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External urethral sphincter (female)

The external urethral sphincter in females is a circular band of striated muscle that surrounds the urethra. It is part of the pelvic floor musculature and contributes to voluntary urinary continence. In women, the sphincter is less robust than in men and is supplemented by associated urethral and pelvic floor muscles. Its clinical relevance lies in urinary continence disorders, pelvic floor dysfunction, and surgical approaches to the lower urinary tract.

Synonyms

  • Female urethral sphincter

  • Sphincter urethrae externus

  • Voluntary urethral sphincter

Location and Structure

  • Surrounds the mid-portion of the female urethra

  • Lies within the deep perineal pouch (urogenital diaphragm)

  • Extends slightly along the ventral urethra as longitudinal fibers

  • In females, often described as part of the striated urogenital sphincter complex, consisting of:

    • True external sphincter: circular fibers encircling the urethra

    • Compressor urethrae muscle: horizontal fibers compressing the urethra against the vagina

    • Urethrovaginal sphincter muscle: fibers encircling both the urethra and anterior vaginal wall

Relations

  • Anteriorly: Pubic symphysis

  • Posteriorly: Anterior vaginal wall

  • Superiorly: Bladder neck and proximal urethra

  • Inferiorly: Perineal membrane

  • Laterally: Ischiopubic rami and perineal muscles

Function

  • Provides voluntary control of micturition

  • Compresses urethra to maintain urinary continence

  • Works synergistically with levator ani muscles to support pelvic organs

  • Prevents leakage during increases in intra-abdominal pressure (e.g., coughing, sneezing, exertion)

Clinical Significance

  • Weakness or damage may cause stress urinary incontinence (e.g., childbirth trauma, aging, pelvic surgery)

  • Important landmark in female urethral sling surgeries and continence procedures

  • Can be affected in neurological conditions (e.g., spinal cord injury, multiple sclerosis)

  • Evaluated in pelvic floor MRI for continence disorders

  • May be injured during pelvic reconstructive or urogynecologic surgery

MRI Appearance

T1-weighted images:

  • Muscle appears as low-to-intermediate signal intensity

  • Fat around sphincter appears bright, aiding in outlining boundaries

T2-weighted images:

  • Muscle shows low signal intensity

  • Urethral lumen, if fluid-filled, appears bright

  • Surrounding fat appears bright 

STIR (Short Tau Inversion Recovery):

  • Muscle remains low signal intensity

  • Inflammatory or edematous changes in pathology may appear bright

  • Fat is suppressed and appears dark

T1 Fat-Sat Post-Contrast:

  • Normal muscle shows mild, uniform enhancement

  • Pathological changes (inflammation, tumor infiltration) may show focal or heterogeneous enhancement

CT Appearance

Non-Contrast CT:

  • Muscle not well visualized due to small size

  • Appears as soft tissue density surrounding the urethra

  • Fat planes around the urethra help localize the sphincter

Post-Contrast CT:

  • Normal sphincter shows mild, homogeneous enhancement

  • Pathology (tumor, infection, inflammation) may appear as asymmetric or irregular enhancement

  • Difficult to evaluate on CT alone; better seen on MRI

MRI image

External urethral sphincter female mri axial anatomy image-img-00000-00000

CT image

External urethral sphincter female CT axial anatomy image-img-00000-00000