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External urethral sphincter (male)

The external urethral sphincter, also known as the rhabdosphincter, is a voluntary skeletal muscle component of the male urethra. It encircles the membranous portion of the urethra and plays a key role in urinary continence. Unlike the internal urethral sphincter (which is smooth muscle), the external sphincter is under voluntary somatic control via the pudendal nerve. Its clinical importance lies in continence, prostate surgery, pelvic trauma, and imaging evaluation of urinary dysfunction.

Synonyms

  • Rhabdosphincter

  • Voluntary urethral sphincter

  • Striated urethral sphincter

Location and Boundaries

  • Surrounds the membranous urethra within the deep perineal pouch

  • Lies inferior to the prostate and superior to the bulb of the penis

  • Extends from the apex of the prostate to the perineal membrane

  • Closely related to the pelvic floor muscles and fascia

Relations

  • Superiorly: Apex of the prostate

  • Inferiorly: Bulb of penis and perineal membrane

  • Anteriorly: Pubic symphysis and puboprostatic ligaments

  • Posteriorly: Rectourethralis muscle and anterior rectal wall

  • Laterally: Levator ani muscles and ischioanal fossa

Function

  • Provides voluntary control of urination (continence mechanism)

  • Contracts during coughing, sneezing, or lifting to prevent leakage

  • Works synergistically with pelvic floor muscles and internal urethral sphincter

  • Important in preserving continence after prostatectomy

Clinical Significance

  • Injury during radical prostatectomy may result in stress urinary incontinence

  • Weakness or dysfunction can cause urinary leakage or incontinence

  • Hypertonicity may contribute to voiding dysfunction or urinary retention

  • Evaluated in urodynamic studies, pelvic MRI, and surgical planning

MRI Appearance

T1-weighted images:

  • Sphincter appears as a thin, low-signal intensity ring surrounding the urethra

  • Fat in adjacent perineal tissues appears bright, outlining the sphincter

  • No fluid: sphincter well defined as dark band

  • Hemorrhage or fibrosis may appear as higher signal intensity

T2-weighted images:

  • Normal sphincter demonstrates low signal intensity

  • Urethral lumen shows bright signal if fluid is present

  • Surrounding fat appears bright, contrasting with dark sphincter ring

  • Inflammation or edema may increase signal intensity in affected areas

STIR (Short Tau Inversion Recovery):

  • Normal sphincter remains dark (low signal)

  • Fluid in urethra appears bright

  • Fat suppressed and appears dark, increasing contrast between sphincter and surroundings

  • Pathology (edema, infection) appears bright

T1 Fat-Sat Post-Contrast:

  • Sphincter enhances mildly and uniformly

  • Inflammatory or neoplastic lesions may show more intense enhancement

  • Fluid does not enhance

CT Appearance

Non-Contrast CT:

  • Sphincter appears as a soft tissue density encircling the membranous urethra

  • Urethral lumen, if fluid-filled, appears hypodense

  • Surrounding fat appears as low attenuation, outlining the sphincter

Post-Contrast CT:

  • Sphincter shows mild, uniform enhancement

  • Pathological changes (infection, tumor, fibrosis) may cause focal or irregular enhancement

  • Fluid in urethra remains low attenuation, non-enhancing

MRI image

External Urethral Sphincter (Male) MRI axial anatomy image-img-00000-00000

MRI image

External Urethral Sphincter (Male) MRI sag anatomy image-img-00000-00000