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Perineal nerves

The perineal nerves are branches of the pudendal nerve, which arises from the sacral plexus (S2–S4 ventral rami). They course through the perineum after branching in the pudendal (Alcock’s) canal. The perineal nerves divide into superficial branches and deep branches, supplying the perineal muscles, perineal skin, external genitalia, and providing part of the sensory innervation of the urogenital region.

They are clinically important in pelvic surgery, obstetrics, urogynecology, and chronic pelvic pain syndromes.

Synonyms

  • Pudendal perineal branches

  • Superficial and deep perineal nerves

  • Perineal division of pudendal nerve

Origin, Course, and Branches

  • Origin: Arises from the pudendal nerve in the pudendal canal (Alcock’s canal), formed by S2–S4

  • Course:

    • Travels anteriorly within the pudendal canal alongside the internal pudendal vessels

    • Divides into superficial and deep perineal branches near the urogenital diaphragm

  • Branches:

    • Superficial perineal nerves: Provide cutaneous innervation to the posterior scrotum in males or posterior labia in females (often via posterior scrotal/labial branches)

    • Deep perineal nerves: Supply the superficial and deep perineal muscles (e.g., bulbospongiosus, ischiocavernosus, superficial transverse perineal muscle), and mucosa of the vestibule/vagina in females

Relations

  • Superiorly: Levator ani muscles and pelvic diaphragm

  • Inferiorly: Superficial perineal pouch

  • Laterally: Ischiopubic ramus and obturator internus muscle (via pudendal canal)

  • Medially: Urogenital diaphragm and perineal body

Function

  • Motor:

    • Deep perineal nerves supply perineal muscles: bulbospongiosus, ischiocavernosus, superficial/deep transverse perineal muscles, external urethral sphincter

  • Sensory:

    • Superficial perineal nerves supply posterior scrotum (male) or posterior labia (female)

    • Contribute to sensation of perineal skin, mucosa of vestibule (female), and urethra

  • Autonomic (via pudendal nerve): Contribute to erectile tissue function

Clinical Significance

  • Obstetrics/gynecology: Target for perineal infiltration anesthesia during episiotomy

  • Pelvic surgery: At risk of injury during perineal or urogenital surgery

  • Neuropathy: Entrapment or damage may cause chronic perineal pain or sexual dysfunction

  • Urology: Important in continence mechanisms via external urethral sphincter innervation

  • Oncology: Evaluated in pelvic malignancies with perineal spread

MRI Appearance

T1-weighted images:

  • Appear as thin, low-to-intermediate signal nerves within bright ischiorectal and perineal fat planes

T2-weighted images:

  • Nerves show intermediate to mildly hyperintense signal compared to muscle

  • Pathological nerves (edema, entrapment) appear more hyperintense

STIR (Short Tau Inversion Recovery):

  • Normal perineal nerves: low signal

  • Inflamed or edematous nerves: bright hyperintensity

T1 Fat-Sat Post-Contrast:

  • Normal nerves: minimal or no enhancement

  • Pathological nerves: show focal or diffuse enhancement (neuritis, infiltration, tumor spread)

3D T2 SPACE / CISS:

  • Nerves show intermediate to mildly hyperintense signal compared to muscle

  • Surrounded by bright fat/CSF providing excellent contrast

  • Useful for delineating branches in the pudendal canal and perineum

CT Appearance

Non-Contrast CT:

  • Nerves not directly seen, inferred by course within fat of ischiorectal fossa and pudendal canal

  • Surrounding fat planes highlight expected location

Post-Contrast CT:

  • Nerves themselves do not enhance

  • Pathology (inflammation, tumor infiltration, abscess) may appear as soft tissue thickening or enhancing masses along their course

  • Fat stranding may indicate inflammatory or infectious processes

MRI image

Perineal nerves mri axial anatomy image-img-00000-00000