric plexus (pelvic plexus)

  • Connections:

    • Communicate with parasympathetic pelvic splanchnic nerves (S2–S4)

    • Contribute to mixed autonomic plexuses supplying pelvic viscera

Relations

  • Anteriorly: Pelvic viscera (rectum, bladder, uterus/prostate) via inferior hypogastric plexus

  • Posteriorly: Sacrum and sacral foramina

  • Laterally: Sacral sympathetic trunk and sacral spinal nerves

  • Medially: Pelvic autonomic plexus at the rectouterine or rectovesical pouch region

Function

  • Provide sympathetic innervation to pelvic viscera

  • Control vasomotor tone of pelvic blood vessels

  • Contribute to contraction of internal urethral and anal sphincters

  • Modulate reproductive organ function (ejaculation in males, uterine contraction in females)

  • Balance parasympathetic activity in pelvic organs

Clinical Significance

  • Pelvic surgery: Must be preserved in nerve-sparing gynecological and urological procedures

  • Chronic pelvic pain: May be involved in autonomic pelvic pain syndromes

  • Oncology: Pelvic tumors may infiltrate or compress these nerves, causing autonomic dysfunction

  • Sympathetic blocks: Target for pelvic pain relief (e.g., hypogastric plexus block)

  • Trauma: Sacral fractures or pelvic injuries may damage these nerves

MRI Appearance

T1-weighted images:

  • Sacral splanchnic nerves appear as thin, low-to-intermediate signal intensity structures

  • Surrounded by bright pelvic fat, aiding visualization

T2-weighted images:

  • Appear with intermediate to mildly hyperintense signal compared to muscle

  • Pathological changes (edema, infiltration) appear brighter

STIR (Short Tau Inversion Recovery):

  • Normal nerves remain low signal

  • Inflamed or infiltrated nerves show bright hyperintensity

T1 Fat-Sat Post-Contrast:

  • Normal nerves: minimal or no enhancement

  • Pathological nerves: diffuse, focal, or nodular enhancement (neuritis, tumor, infection)

3D T2 SPACE / CISS:

  • Nerves appear as intermediate to mildly hyperintense linear structures compared to muscle

  • Surrounded by bright pelvic fat or fluid, enhancing contrast

  • Useful for identifying their course into the inferior hypogastric plexus and detecting compressive lesions

CT Appearance

Non-Contrast CT:

  • Sacral splanchnic nerves are usually not directly visualized

  • Course inferred as fine soft tissue strands near sacral sympathetic trunk in pelvic fat

  • Pathology may appear as soft tissue thickening or obliteration of fat planes

Post-Contrast CT:

  • Nerves themselves do not enhance

  • Surrounding plexus or pathological processes (tumor, infection, fibrosis) may show enhancement

  • Fat stranding in pelvic fat planes may suggest neuritis or infiltrative disease

MRI image

Sacral splanchnic nerves MRI coronal image